FUNDAMENTALS OF THE THEORY OF OPERATOR ALGEBRAS VOLUMEI Elementary Theory
This is a volume in PURE AND APPLIED MATHEMATICS A Series of Monographs and Textbooks
Editors: SAMUEL EILENBERG AND HYMAN BASS A list of recent titles in this series appears at the end of this volume.
FUNDAMENTALS OF THE THEORY OF OPERATOR ALGEBRAS VOLUMEI Elementary Theory Richard V. Kadison
John R. Ringrose
Department of Mathematics University of Pennsylvania Philadelphia, Pennsylvania
School of Marhematics University of Newcastle Newcastle upon Tyne, England
1983
ACADEMIC PRESS A Subsidiary of Harcourt Brace Jovanovich, Publishers
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Library of Congress Cataloging in Publication Data Kadison, Richard V., date Fundamentals of the theory of operator algebras. (Pure and applied mathematics) Includes index. 1. Operator algebras. I. Ringrose, John R. 111. Series: Pure and applied mathematics 11. Title. (Academic Press) QA3.P8 [QA326] 512’.55 82-13768 ISBN 0-12-393301-3 (v. 1)
PRINTED IN THE UNITED STATES OF AMERICA a3 a4 a5 86
9
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7 6 5 4 3 2 1
CONTENTS
vii xiii
Preface Contents of Volume I1
Chapter 1 . Linear Spaces 1.1. 1.2. 1.3. 1.4. 1.5. 1.6. 1.7. 1.8. 1.9.
Algebraic results Linear topological spaces Weak topologies Extreme points Norrned spaces Linear functionals on normed spaces Some examples of Banach spaces Linear operators acting on Banach spaces Exercises
1 12 28 31 35 43 48 59 65
Chapter 2. Basics of Hilbert Space and Linear Operators 2.1. 2.2. 2.3. 2.4.
2.5. 2.6.
2.7. 2.8.
Inner products on linear spaces Orthogonality The weak topology Linear operators General theory Classes of operators The lattice of projections Constructions with Hilbert spaces Subspaces Direct sums Tensor products and the Hilbert-Schmidt class Matrix representations Unbounded linear operators Exercises V
75 85 97
99 100 103 109 120 120 121 125 147 154 161
C0N TEN TS
vi Chapter 3. Banach Algebras
3.1. Basics 3.2. The spectrum The Banach algebra L,(W) and Fourier analysis 3.3. The holomorphic function calculus Holomorphic functions The holomorphic function calculus 3.4. The Banach algebra C(X) 3.5. Exercises
173 178 187 202 202 205 210 223
Chapter 4. Elementary C*-Algebra Theory 4.1. 4.2. 4.3. 4.4. 4.5. 4.6.
Basics Order structure Positive linear functionals Abelian algebras States and representations Exercises
236 244 255 269 275 285
Chapter 5. Elementary von Neumann Algebra Theory 5.1. 5.2. 5.3. 5.4.
The weak- and strong-operator topologies Spectral theory for bounded operators Two fundamental approximation theorems Irreducible algebras-an application 5.5. Projection techniques and constructs Central carriers Some constructions Cyclicity, separation, and countable decomposability 5.6. Unbounded operators and abelian von Neumann algebras 5.7. Exercises
304 309 325 330 332 332 334 336 340 370
Bibliography
384
Index of Notation Index
387 391
PREFACE
These volumes deal with a subject, introduced half a century ago, that has become increasingly important and popular in recent years. While they cover the fundamental aspects of this subject, they make no attempt to be encyclopaedic. Their primary goal is to teach the subject and lead the reader to the point where the vast recent research literature, both in the subject proper and in its many applications, becomes accessible. Although we have put major emphasis on making the material presented clear and understandable, the subject is not easy; no account, however lucid, can make it so. If it is possible to browse in this subject and acquire a significant amount of information, we hope that these volumes present that opportunity-but they have been written primarily for the reader, either starting at the beginning or with enough preparation to enter at some intermediate stage, who works through the text systematically. The study of this material is best approached with equal measures of patience and persistence. Our starting point in Chapter 1 is finite-dimensional linear algebra. We assume that the reader is familiar with theresults of that subject and begin by proving the infinite-dimensional algebraic results that we need from time to time. These volumes deal almost exclusively with infinitedimensional phenomena. Much of the intuition that the reader may have developed from contact with finite-dimensional algebra and geometry must be abandoned in this study. It will mislead as often as it guides. In its place, a new intuition about infinite-dimensional constructs must be cultivated. Results that are apparent in finite dimensions may be false, or may be difficult and important principles whose application yields great rewards, in the infinite-dimensional case. Almost as much as the subject matter of these volumes is infinite dimensional, it is non-commutative real analysis. Despite this description, the reader will find a very large number of references to the “abelian” or “commutative” case-an important part of this first volume is an analysis of the abelian case. This case, parallel to function theory and measure theory, provides us with a major tool and an important guide to our vii
viii
PREFACE
intuition. A good part of what we know comes from extending to the noncommutative case results that are known in the commutative case. The “extension” process is usually dimcult. The main techniques include elaborate interlacing of “abelian” segments. The reference to “real analysis’’ involves the fact that while we consider complex-valued functions and, non-commutatively, non-self-adjoint operators, the structures we study make simultaneously available to us the complex conjugates of those functions and, non-commutatively, the adjoints of those operators. In essence, we are studying the algebraic interrelations of systems of real functions and, non-commutatively, systems of self-adjoint operators. At its most primitive level, the non-commutativity makes itself visible in the fact that the product of a function and its conjugate is the same in either order while this is not in general true of the product of an operator and its adjoint. In the sense that we consider an operator and its adjoint on the same footing, the subject matter we treat is referred to as the “self-adjoint theory.” There is an emerging and important development of non-selfadjoint operator algebras that serves as a non-commutative analogue of complex function theory-algebras of holomorphic functions. This area is not treated in these volumes. Many important developments in the selfadjoint theory-both past and current-are not treated. The type I C*algebras and C*-algebra K-theory are examples of important subjects not dealt with. The aim of teaching the basics and preparing the reader for individual work in research areas seems best served by a close adherence to the “classical” fundamentals of the subject. For this same reason, we have not included material on the important application of the subject to the mathematical foundation of theoretical quantum physics. With one exception, applications to the theory of representations of topological groups are omitted. Accounts of these vast research areas, within the scope of this treatise, would be necessarily superficial. We have preferred instead to devote space to clear and leisurely expositions of the fundamentals. For several important topics, two approaches are included. Our emphasis on instruction rather than comprehensive coverage has led us to settle on a very brief bibliography. We cite just three textbooks (listed as [HI, [K], and [R]) for background information on general topology and measure theory, and for this first volume, include only 25 items from the literature of our subject. Several extensive and excellent bibliographies are available (see, for example, [2,24,25]), and there would be little purpose in reproducing a modified version of one of the existing lists, We have included in our references items specifically referred to in the text and others that might provide profitable additional reading. As a consequence, we have made no attempt, either in the text or in the exer-
PREFACE
ix
cises, to credit sources on which we have drawn or to trace the historical background of the ideas and results that have gone into the development of the subject. Each of the chapters of this first volume has a final section devoted to a substantial list of exercises, arranged roughly in the order of the appearance of topics in the chapter. They were designed to serve two purposes: to illustrate and extend the results and examples of the earlier sections of the chapter, and to help the reader to develop working technique and facility with the subject matter of the chapter. For the reader interested in acquiring an ability to work with the subject, a certain amount of exercise solving is indispensable. We do not recommend a rigid adherence to ordergach exercise being solved in sequence and no new material attempted until all the exercises of the preceding chapter are solved. Somewhere between that approach and total disregard of the exercises a line must be drawn congenial to the individual reader’s needs and circumstances. In general, we do recommend that the greater proportion of the reader’s time be spent on a thorough understanding of the main text than on the exercises. In any event, all the exercises have been designed to be solved. Most exercises are separated into several parts with each of the parts manageable and some of them provided with hints. Some are routine, requiring nothing more than a clear understanding of a definition or result for their solutions. Other exercises (and groups of exercises) constitute small (guided) research projects. On a first reading, as an introduction to the subject, certain sections may well be left unread and consulted on a few occasions as needed. Section 2.6, Tensor products and the Hilbert-Schmidt class (this “subsection” is the largest part of Section 2.6) will not be needed seriously until Chapter 11 (in Volume 11). All the material on unbounded operators (and the material related to Stone’s theorem) will not be needed until Chapter 9 (in Volume 11). Thus Section 2.7, Section 3.2, The Banach algebra L,(R) and Fourier analysis, the last few pages of Chapter 4 (including Theorem 4.5.9), and Section 5.6, can be deferred to a later reading. Some readers, more or less familiar with the elements of functional analysis, may want to enter the text after Chapter 1 with occasional back references for notation or precise definitions and statements of results. The reader with a good general knowledge of basic functional analysis may consider beginning at Section 3.4 or perhaps with Chapter 4. The various possible styles of reading this volume, related to the levels of preparation of the reader, suggest several styles and levels of courses for which it can be used. For all of these, a good working knowledge of point-set (general) topology, such as may be found in [K],is assumed. Somewhat less vital, but useful, is a knowledge of general measure the-
X
PREFACE
ory, such as may be found in [HI and parts of [R]. Of course, full command of the fundamentals of real and complex analysis (we refer to [R]for these) is needed; and, as noted earlier, the elements of finite-dimensional linear algebra are used. The first three chapters form the basis of a course in elementary functional analysis with a slant toward operator algebras and its allied fields of group representations, harmonic analysis, and mathematical (quantum) physics. These chapters provide material for a brisk one-semester course at the first- or second-year graduate level or for a more leisurely one-year course at the advanced undergraduate or beginning graduate level. Chapters 3, 4, and 5 provide an introduction to the theory of operator algebras and have material that would serve as a onesemester graduate course at the second- or third-year level (especially if Section 5.6 is omitted). In any event, the book has been designed for individual study as well as for courses, so that the problem of a wide spread of preparation in a class can be dealt with by encouraging the better prepared students to proceed at their own paces. Seminar and reading-course possibilities are also available. When several (good) terms for a mathematical construct are in common use, we have made no effort to choose one and then to use that one term consistently. On the contrary, we have used such terms interchangeably after introducing them simultaneously. This seems the best preparation for further reading in the research literature. Some examples of such terms are weaker, coarser (for topologies on a space), unitary transformation, and Hilbert space isomorphism (for structure-preserving mappings between Hilbert spaces). In cases where there is conflicting use of a term in the research literature (for example, “purely infinite” in connection with von Neumann algebras), we have avoided all use of the term and employed accepted terminology for each of the constructs involved. Since the symbol * is used to denote the adjoint operations on operators and on sets of operators, we have preferred to use a different symbol in the context of Banach dual spaces. We denote the dual space of a Banach space 3 by 3”.However,,we felt compelled1by usage to retain the terminology “weak *” for the topology induced by elements of 3E (as linear functionals on 3’ ), Results in the body of the text are italicized, titled Theorem, Proposition, Lemma, and Corollary (in decreasing order of “importance”though, as usual, the “heart of the matter’’ may be dealt with in a lemma and its most usable aspect may appear in a corollary). In addition, there are Remarks and Examples that extend and illuminate the material of a section, and of course there are the (formal) Definitions. None of these items is italicized, though a crucial phrase or word frequently is. Each of these segments of the text is preceded by a number, the first digit of which
PREFACE
xi
indicates the chapter, the second the section, and the last one- or twodigit number the position of the item in the section. Thus, “Proposition 5.5.18” refers to the eighteenth numbered item in the fifth section of the fifth chapter. A back or forward reference to such an item will include the title (“Theorem,” “Remark,” etc.), though the number alone would serve to locate it. Occasionally a displayed equation, formula, inequality, etc., is assigned a number in parentheses at the left of the display-for example, the “convolution formula” of Fourier transform theory appears as the display numbered (4) in the proof of Theorem 3.2.26. In its own section, it is referred to as (4) and elsewhere as 3.2(4). The lack of illustrative examples in much of Chapter 1 results from our wish to bring the reader more rapidly to the subject of operator algebras rather than to dwell on the basics of general functional analysis. As compensation for their lack, the exercises supply much of the illustrative material for this chapter. Although the tensor product development in Section 2.6 may appear somewhat formal and forbidding at first, it turns out that the trouble and care taken at that point simplify subsequent application. The same can be said (perhaps more strongly) about Section 5.6. The material on unbounded operators (their spectral theory and function calculus) is so vital when needed and so susceptible to incorrect and incomplete application that it seemed well worth a careful and thorough treatment. We have chosen a powerful approach that permits such a treatment, much in the spirit of the theory of operator algebras. Another (general) aspect of the organization of material in a text is the way the material of the text proper relates to the exercises. As a matter of specific policy, we have not relegated to the exercises whole arguments or parts of arguments. Reference is occasionally made to an exercise as an illustration of some point-for example, the fact that the statement resulting from the omission of some hypothesis from a theorem is false. During the course of the preparation of these volumes, we have enjoyed, jointly and separately, the hospitality and facilities of several universities, aside from our home institutions. Notable among these are the Mathematics Institutes of the Universities of Aarhus and Copenhagen and the Theoretical Physics Institute of Marseille-Luminy. The subject matter of these volumes and its style of development is inextricably interwoven with the individual research of the authors. As a consequence, the support of that research by the National Science Foundation (U.S.A.) and the Science Research Council (U.K.) has had an oblique but vital influence on the formation of these volumes. It is the authors’ pleasure to express their gratitude for this support and for the hospitality of the host institutions noted.
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CONTENTS OF VOLUME 11 Advanced Theory
Chapter 6. Comparison Theory of Projections 6.1. 6.2. 6.3. 6.4. 6.5. 6.6. 6.7. 6.8. 6.9.
Polar decomposition and equivalence Ordering Finite and infinite projections Abelian projections Type decomposition Type I algebras Examples Ideals Exercises
Chapter 7. Normal States and Unitary Equivalence of von Neumann Algebras 7.1. 7.2. 7.3. 7.4. 7.5. 7.6.
Completely additive states Vector states and unitary implementation A second approach to normal states The predual Normal weights on von Neumann algebras Exercises
Chapter 8. The Trace 8.1. Traces
The trace in finite algebras The Dixmier approximation theorem The dimension function Tracial weights on factors Further examples of factors An operator-theoretic construction Measure-theoretic examples 8.7. Exercises 8.2. 8.3. 8.4. 8.5. 8.6.
xiii
xiv
CONTENTS OF VOLUME I1
Chapter 9. Algebra and Commutant 9.1. The type of the commutant 9.2. Modular theory A first approach to modular theory Tomita's theorem-a second approach A further extension of modular theory 9.3. Unitary equivalence of type I algebras 9.4. Abelian von Neumann algebras 9.5. Spectral multiplicity 9.6. Exercises
Chapter 10. Special Representations of C*-Algebras 10.1. The universal representation 10.2. Irreducible representations 10.3. Disjoint representations 10.4. Examples Abelian C*-algebras Compact operators !a(,#)and the Cakin algebra Uniformly matricial algebras 10.5. Exercises
Chapter 11. Tensor Products 11.1. Tensor products of represented C*-algebras
11.2. Tensor products of von Neumann algebras Elementary properties The commutation theorem The type of tensor products Tensor products of unbounded operators 1I .3. Tensor products of abstract C*-algebras The spatial tensor product C*-norms on 'LI 0 d Nuclear C*-algebras 11.4. lnfinite tensor products of C*-algebras 113. Exercises
Chapter 12. Approximation by Matrix Algebras 12.1. 12.2. 12.3. 12.4.
Isomorphism of uniformly matricial algebras The finite matricial factor States and representations of matricial C*-algebras Exercises
CONTENTS OF VOLUME I1
Chapter 13. Crossed Products 13.1. 13.2. 13.3. 13.4.
Discrete crossed products Continuous crossed products Crossed products by modular automorphism groups Exercises
Chapter 14. Direct Integrals and Decompositions 14.1. 14.2. 14.3. 14.4.
Direct integrals Decompositions relative to abelian algebras Appendix-Bore1 mappings and analytic sets Exercises
Bibliography
xv
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CHAPTER I
LINEAR SPACES
This chapter contains an account of those basic aspects of linear functional analysis that are needed, later in the book, in the study of operator algebras. The main topics - continuous linear operators, continuous linear functionals, weak topologies, convexity - are studied first in the context of linear topological spaces, then in the more restricted setting of normed spaces and Banach spaces. In preparation for this, some related material is treated in the purely algebraic situation (that is, without topological considerations). 1.1. Algebraic results
In this section we shall consider linear spaces (that is, vector spaces) over a field K, and it will be assumed throughout that K is either the real field R or the complex field C. We sometimes distinguish between these two cases by referring to real vector spaces or complex vector spaces. Our main concern is with linear functionals, convex sets, and the separation of convex sets by hyperplanes. Suppose that $ . is a linear space with scalar field K. If X and Y are nonempty subsets of $; and U E K, we define further subsets a X , X f Y by ax= (as:sEXJ,
x+
Y = {s+y:xEX,yEY},
and
x-
Y = X + ( - 1)Y.
When X consists of a single element s,we write x f Y in place of X f Y. To avoid ambiguity in the use of the symbol-, the set theoretic difference { ~ E A : ~ $oftwosetsGandBwillbedenotedbyA\B. BJ Avectoroftheform a 1 x , + . . . + a,x,, where x I ,. . . ,x, E X and a l ,. . . ,a, E 06, is called a (finite) linear combination of elements of X . The zero vector is always of this form (in a . . . ,x,) an arbitrary finite subset of X , and aj = 0 for trivial way), with {s,, each j . If it can be expressed as a non-triuiul linear combination of elements of X (that is, with x l , .. . ,x, distinct, and at least one aj non-zero), then Xis said to be linearly dependent; otherwise X is linearly independent. The set of all
2
1. LINEAR SPACES
finite linear combinations of elements of X is a linear subspace of the smallest containing X ;we refer to it as the linear subspace generated by A'. If 9; is a linear subspace of % we denote by Y-/Y,j the set of all cosets x + Yo(XE Y ) in the additive group $C Of course, Y/%, is a group, with addition defined by (x + Yo)+ ( y + Yo)= (x + y ) + 9';. If a € H, and x1 + %$= x2 + G, we have axl - axz = a(xl - x 2 ) ~ V o ,so axl + $; = ax2 + 6. From this it follows easily that $'-/Yobecomes a linear space over 06, the quotient of Y'. by Y;, when multiplication by scalars is defined (unambiguously) by a(x + G) = ax + Yo.If V / V i has finite dimension n, we say that YG has finite codimension n in 9'.' Suppose that Y and W are linear spaces over K. By a linear operator (or linear transformation) from Y into .Iyc we mean a mapping T : Y + W such that T(ax + by) = aTx
+ bTy
whenever x, Y E V and a, b~ K (the notation T : Y + W indicates that T is defined on Y and takes values in $6'"; it can be read " T , from Y into W " ) .If Y; is a linear subspace of K the equation Q x = x + Yo defines a linear operator Q from Y onto V - / P ; ,the quotient mapping. When T : Y + Y#" is a linear operator, the nullspace of T i s the linear subspace { X E Y :T x = 0 ) of Y, and the image (or range) T ( Y ) = { T X : X E V } is a linear subspace of %KIf T( Pi) = { O } , the condition x + = y + 6 entails x - Y E U;, and hence T X - Ty = 0 ; moreover, if 6 is the null space of T, Tx = 0 entails X E Yo. From this, the equation T,(x + Yo)= T x defines (unambiguously) a linear operator To from V / V ; onto T ( Y )( E W"),when T(Yo)= ( 0 ); and To is oneto-one if $;' is the null space of T. Note that T = T o e , a fact sometimes described by saying that Tfactors through Y/Yowhen T ( 6 ) = { O } . Given any linear operators S, T : Y- + W" and scalars a, 6, the equation (aS + b T ) x = aSx + bTx ( X E V ) defines another such operator aS + bT, and in this way, the set of all linear operators from 9- into W becomes a linear space over K. By a linearfunctionalon Y we mean a linear operator p : *Y' + H (of course, K is a one-dimensional linear space over W). The set of all linear functionals on 3' is itself a linear space over K, the algebraic dualspace of Y': When p is a nonzero linear functional on V (that is, p does not vanish identically on U')the image p ( V )is K. 1.1.1. PROPOSITION. If p is a linearfunctionalon a linear space $ ;then every linear functional on Y' that vanishes on the null space Yoof p is a scalar multiple of p . I f ' p # 0, Y; has codimension 1 in $C Conversely each linear subspace of codimension 1 in V - is the null space of a non-zero linearfunctional. U p ,, . . . ,pn are linearfunctionals on %< then every linearfunctional on 9' that vanishes on the intersection ojthe null spaces of p l , . . . ,p . is a linear combination of p l , . . . ,pn.
3
1 . 1 . ALGEBRAIC RESULTS
Proof We may suppose that p # 0. The equation p o ( x + Yo)= p ( x ) defines a one-to-one linear operator p o from *Y /YG onto the one-dimensional linear space K ;so 9 ‘/Yois one dimensional. Of course, p o is a non-zero linear functional on $ ’/$i; and in the same way, if a linear functional B on 3’vanishes on T i , there is a linear functional go on V/Yo,defined by o0(x + r’;) = cr(x). Since $//Yo is one dimensional, o0 = upo for some scalar a, and B = o0Q = apoQ = up, where Q is the quotient mapping from Y onto Y/Y&
If 9 ; is a linear subspace with codimension 1 in
,
o=alpl
+
” ’
,
,
,
+akPk+ak+lPk+l.
Suppose that V is a linear space over K ( = R or C), and X , Y E 7T By a (finite) convex combination of elements of X, we mean a vector of the form a l x l + . . . + a,x,, where x, ,. . . ,x, E Xand a,, . . . ,a, are real scalars satisfying a, > 0 ( j = 1, . . . ,n) and 2 a, = 1. It makes no difference in this definition if the condition a, > 0 is relaxed to a, b 0 (because zero terms can be deleted), but strict inequality is slightly more convenient for our present purposes. We say that Y is conres if b , y , + h2y2E Y whenever y , , y 2 E Y and b , , 6, are positive real numbers with sum 1 (that is, Y contains each convex combination ofjust two elements of Y ;geometrically, this means that each line segment with endpoints in Y lies wholly in Y ) .A simple proof, by induction on n, shows that a convex set Y contains every convex combination a l x l + * . . + a,x, of elements s1, . . . , x, of Y ; the “inductive step up,” from n - 1 to n, depends on the observation that a , x , + . . . + a,x, = b , y , b,y,, where b , = a , , b2 = a2 + . * . + a n r y l= sI, andy, is theconvexcombination b; l(U2.Y2 + . . . + a,x,) of x 2 , . . . ,x,. It is sometimes useful to note that a subset Y of Y is convex if and only if a, Y a, Y = ( a , + a 2 )Y whenever a, and a, are non-negative scalars; for this
+
+
4
I . LINEAR SPACES
isequivalenttn(ai + u 2 ) - l ( u ,Y + a 2 Y )= Y, whena, anda2arenon-ncgativc scalars, not both 0. When X G $',.we denote by co X the set of all finite convex combinations of elements of X . A straightforward calculation shows that if y , , . . . ,y . E C O X, then every convex combination of y , ,. . . ,y . lies in co X. Thus co Xis a convex set, the smallest one containing X; it is called the convex hullof X . By an internal point of X we mean a vector x in Xwith the following property: given any y in Y'; there is a positive real number c such that x + ayE X whenever 0 6 a < c. Our next result is concerned with real vector spaces. By a hyperplane, in a linear space V over R, we mean a set of the form xo + Yo,where xoE V and U,j is a linear subspace with codimension 1 in K From Proposition 1.1.1, a subset H of V ' is a hyperplane if and only if it can be expressed in the form
H
=
{ x E Y : ~ (=xk)} ,
where p is a non-zero linear functional on Y and k E R; of course, p and k are not uniquely determined by H, but the only possible variation is to replace them by ap and ak, respectively, where a is a non-zero real number. With the hyperplane H we can associate the two closed half-spaces, { X E f " : . p ( x )2 k } and { X E V : p ( x )< k } , and the two open half-spaces, which are defined similarly but with strict inequalities. We say that H separates two subsets Y and Z of YT if Y is contained in one of the closed half-spaces determined by Hand Z is contained in the other; strict separation is defined similarly in terms of the open half-spaces. If the hyperplane is described in terms of ap and ak, the property of separation remains unchanged (although the two half-spaces are interchanged if a < 0). 1.1.2. THEOREM.I f Y and Zare non-empty disjoint convex subsets of a real rector space V;at least one of which has an internalpoint, they are separated by a hyperplane H in 9.:I f either Y or Z consists entirely of internal points, it is contained in one of the open half-spaces determined by H . Ifboth Y and Z consist entirely of internal points, they are strictly separated by H . Proof: We may suppose that Y has an internal point, and denote by Yi the set of all internal points of Y. It is easily verified that Yi is a convex subset of Y, and that (1 - a)yl + a y e Yi whenever y , E Yi, Y E Y , and 0 < a < 1 . We assert that every point of Yi is an internal point of Yi, and that a hyperplane which separates Yi and Z also separates Y and Z . For this, suppose that y , E Yi and X E K Since y , is an internal point of Y , y , + C X E Y for some positive scalar c. From the preceding paragraph,
+ a c x = ( I a)yl + a ( y l + C X ) E Yi when 0 G a < 1 ;so y, + bx E Yi whenever 0 G b < c, and thus y , is an internal y1
-
point of Yi. If H is a hyperplane separating Yi and Z , there is a non-zero linear
functional p on Y and a scalar k such that H p(yi) 2 k k p(=)
(4'1 E
U)P(Yl)
+ w(i)= P(( 1
and
Y,, ZEZ).
Given y in Y , choose any y , in Y ,. Since ( 1 - a)y have -
f : p ( ~= ) k}
=
'
(1
5
ALGEBRAIC RESULTS
1.1.
, + ay
- Q)Yl
E
Y, when 0 < a < I , we
+ ay) 2 k ;
and when a + I , we obtain p ( y ) 2 k . Thus H separates Y and Z . Upon replacing Y by Y , , it now suffices to prove the theorem under the additional assumption that each point of Y is an internal point of Y. In this case, Y - Z is a convex subset of I . consisting entirely of internal points and not containing 0. Let @? be the family of all convex subsets C of Y for which 0 4 C, Y - Z c C, and each point of C is an internal point of C. Then W is partially ordered by the inclusion relation G . If Wo is a totally ordered subfamily of @?, let C1 be the union of all the sets in go.It is apparent that O $ C , , Y - Z c C , , and C, consists entirely of internal points. Given u and u in C , ,there is a single set Coin 'Z0containing both u and u (because V 0is totally ordered by G ) . Thus Co (and hence, also C,) contains every convex combination of u and u, and so C , is convex. Accordingly, C1EV, and C, is an upper bound for % o. It now follows from Zorn's lemma that there is an element C of % that is maximal with respect to inclusion. It is immediately verified that the set { a u : u ~ Ca ,> 0 )
is an element of $5 and contains C. By maximality, it coincides with C ; so U U E C whenever U E C and a > 0. From this, and since C is convex and 0 4 C , it now follows that C n - C = 0 (the empty set), and U U E C , au + bilEC,
bwE T \ C
whenever u, LIEC, W E 9-\C, a > 0, and b 2 0. We assert next that au + be E Y -\C whenever u, I ) E .Y\C and a, b 2 0. For this, suppose the contrary, so that au + be E C for some a, b, u , u satisfying the stated conditions. From the preceding paragraph, au, bue Y \ C ; so, upon replacing u by au and c by bc, we may suppose that u, u E Y \ C and u + c E C. When r 2 0, we have 2rr~"Y'\C,
2rc
= r(r
+ u) + r(u - u)
and L' + U E C ; so, again from the preceding paragraph, Accordingly, if C, = {x
then 0 4 C, , C,
2
C (3Y
-
T(L'
- u ) $ C.
+ r(u - u ) : x ~ Cr ,k 0 ) .
Z ) , and C1 is convex and consists entirely of
6
I . LINEAR SPACES
internal points. Thus C , EV, by maximality C1 = C, and hence
2u = ( u + u )
+ ( u - U ) E C 1 = c,
a contradiction (since U U E Y‘\C for all a 2 0). This proves our assertion that au + ~ V Y’\C E whenever u, L ~ E Y \ and C a, b 2 0. It follows that the set VO = { x E ” / ” : x, - X E V . \ C } = Y-\(cu- C )
is a linear subspace of V (and Von C = 0, whence Vo# V - ) . We prove next that has codimension 1 in V:To this end, we have to show that any two non-zero elements of Y‘/Y; are linearly dependent; that is, if u, L‘ E Y T \ %$, then au + bu E Yo for suitable non-zero scalars a, b. Since $/\YO = C u - C, we may suppose (upon replacing u by - u, or L] by - t i , if necessary) that u E C and L’ E - C. Since C consists entirely of internal points, the same is true of - C. From this, the disjoint subsets
+ s(u - U ) E C } , 1]:u + s(u - U)E - C }
So = {SE[O, I]:u S1 =
{SE[O,
of the real interval [0, 13 are both open; indeed, if u + so(u - U ) E C (or - C), then u - u ) E C (or - C) for all s sufficiently close to so, since both the points u s0(tl - u) f t(u - u ) lie in C (or - C) for all sufficiently small non-negative 1. Since 0 E So, 1 E S1,and [0,1] is connected, So u S1is not the whole of [0, I]; so there is a real number s such that 0 < s < 1 and
+ +
s(tl
( I - s)u
+ su = u + s(v-
u)EY‘\(Cu - C ) =
%O.
This completes the proof that V; has codimension 1 in %’: Let p be a (non-zero) linear functional on Y‘ whose null space is %$.Since C is convex and Y 0 n C = 0, the subset p(C)of R is convex and does not contain 0 ; so either p(C) G ( 0 , ~ or ) p(C) G ( - o 0 , O ) . Upon replacing p by - p if necessary, we may suppose that p(u) > 0 for all u in C. Since Y - Z E C , it follows that p ( y ) > p(z) whenever y e Yand Z E Z From . this, the subset p ( Z ) of R is bounded above, and its least upper bound k satisfies P(Y) 2 k 2 p ( 4
( Y E Y,
zEZ )
Thus the hyperplane { X E Y . :p ( x ) = k} ( = H ) separates Y and Z . From the assumption that Y consists entirely of internal points, we now deduce that it is contained in the open half-space { x E Y - : p ( x ) > k ) . For this, suppose that Y E Y, and choose xo in V such that p(x,) > 0. Then y - axo E Y (and therefore p(y) - ap(xo)2 k ) for all sufficiently small positive scalars a, and thus p ( y ) > k. If Z (as well as Y) consists entirely of internal points, a similar argument shows that p(z) < k for each z in Z ; so in this case Y and Z are strictly separated by H .
7
1 . 1 . ALGEBRAIC RESULTS
Theorem 1.1.2 is our first example from a group of related results, described loosely as Hahn-Banach theorems. These results, which occur both in the present algebraic setting and also in the context of linear topological spaces, can be divided broadly into two main types. The first group (separation theorems) is concerned with separation of convex sets; closely related to this, there is a second group (extension theorems for linear functionals). A complex vector space Y can be viewed also as a real vector space simply by restricting attention to real scalars. Occasionally, for emphasis, we shall denote the real vector space so obtained by K-. A linear functional on % is described as a real-linearfunctional on V,and linear subspaces of %are called real-linear subspaces of .Y: A set X ( E V )is convex if and only if it is convex and the internal points of Xare the same in both when viewed as a subset of cases, since the concepts of “convex set” and “internal point” depend only on real scalars. In proving Hahn-Banach theorems for complex vector spaces, we shall require the following simple result.
<,
1.1.3. LEMMA.I f p is a linearjunctional on a complex vector space V; the equation p r ( x ) = Re p ( x ) defines a real-linear functional pr on and P ( X ) = PAX) -
ipr(ix)
(XE
v).
Every real-linear functional on Y‘ arises, :n this way, from a linear functional. Proof. It is apparent that, given a linear functional p on equation defines a real-linear functional pr . Moreover
< the stated
Im p ( x ) = - Re ip(x) = - Re p(ix) = - pr(ix), whence p ( x ) = Re p ( x ) + i Im p ( x ) = p,(x) - ipr(ix). Suppose next that o is a real-linear functional on V, and define a function oc: %’‘ + C by ( X E V). oc(x)= a(x) - io(ix) It is clear that o(x) = Re a,(x), aC(x+ y ) = o,(x) + a&), whenever x, y E 9” and a is a real scalar. Since, also, a,(ix) = o(ix) - io( - x) = i [ a ( x )-
=
o(ix)
and o,(ax)
= aa,(x),
+ ia(x)
ia(ix)] = iac(x),
it follows that o, is a linear functional on K We now obtain the analogue, for complex vector spaces, of Theorem 1.1.2. 1.1.4. THEOREM.If Y and Z are non-empty disjoint convex subsets of a complex vector space %at least one of which has an internalpoint, there is a nonzero linear functional p on V; and a real number k, such that
8
I . LINEAR SPACES
Moreover, Rep(y) > k ( Y E Y ) if Y consists entirely of internal points, and k > Re p(z) ( z E Z ) if Z consists entirely of internal points. Proof. By considering Y and 2 as subsets of the real vector space obtained from K i t follows from Theorem 1.1.2 that there is a non-zero reallinear functional a on 9" and a real number k such that a(y) > k 3 ~ ( z ) whenever y E Y and z E Z. Moreover, if either Y or Zconsists entirely of internal points, the corresponding one of the inequalities 2 can be replaced by > . By Lemma 1.1.3, there is a linear functional p on Y such that a(x) = Re p ( x ) for each x in % H Let 3' be a linear space with scalar field H ( = R or C). By a sublinear functional on V - we mean a function p : Y -,R such that
+ Y ) G P(X) + P(Y),
p(x
p(ax)= d x )
whenever x , y ~ V "and a is a non-negative real number. If, further, (x E K
p ( a 4 = lalp(x)
a E H),
p is described as a semi-norm on % If p is a semi-norm, then P(X) 2 09
Indeed, 2p(x) = p ( x )
I&)
- p(y)l
G A x - v)
(X,YE %
-1.
+ p ( - x) > p ( x - x) = 0; while
P(X) = P ( ( X
- Y ) + v)d p ( x - y ) + P(Y)?
whence p ( x ) - p ( y ) d p ( x - y ) , and similarly P ( Y ) - P(4
< p ( y - x) = p ( x - y ) .
By a norm on V i we mean a semi-norm p such that p ( x ) > 0 whenever X E $1 x # 0.
As an example note that if H is R or C and n is a positive integer, the set H" consists of all ordered n-tuples ( a l , . . . ,an)of elements of 06, and is an ndimensional vector space when the algebraic structure is defined by a(a1,. . . ,a,)
+ b(b1,. . . ,b,) =
(a01
+ bbl, . . . ,UU, + bb,).
The equations ~~((a1~...~an))=Ia11 + ...
+Ian13
p,((al?...,a,)) = max{la,l,. . . la,l> 9
define norms, p1 and p a , on K". In particular, the modulus function is a norm on K. A subset Y of V is said to be balanced if ay E Y whenever y E Y , a E H,and la1 < 1. Ifp is a sublinear functional on Kit is immediately verified that the set V,, = {XE V : p ( x )< I } is convex, contains 0, and consists entirely of internal points; V p is balanced if p is a semi-norm.
9
ALGEBRAIC RESULTS
1.1.
1.1.5. PROPOSITION. Suppose that V is a convex subset of a linear space ooer K ( = R or C ) , and 0 is an internal point of V. Then the equation
v
p ( x ) = inf{c:cER, c > 0, X E C V }
(xEV) dejines a sublinearjunctionalp on 9; I f V consists entirely of internalpoints, then V = {.YE ^I : p ( x ) < 1 }. If V is balanced, p i s a semi-norm.
Proof: Given .Y in %; c- ‘ X E V (and thus X E C V )for all sufficiently large positive scalars c, since 0 is an internal point of V ; so p ( x ) , as defined in the proposition, is a non-negative real number. Suppose that x , y ~ Vand a > 0. Since X E C Vif and only if axEacV, it follows that p ( a x ) = ap(x) (and this remains true when a = 0, since it is apparent that p(0) = 0). Given any positive real number E , we can choose real numbers b and c so that
0 < b
0
+ E,
X E ~ V ~, E c V .
Since V is convex, .r + y E b V + c V = (b + c) V , and
+ y ) d b + c
0) is arbitrary,p(x + y ) d p ( x ) + p ( y ) ;sop is a sublinear functional p(x
Since E on 3: If Vis balanced, thecondition axe ~Visequivalentto IalxEcV, when X E U E K, and c > 0. Thus p ( a x ) = p(lalx) = lalp(x), and p is a semi-noh. Suppose finally that V consists entirely of internal points. Given y in V , y + C ~ VE (and hence y ~ ( +l c ) - ’ V ) for all sufficiently small E ( > 0); so p ( y ) < ( 1 + c)- < 1. Conversely, if z E Y- and p ( z ) < 1, there is a real number c s u c h t h a t O < c < 1 andzEcV.Since Visconvex,c-’ > I , a n d O , c - ’ z ~ V , i t now follows that z E V . Accordingly, V = { x E V : p ( x )< 1 }. The sublinear functional p occurring in Proposition 1.1.5 is called the support ,functional of V. Our next two results are Hahn-Banach theorems of the “extension” type. 1.1.6. THEOREM. I f p is a sublinear .functional on a real vector space V; while po is a linear,functionalon a linear subspace of Y< and
Po(Y) G P O )
(YE
m r
there is a linear functional p on P- such that p ( s ) G p ( x ) (sE
f’T
p ( y ) = po(y)
( y E PO).
Proof: The product set R x V ” becomes a real vector space when addition and scalar multiplication are defined by (r,x)
+ b y ) = (r + s,.Y + y).
a(r,x)
= (ar,ax),
10
I . LINEAR SPACES
for x , y in Y and a, r, s in R. From the defining properties of sublinear functionals, it is immediately verified that the set V = { ( r , x ) E R x P . : r > p ( x ) } (ER x 3 . )
is non-empty, convex, and consists entirely of internal points. The set
w = {(Po(Y)7Y ): Y E VO} is a linear subspace of R x Y (and is therefore convex), and V n W = $3. From Theorem 1.1.2, there is a linear functional c o n R x $‘and a real number k such that a(u) > k b a ( w )
If
W E W , then
(U€
V,
W).
WE
awE W , and thus aa(w) = a(aw) < k , for every scalar a ; so a(w) = 0
(W€
W),
and k 3 0. From this, and since (1,O) E V , it follows that a(( 1,O)) > k 2 0 ; upon replacing a by a suitable positive multiple of a, we may assume that a((1,O)) = 1. The equation p ( x ) = - a ( ( 0 , x ) ) defines a linear functional p on f ; and a(@, x ) ) = a(r( 1,O)
+ (0,x)) = r - p ( x )
(r E R, x E $ ‘).
Given any x in V; we have ( r , x ) E V , and therefore r - p ( x ) = a ( ( r , x ) )> k 2 0 , whenever r > p ( x ) ; so p ( x ) < p ( x ) . When
YE
9$, ( p 0 ( y ) , y ) € W , and thus
Po(Y) - P(Y) = a((po(r),y)) = 0. H
I . 1.7. THEOREM. I f p is a semi-norm on a linear space P over K ( = R or C),while p o is a linear functional on a linear subspace 3 0 of’ 3 ;and IPo(Y)l
< P(Y)
( Y E YO),
there is a linear functional p on Y‘ such that IP(4l
(.ye
V-L
P(Y) = Po(Y)
(.YE YO).
Proof: If 116 = R,p is a sublinear functional on %’,and p o ( y ) < p ( y ) for all y in 90.By Theorem 1.1.6, there is a linear functional p on 3 such that p ( y ) = p o ( y )when Y E Yoand p ( s ) d p ( x ) for each x in $: Since, also, - p ( x ) = p ( - x) 6 p( - x) = p(x),
it follows that Ip(x)l < p ( x ) when X E 3 : Suppose now that K = @, and let $; be the real vector space obtained from f’. by restricting the scalar field. Then p is a sublinear functional on f r , the
1.1.
11
ALGEBRAIC RESULTS
equation ao(y) = RePo(Y) defines a linear functional a. on the linear subspace Yoof K , and ao(y) < p ( y ) for each y in Y ; . By Theorem 1.1.6, there is a linear functional a on <such that
.(V)
=
ao(Y)
( Y E YO),
a(4
(XE
m.
Thus a is a real-linear functional on 3 ; By Lemma 1.1.3, there is a linear functional p on Y ’ such that a(x) = Rep(x), and P(Y) = O ( Y ) - i.(iy> = ao(y) - iao(iy)= po(y)
When
.YE $;
( Y € Yo).
we can choose a scalar a so that la1 = 1, Ip(x)l = a p ( x ) ; and Ip(x)l = p(ax) = Rep(ax) = a(ax) Q p(ax) =
lalp(x) = p ( x ) .
If 42, 9’; W- are vector spaces over the same scalar field K, and S : Y + “w; T: 4! V are linear operators, the composition of Sand Tis a linear operator, which we write as a product ST, from ud! into W This applies, in particular, when ’@ = $/ = W ; and with the multiplication so obtained, the linear space of all linear operators T: V . -+ 9’’ becomes an associative linear algebra. It has a unit, the identity mapping I on $:’ We now identify the idempotents in this algebra (those elements E such that E 2 = E ) . If E : Y’. + 9” is a linear operator and E 2 = E, the sets -+
Y = {.YE f ’ - : E-Y = x},
(1)
Z = { x E VEX : = 0)
are linear subspaces of K Given x in % let’; y = Ex, z = x - E x ; note that x = y + z, Y E Y (because Ey = E 2 x = E,Y = y ) and Z E Z (because Ez = Ex - E 2 s = 0). If x has another expression as x = y , + z,, with yl in Yand z 1 in Z , then Ey, = y , and Ez, = 0 from (l), and thus y,
= E(y,
+ z , ) = ES = y ,
ZI =
x -y,
=x -
EX = Z.
Accordingly, the subspaces Y and Z of 3 . have the following property: each element .Y of F‘ can be expressed uniquely in the form ?c = y + z , with y in Y and z in Z . Two linear subspaces of V“ with this property are described as complementary subspaces of 3 : We assert that two subspaces Y and Z of Y are complementary if and only if Y + Z = % . and Y n Z = { O ) . Indeed, the first of these conditions asserts that each .Y in 3’ has at least one expression as ?c = y + z , with y in Y and z in Z. If it has another such expression, .Y = y , + z , (= y + z), then y, - y = z - z , ~ Y n Z ;
12
I . LINEAR SPACES
so the most general expression for X, in the stated form, is x = ( y + u) + ( z - u), where u E Y n Z. Accordingly, x is uniquely expressible in this form if and only if Y n Z = ( 0 ) . Now suppose that Y and Z are complementary subspaces of Y ;we shall show that they can be obtained as in (1) from an idempotent linear operator acting on -KWe may define a mapping E : Y + Y as follows: when x E Y;take theuniqueexpressionforxin theformy + z(with yin YandzinZ),and let Ex be y . It is then apparent that Y and Z are related to E as in ( I ) ; moreover, E X EY, and therefore E(Ex) = Ex, for each x in $C If, for j = 1,2, we have .xi = y j + z j (where y j e Y and Z ~ Z) E and a j e K, then 01x1
+ a2x2 = @ l Y l + azY2) + (alzl + a2221
and a l y l + a 2 y 2 eY, a , z l E(alxl
+ a 2 z 2 e Z .Thus
+ a 2 x 2 )= alyl + azyz = a l E x l + azEx2;
so E : Y" + Y is a linear operator and E 2 = E. The following theorem embodies the main results of the preceding discussion.
1.1.8. T H E O R E M . Two linear subspaces Y and Z of a linear space Y are complementary if and only if Y Z = Y and Y n Z = ( 0 ) . When these conditions are satisfied, the equation
+
E(y+z)=y dejines a linear operator E : V - + Y = { x E YEX : = x},
(YEY,
Z E Z )
and E 2 = E,
Z = { x E ~ ' "EX : = 0).
Conversely, every linear operator E : V + Y satisfying E 2 = E arises in the abooe manner from a pair of complementary subspaces of *y:
The operator E occurring in Theorem 1.1.8 is described as the projection from Y onto Y, parallel to Z. 1.2. Linear topological spaces
Suppose that a set Y is both a linear space with scalar field K ( = R or C) and also a Hausdorff topological space. If the algebraic and topological structures are so related that the mappings (.u,y)+x+y : Y x Y+< (a, X) + ax : K x V
+ 3'
are continuous (,whenY" x 9- and K x Y have their product topologies), then Y is said to be a linear topological space.
I.?.
LINEAR TOPOLOGICAL SPACES
13
The simplest examples of linear topological spaces over K are the sets K” ( n = I , 2 , . . .), with their usual vector space structure and with the product
topology. I t is apparent that a complex linear topological space can be viewed also as a real one, simply by restricting the scalar field. A linear subspace of a linear topological space is itself a linear topological space, with the relative topology. The given topology, in a linear topological space ${is sometimes described as the inirial topology in order to distinguish it from other topologies that can naturally (and usefully) be introduced, such as the weak topologies described in Section 1.3. It is usual to develop the early parts of the theory without the assumption that the initial topology is Hausdorff. However, for most purposes, an easy quotient procedure permits an immediate reduction to the Hausdorff case; and the initial topology is Hausdorff in all the cases we shall encounter in later chapters. Accordingly, we have included this condition as part of our definition of linear topological spaces. If % ;is a linear subspace of a linear topological space %the closure Y’i of Y ; is also a linear subspace. Indeed, suppose that x, y o E 9 ; and a, b are scalars. , is therefore the limit Then (x,,,y,,) lies in the closure Y; x 9; of $1 x %
and a.\-o + by, lies in the closure 9 ; of d : . Similar arguments show that if a subset of -iis balanced (or convex), then the same is true of its closure. Note also that an open set G in Y . consists entirely of internal points. For this, suppose that ?I E G, J E d :Since the mapping a + x + ay: R + V is continuous and takes 0 into the open set G , it carries some real interval ( - c, c) into G ; and in particular, x + ay E G whenever 0 6 a < c. Suppose that 3” is a linear topological space with scalar field K, x, E V ; and V G d :Since the continuous mapping x -+ .Y + x, : V”+ V” has a continuous inverse mapping x -+ x - -yo, it follows that V is a neighborhood of 0 if and only if ,yo + V is a neighborhood of xg. Accordingly, the topology of dr is determined once a base of neighborhoods of 0 has been specified. If V is a neighborhood of 0, then so is a V for each non-zero scalar a, since the one-toone mapping .Y -+ as, from V onto Y’; is bicontinuous; in particular, - V is a neighborhood of 0. From continuity at (0,O) of the mapping ( x , y ) -+ x + y , there is a neighborhood V , of 0 such that Vo + V , c V . From continuity at (0,O)of the mapping (a,x) + ax, there exist a neighborhood V , of 0, and a positive real number E , such that ax E Vwhenever .Y E V1and la1 < E . From this, u { a V , : 0 < la1 6 E } is a balanced open subset of V ; so every neighborhood of 0 contains a balanced neighborhood of 0.
14
I . LINEAR SPACES
Let .V denote the set of all neighborhoods of 0 in
1.2.1. PROPOSITION. Suppose that Y‘ and W are linear topological spaces with the same scalar ,field I6 ( = R or C), and T :Y‘ + W is a linear operator.
v
(i) .yo E $ and T is continuous at xo, then T is uniformly continuous on ^I: (ii) C is a balanced convex subset of Y and the restriction TIC is continuous a( 0, then TIC is uniformly continuous on C . “
Proof. (i) Since Tis continuous at xo,given any neighborhood Wof 0 in V such that T X ETx, + W whenever ?c E x, + V. Let b( V ) ( E Y x V )be the set occurring in the above discussion of the uniform structure on Y< and let b( W) (cW x W )be defined similarly. When (x, y ) ~ dV() , we have y - X E V , xo y - X E xo + V , and therefore Tx, + Ty - T x € T x o + W ; so Ty - T ~ WE, and (Tx, T y ) ~ b ( w ) . The above argument shows that, given any neighborhood W of 0 in there is a neighborhood V of 0 in V’. such that (Tx, Ty)~ bW() whenever (x, y ) E &( V ); so T is uniformly continuous on Y“. (ii) Since the restriction TIC is continuous at 0, given any neighborhood W of 0 in W, there is a balanced neighborhood V of 0 in Y‘ such that T X E ~ W whenever x E V n C .
K there is a neighborhood V of 0 in
+
1.2. LINEAR TOPOLOGICAL SPACES
15
Suppose that x , y C ~ and y - X E V. Since C is convex, and both C and V are balanced, we have i y - ~ X VE n C ; hence $Ty - + T x E $ W , and Tv - TXE W. Accordingly, ( T x , TI')E a(W ) whenever x,y E C and (x, y ) E a( V ) ;so TIC is uniformly continuous on C . H 1.2.2. R E M A R K . For complex linear topological spaces, Proposition 1.2.1 remains true under the weakened assumption that T : Y + W is a real-linear operator, since Tcan then be viewed as a linear operator between the real linear topological spaces obtained from %'. and W ' by restricting the scalar field. In particular, therefore, Proposition 1.2.1 applies to conjugate-linear operators (those mappings T : P--, w' satisfying T(ax + by) = GTx + 6Ty, where G denotes the complex conjugate of a). 1.2.3. C O R O L L A R Y . If V" and W are linear topological spaces, W is complete, 6 is an eoerywhere-dense subspace of Iv; and To:Yo-+ W is a continuous linear operator, then To extends uniquely to a continuous linear operator T : Y -+ W . Proof. By Proposition 1.2.I , To is uniformly continuous on Yo,and so extends uniquely to a uniformly continuous mapping T : Y + YK Given any scalars a, b, the equation y(x, y ) = T(ax
+ by) - a T x
-
bTy
defines a continuous mapping 9 : Y x Y' -, w'; and g vanishes on Y x Y since it vanishes on the everywhere-dense subset Yox Yo.Thus Tis linear. H The lemma that follows, concerning continuity of linear functionals and semi-norms, is written in a form that applies to both real and complex linear topological spaces; the notation Re, occurring in part (i), is redundant in the real case. 1.2.4. L E M M A . Suppose that .Y- is a linear topological space, p is a linear functional on V,- and p is a semi-norm on K (i) If there is a non-empty open set G in Y and a real number c such that Rep(x) < c wheneaer ~ E G then , p is (uniformly) continuous on K (ii) I f p is bounded on some neighborhood of 0 in *y; then p is (uniformly) continuous on Y..
Proof. (i) If G and c have the stated properties, we can choose xo in G and a balanced neighborhood Vo of 0 in Y such that xo + Vo G G . Given x in Vo, let a be a scalar such that (a(= 1 and Ip(x)l = p(ax). Then a x € Vo, xo + a x E x O + Vo E G, and therefore c > Rep(xo ax) = Rep(xo) + Ip(x)l.
+
Hence Ip(x)l < b for all x in Vo, where b = c - Rep(xo) ( > 0).
16
I . LINEAR SPACES
Given any positive E , Ip(x)l < E for all x in the neighborhood Eb- Vo of 0. Thus p is continuous at 0, and is therefore uniformly continuous on V” by Proposition 1.2.1. (ii) Suppose that there is a neighborhood Vof 0 in ^Y- and a positive real number b such that p ( x ) < b whenever X E V . Given any positive E , the set Eh- V is a neighborhood of 0 in % and
’
IP(Y) - P ( X ) l
P(Y - x ) < E
’
whenever x, y e Y‘ and y - X E Eb- V. Thus p is uniformly continuous on 9: rn A linear functional p on a linear topologicalspace 9”is 1.2.5. COROLLARY. continuous if and only if its null space p - ‘(0) is closed in K
Proof. We may assume that p # 0 ; it is evident that p - ‘(0) is closed if p is continuous. Conversely, suppose that p - ‘ ( 0 )is closed. We can choose xo in V ’ so that p ( x o )= 1. Since x o 4 p - l(O), there is a balanced neighborhood Vo of 0 in f . such that xo + Vo does not meet p - ’ ( 0 ) . If X E Vo and Ip(x)l 3 1, we can choose a scalar a so that (a(< 1 and p(ax) = - 1. Then U X E Vo,xo + a x E x , + Vo, and p(xo + ax) = 0, contrary to our assumption that x,, + Vo does not meet p - ‘(0). From this, it follows that Ip(x)l < 1 whenever X E Vo, and p is continuous by Lemma 1.2.4(i). rn
If p is a non-zero continuous linear functional on a (real or complex) linear topological space V; the set A = { X G Y :Rep(x) < 1 ) is a non-empty convex open subset of K but is not the whole of K There are examples of linear topological spaces that have no subset A with the properties just listed, and these spaces therefore have no non-zero continuous linear functionals. We now introduce a class of linear topological spaces, each of which (as we shall see in Corollary 1.2.11) has an abundance of continuous linear functionals. A locally conuex space is a linear topological space in which the topology has a base consisting of convex sets. By a locally convex topology, on a (real or complex) vector space K we mean a topology with which ^Ir becomes a locally convex space. The vector spaces R“ and @” provide the simplest examples of locally convex spaces, since the product topology has a base (for example, the open balls) consisting of convex sets. In the theorem that follows we show that locally convex topologies, on a linear space Y over K ( = R or C),are closely associated with certain families of semi-norms. A semi-norm p on ^Y- can be regarded as an analogue of the “modulus” function on K; the “triangle inequality,” p ( x + y ) < p ( x ) + p ( y ) , and its consequences (for example, the inequality ( p ( x )- p(y)l d p ( x - y ) )are used in analysis in locally convex spaces, in much the same way that the
17
1.2. LINEAR TOPOLOGICAL SPACES
corresponding properties of the modulus are used in elementary real or complex analysis. 1.2.6. THEOREM. Suppose that V is a (realor complex) vector space, and is a family of semi-norms on V - that separates the points of Y in the following sense: i f s V“ ~ and x # 0 , there is an element p of l- for which p ( x ) # 0. Then there is a locally convex topology on V in which,for each xoin K thefamily of aII sets V(x0 : p l , . . . ,pm;E ) = {xE Y“: p j ( x - xo) < E ( j = 1, . . . ,m ) }
(where c: > 0 and p i , . . . , p mE r ) is a base of neighborhoods of xo. With this topology, each of the semi-norms in r is continuous. Moreover, every locally convex topology on V arises, in this way,from a suitable family of semi-norms. Proof.
If xo E V(YO:P I , . . * 7pm;6) n V(ZO:41,. . .,q n ; q),
we can choose c ( > 0 ) so that
pj(xo -yo)
+
E
< 6,
4k(xO - zo)
+E
( j = i,. . . , m , k = 1,..., n).
It then follows easily, from the triangle inequality, that
V(-xo:pi,.. . )Pmrqlr.. ., 4 n ; E ) E V(yo:p i , . . . , P m ; d ) n V(zo: 41,. . .. q.;q). In particular, if V(y,: p i , . . . , p m ; 6 )contains x,,, it also contains a set of the form V ( x o :p i , . . .,pm;c:).From thesefacts,andsincex,E V(xo: p l , . . .,pm;&), it follows that the family
{ v ( x 0 p: l , . . .,pm;E):X O E q p , , . . . , p , , , ~ r E, > 01 is a base for a topology on V [ K : p. 471, in which { V ( x o :P I , . . . , P ~ ; E ) p1,. : .., P , , , E ~ .
E
>O}
is a base of neighborhoods of xo. If xo,y o E 3” and xo # yo, we can choose p in r so that p(x0 - y o ) > 0. When 0 < E < +p(xo- y o ) , the sets V(x,: p ; ~and ) V ( y o :p ; ~are ) disjoint neighborhoods of xo and yo, respectively; so the topology is Hausdorff. Since P(X
+ I’ - -‘co - Y o ) < P(X
- xo)
+ P(Y - Y o ) ,
p(ax - ao x o )= p(ao(x - xo)
+ (a - ao)xo+ (a
-
ao)(x - xo))
< laolP(x - xo) + la - aolp(xo) + la - aolp(x - xo), for every semi-norm p , it follows that x
+ Y E V(x-0 + yo : p i , . . . ,pm; E ) , axe V(aoxo:P I , .. . ,P,,,;E),
18
1 . LINEAR SPACES
whenever x~ V(xo:~ 1 ,. ..,pm; 6), Y E V(.YO:P I , . . . YPm; 6) and la - a,[ < 6, provided that the positive real number 6 is sufficiently small to ensure that
26 < c ,
la016
+ 6p,(xo) + 6, <
E
( j = 1,.
. . ,m).
This establishes the continuity of the mappings ( x , y ) + x + y , (a, x) -+ ax, and so shows that Y is a linear topological space. It is locally convex, since each of the basic neighborhoods V ( x o :p,, . . . , p m E; ) is convex. When p ~ f p, is bounded on the neighborhood V(0:p; I), so that p is (uniformly) continuous on V" from Lemma 1.2.4(ii). Conversely,suppose that z is a locally convex topology on V: We shall show that z can be obtained, by the process described in the theorem, from a suitable family of semi-norms. To this end, let V be a z-neighborhood of 0 in YJC Then V contains a convex z-neighborhood Vo of 0. Moreover, V o contains a balanced r-neighborhood V1 of 0, and thus contains the convex hull V , of V , ; so V 2 G V . Since V2 consists of all finite convex combinations a l x l + . . . a,x, of elements of V , (with each aj > 0 ) ,it too is balanced. It is r-open, and so consists entirely of internal points, since it can be expressed as a unionofsetsoftheforma,x, + . . . + u n - , x ~ - + , a,V,.ByProposition 1.1.5 there is a semi-norm p on V" such that
+
V,
={ X E Y :
p(x)< I},
and since V , is a z-neighborhood of 0, it results from Lemma 1.2.4(ii)that p is z-continuous. Let f denote the set of all r-continuous semi-norms on Y: The preceding paragraph shows that every z-neighborhood Vof 0 contains a set of the form { x E Y ' :p(x) < I } , w i t h p i n f , . From this,ifxEVandp(x) = Oforeverypin f , then x lies in each r-neighborhood of 0, and hence x = 0 ; so f separates the points of Kin the sense stated in the theorem. Moreover, when x, E P; the z-neighborhood x, + V contains the set
,
,
{ x o + x : x E ~ ; p ( X ) < 1 } = { X E v " : p ( x - x ~ )
= V ( x , :p; I ) ;
and V(x,: p l , . . . ,pm;E ) is a r-neighborhood of x,, whenever p,, . . . ,pmE T o and E > 0, since p , , . . . , p m are z-continuous. It follows that the sets V ( x , : p,, . . . ,pm;E ) form a base of z-neighborhoods of x,; therefore T coincides with the topology obtained from T o by the process described in the theorem.
1.2.7. COROLLARY. In a locally convex space there is a base of neighborhoods of 0 consisting of balanced convex sets.
19
1.2. LINEAR TOPOLOGICAL SPACES
Proof
The sets V(0:p I , .. . ,pm;c)are balanced and convex.
Observe that the topology on the locally convex space K" (where K = R or C) can be obtained, as in Theorem 1.2.6, from a family f consisting ofjust one norm. For example, either of the norms p , and p a , defined in the discussion preceding Proposition 1. I .5, will suffice for this purpose. We now give criteria for the continuity of semi-norms and linear operators on locally convex spaces, in terms of families of semi-norms defining the locally convex topologies. 1.2.8. PROPOSITION. Suppose that #';and fi are locally convex spaces with the same scalarfield K ( = FS or C );and for j = 1,2, let r,be a separatingfamiiy of semi-norms on 9, that gives rise to the topology of $<. (i) A semi-norm p on fi is continuous ifand only ifthere is apositire real number C and a finite set p l , . . . ,pm of elements off such that P ( X ) Q Cmax{p,(x), . . . ,pm(x)}
( x %). ~
(ii) A linear operator T: Y, -, $; is continuous ifandonly
given any q in
r2,there is a positice real number C and afinite set p , , . . . ,pm of elements of rl such that ~ ( T xQ) C m a x { p l ( 4 , .. . , p , , , ( ~ ) ) ( x 6). ~ (iii) A linearfunctional p on -I ; is continuous ifand only ifthere is apositiue real number C and a finite set p I ,. . . ,pm of elements off such that Ip(X)l
< Cmax{P l ( X ) , . . . *Pm(*v)}
(XE
%I.
Proof: (i) If p is continuous, the set { X E % : p ( x )< l} is a neighborhood of 0 in $;; so it contains one of the basic neighborhoods V(O:p,, . . . , p , , , ; ~ ) ,where E > 0, and p 1, . . . . p m e f l . If X E and ~ p ( x ) > E - I max{ p l ( x ) ,. . . ,pm(x)}, we may assume (upon replacing x by cx, for some positive c) that p(x) = 1 and pj(,v) < E ( j = 1,. . . , m ) ; that is, p(x) = 1 and .YE V(O:p,,. . . ,pm;&), contradicting our assumption concerning this basic neighborhood. Accordingly, the stated condition p(.v) < Cmax{p,(.x), . . . ,pm(x)}
(XE
fi),
is satisfied (with C = E - '). Conversely, if this condition is satisfied, then p is bounded on V ( O : p , ,. . . ,pm;1 ) ; by Lemma 1.2.4(ii),p is continuous on Y i . (ii) When y is a semi-norm on V2,the composite mapping q Tis a seminorm on $ In view of this, and taking into account part (i) of the proposition, we have to show that Tis continuous if and only if q o Tis continuous whenever y E f 2 . By Theorem 1.2.6, each y in T 2 is continuous on 6 ;so continuity of T entails continuity of y c T. 0
20
I . LINEAR SPACES
Conversely, suppose that 4 0T is continuous, for each y in r2.Every neighborhood V of 0 in V2contains a basic neighborhood V(0:yl, . . . ,q m ;E ) , where E > 0 and q l , . . . ,qmE r2.Since qjo T is continuous ( j = 1 , . . . ,m), the set W = {,re% :qj(Tx) < E ( j = 1,. . . , m ) } is a neighborhood of 0 in
K , and it is apparent that
T ( W )E V(0:q1,...,ym;&) G v. Thus T is continuous (at 0, and therefore throughout %). (iii) The scalar field H is a locally convex space, its (usual) topology being obtained from a single norm, the modulus function; and p : % + I6 is a linear operator. Thus (iii) is a special case of (ii). H
Our next two results are Hahn-Banach (separation) theorems. They are formulated so as to apply to both real and complex linear topological spaces, the notation Re being redundant in the real case. 1.2.9. THEOREM.r f Y and Z are disjoint non-empty convex subsets of a linear topologicalspace and Y is open, there is a continuous linearfunctional p on V and a real number k such that
e
Rep(y) > k 2 Rep@)
(.YE
Y, ~ € 2 ) .
%further, Z is open, then k > Rep(z) for each z in Z . Proof. In view of the fact that an open set consists entirely of internal points, the assumptions of Theorem 1.1.2 are fulfilled in the real case, while those of Theorem 1.1.4obtain in the complex case. From those theorems, there is a linear functional p on V that satisfies the stated inequalities; and by applying Lemma 1.2.4(i),with - p and Yin place of p and G , it follows that p is continuous. H
1.2.10. THEOREM.If Y and Z are disjoint non-empty closed convex subsets of a locally convex space K at least one of which is compact, there are real numbers a, b and a continuous linear functional p on .lr such that Rep(y) 2 a > b 2 Rep(z)
(YE
Y, Z E Z ) .
Prooj,f: We may suppose that Y is compact. For each y in Y, there is a balanced convex neighborhood V y of 0 such that ( y + V,,) n Z = 0, since Z is closed and y $ Z . The open covering { y + i V , , : y Y~ } of Y has a finite subcovering { y ( j )+ : V y c n f = j I , . . . ,m } , and the set V = $Vycjr is a balanced convex neighborhood of 0. The convex sets Y + V and Z + V are open since, for example, Y + V = U { y + V : Y EY ) ; we assert also that they are disjoint. For this,
ny=
I.?.
21
LINEAR TOPOLOGICAL SPACES
suppose the contrary, and choose y in Y, i‘ in Z , and cl, u2 in V , so that J’ + 1’1 = + “ 2 . For Some j ( 1 6 j < nz), y ~ y ( j ) :vycj,; moreover 1.1 ,- 1.2 E V E j VyI,). Thus
+
7
=y
+ v1 -
1’2
= y(.j)
+ ( y - y ( j ) ) + c 1 - u2 +
contradicting our assumption that (y(.j) Vylj,)n Z = 0. This proves our assertion that Y + V , Z + V are disjoint. From Theorem 1.2.9, we can choose a continuous linear functional p on ’I‘ and a real number k such that Rep(y) > k > Re&)
(JJE Y
+ V,
+
ZEZ V);
in particular, these inequalities are satisfied when y E Y and z E Z. Since Y is compact, the continuous function y : Y + R,defined by&) = Rep(y), attains its lower bound at a point y o of Y. Hence Re p(y) 3 Re p ( v d > k > Re p(z) and it suffices to take a
=
Rep(yo) and b
(vE Y ,
z EZ),
= k.
1.2.11. C O R O L L A R Y . I f ’ x is a non-zero rector in a locally convex space V,’ there is a continuous linear functional p on $ such that p ( x ) # 0. ’
Proq/: This follows from Theorem 1.2.10, with Y = {x} and 2 = {O}. 1.2.12. COROLLARY. IfZ is a closed convex subset of a locally convex space ,-and},E $-\Z, there is a continuous linearfunctional p on Y” and a real number b W C / J that Re p(.v) > b, Re p(z) 6 b ( z E Z ) . f
Proof. This follows from Theorem 1.2.10, with Y = { y } . 1.2.13. C O R O L L A R Y . If2 is a closed subspace of a locally convex space Y i andyE $ \Z, there is a continuous linearfunctional p on V- such that p ( y ) # 0, p(z) = 0 ( z E Z ) .
Proof. From Corollary 1.2.12 there is a continuous linear functional p on a real number b such that
f and
Re y ( y ) > b,
Re p ( z ) 6 b
(z E Z ) .
The latter inequality implies that the range of values assumed by the linear functional p 1 Z on Z is not the whole of the scalar field. Hence p I Z = 0, b 2 0, Re p(y) > b 2 0, and thus p(y) # 0. We now prove a Hahn-Banach extension theorem.
22
I . LINEAR SPACES
1.2.14. THEOREM.I f p o is a continuous linear functional on a subspace $ 0 of a locally concex space there is a continuous linear functional p on Y such that p ) 3 ; = po. Proof. Let r be a family of semi-norms that gives rise, as in Theorem 1.2.6, to the topology on Y: By restricting each member of to f 6,we obtain a family of semi-norms that defines the relative topology on 9;. Since po is continuous, it follows from Proposition I .2.8(iii) that there is a positive real number C and a finite set p l , . . . ,pmof elements of f such that
IPO(Y)~
6 Cmax{ P I ( Y ) ,. . . tPm(Y))
(YE
$0).
By Theorem 1.1.7, p o extends to a linear functional p on f ; such that Ip(x)l
< Cmax{ P l ( X ) ,
*
. .,p,(x))
(x E Y, I
since the equation
AX) = Cmax{p,(x), . . . ,Pm(-K)J defines a semi-norm p on K A further application of Proposition 1.2.8(iii) shows that p is continuous. If 3'. is a locally convex space and X E we denote by [ X I the closure of the set of all finite linear combinations of elements of X. It is apparent that [ X I is the smallest closed subspace of V that contains X ; we describe it as the closed subspace generated by X. When X is a finite set, {xl,. . . ,x,,},we write [ x ,,..., x,] in place of [XI. In fact, [xl ,..., x,] is the set of all linear combinations of xl, . . . ,x,, since it results from Theorem 1.2.17 that this set is closed in -K We now consider some properties of finite-dimensional subspaces of locally convex spaces. In fact, the main results obtained remain valid in all linear topological spaces, without the restriction of local convexity. However, we shall not need that degree of generality, and have preferred to derive these results as simple consequences of the Hahn-Banach theorems in the locally convex case. We have already noted that, when Dd is R or C and n is a positive integer, the finite-dimensional vector space K", with its usual (product) topology, is a locally convex space. Each linear functional p on Dd" is continuous, since it is given by a formula p ( ( a l , .. . ,a,)) = alcl
+ . . . + a,c,,
where cl, . . . ,c,, are fixed elements of Dd. From elementary linear algebra, if ./11 is a proper subspace of K", there is a non-zero linear functional on K" that vanishes on ,A.
23
1.2, LINEAR TOPOLOGICAL SPACES
1.2.15. LEMMA.Suppose that $ is a locally convex space with scalarfield x,) is a basis of afinite-dimensionalsubspace ?; o f f Then there are continuous 1inear.functionalsp I ,. . . ,pn on Y such that pj(xj) = 1 and p,(.yk) = 0 when j # k . The equation '
K ( = R or C)and { xl, . . .
+
T.y = (PI(-y), . . . ,pfl(+y)) (x E V) defines a continuous linear operator T : P- -+ K", and the restriction TI% is a oneto-one hicontinuous linear operator from $;' onto K". Proof: The set IT'of all continuous linear functionals on V is a linear subspace of the algebraic dual space of V.'The equation s p = (P(XI),
. . ., p(xfl))
(PE
YU)
defines a linear operator S : Y' + 06". If the subspace S ( P ) is not the whole of K", there is a non-zero linear functional on K" that vanishes on S ( V ' ) ) ;in other words, we can choose c l , . . . ,c, in H,not all 0, so that ClP(X1)
+ . . . + c,p(x,)
=0
( p € V').
Thus every continuous linear functional p on Y vanishes at the non-zero vector c l x l + . . . + c,x, contradicting the conclusion of Corollary 1.2.11. It follows that S( V ' ) = 06". Accordingly, we can find p l , . . . ,pn in V'such that Spj = (0,. . . ,0, 1,0,. . . ,0) (with the 1 in thejth place); that is, p l , . . . ,pn are continuous linear functionals on and pj(xk) is 1 or 0 according a s j = k or j#k. I t is apparent that T, as defined in the lemma, is a continuous linear operator from V into K". Moreover, since pj(clxl
if follows that
+ . . + c.x,,) *
T ( c , x ~+ . . .
( j = 1,. . . ,n),
= cj
+ cp,) =
. . ,c,,).
( ~ 1 , .
Thus T carries V; onto K", and the restriction TlVo has a continuous inverse mapping (c1,
..., c n ) 4 c 1 x l + . . . +c,x,:
K"+YO.
H
I f f is a finite-dimensional linear space, with scalar 1.2.16. PROPOSITION. field K (= R or C),there is a unique locally convex topology on Y..
Proof. Let {x . . ,x,} be a basis of -Y; and define a one-to-one linear mapping T, from Y* onto K", by T(C,X,
+ . . . + c,x,)
= (c1,.
. . ,C").
There is a unique topology T on Y; which makes T a homeomorphism; locally convex, since K" is a locally convex space.
T
is
24
I . LINEAR SPACES
If Y has another locally convex topology q,,we can apply Lemma 1.2.15 (with 9'; = 9'-). Since the mapping T just defined is the same as the one occurring in the lemma, ro makes T a homeomorphism, and thus coincides with t. W 1.2.17. THEOREM.If Yois afinite-dimensional subspace of a locally convex space $; then Yois closed in *I", moreover, there is a closedsubspace % of Y such thaf Y ; and % are complementary subspaces of Y and the projection from Y onto $0 parallel to % is continuous. Proof. Let { x l , . . .,xn} be a basis of Yo.By Lemma 1.2.15, there are continuous linear functionals p, ,. . . ,pn on Y such that pj(xk) is 1 or 0 according as j = k or j # k. Each element y of Yois a linear combination clxl + . . . + cnxn.and p j ( y )= c j ; so n
Y
=
1 Pj(Y)xj
YE^;).
j= 1
The equation n
EX =
C pj(x)xj
(XE
Y)
j= 1
defines a continuous linear operator E : Y -+ f l From the preceding paragraph, Ey = y when y E $$,and it is apparent that E ( Y ) s Yo.It follows easily that
v; = {x E v :Ex = x}, that E Z x = Ex for each x in K and hence that E 2 = E. B y Theorem 1.1.8, 9; has a complementary subspace, "y; = { x E Y : E x = O } ,
<
and E is the projection from Y onto Y ; , parallel to . Moreover, since E is continuous, the above descriptions of 6and fi ,in terms of E, show that both these subspaces are closed. W 1.2.18. THEOREM.A locally convex space Y is locally compact ifand only if it is finite dimensional.
Proof. If Y has finite dimension n, it is homeomorphic to K", where U 6 is the scalar field, by Lemma 1.2.15 (with Yo= Y ) ;so V" is locally compact. Conversely, suppose that Y is locally compact, and let V be a neighborhood of 0 in Y whose closure V - is compact. If xoE Y and xo # 0, the onedimensional subspace [ x o ]is closed in Y (Theorem 1.2.17),but is not compact 6 (Lemma I .2.15). Accordingly, [xo] Q V - ; and since it is homeomorphic to U
1.2. LINEAR TOPOLOGICAL SPACES
25
since, also, [x,] meets V(at 0, for example), there is an element ax, of [x,] in the boundary V - \ V of V . For each y in the compact set V - \ V , since y # 0, there is a continuous linear functional p, on V such that p,(y) # 0 (Corollary 1.2.1 1). We may suppose that Ip,(y)l> 1, and define a neighborhood G, of y by G, = {.YE V :Ip,(x)l > 1). The open covering { G , : ~ EV - \ V } of V - \ V has a finite subcovering; and if the linear functionals p,, corresponding to the sets G, in this subcovering, are enumerated as p l , . . . ,pn, then for each x in V - \ V there is at least one integerj such that 1 < j < n and Ipj(x)l > 1. The equation Tx = (pr(x), . . . ,p,(x)) defines a linear mapping T from Y into K". In order to prove that Y is finitedimensional, it suffices to show that T is one-to-one, and this follows easily from the two preceding paragraphs. Indeed, if x, E ,Y\(O},then ax, E V - \ Vfor some scalar a, and Ipj(ax,)l > 1 for some integerj with 1 < j < n ; so that pj(x,) # 0, and hence Tx, # 0. H We conclude this section with a discussion of nets and (unordered) infinite sums in a linear topological space K Suppose that v E K and ( u j , j € J , 3 ) (or, more briefly, { v j } ) is a net in f: the index set J being directed by the binary relation 2 . Then { i i j } converges to v if and only if, given any neighborhood V of 0, there is an index j , such that uj E v + V(equivalently, tij - E V ) whenever j >j,. From the definition of the uniform structure on C as set out in the discussion preceding Proposition 1.2.1, { u j } is a Cauchy net if and only if the following condition is satisfied: given any neighborhood V of 0, there is an indexj, such that uj - uk E V wheneverj, k 2 j o. If Y is a locally convex space and r is a separating family of semi-norms that determines its topology, each neighborhood V of 0 contains a basic neighborhood
n m
~ ( 0,,..., : ~p m ; E ) =
{xE~'-:pj(X <)
j= 1
In this case, { u j } converges to 1: if and only if, given anyp in r a n d any positive E , there is an indexj, such thatp(vj - v) < E wheneverj 2 j o;and { u j } is a Cauchy net if and only if, given any such p and E , there is an index j , such that p ( c j - ck) < E when j , k 2.jo. Now suppose that {x,: U E A; is a family of vectors in a linear topological space Y,' and denote by .Sthe family of all finite subsets of the index set A. Then 3 is directed by the inclusion relation 2 , and for each [F in 9 we can form the finite sum s(F) =
c
xu.
uc6
In this way we obtain a net (s([F), [FE 9, 2 )(briefly, {s(F)}) of elements of Y': If it converges to a vector x in K we say that the family {x,: a E A} is summable, or that the sum CUEWxu exists, and we write CutAx,= x.
26
I . LINEAR SPACES
The sums considered here are unordered, in that we do not assume any order structure on the index set A. We shall show that, at least in certain elementary respects, such sums can be handled in much the same way as convergent series; moreover, subject to suitable completeness properties in U ; they have some of the properties associated, in elementary analysis, with absolute convergence. Suppose that P" and w' are linear topological spaces and { x u: u E A}, { y , : a c A } are summable families of elements of %; with sums x and y , respectively. When [ F E define ~ s(F) as above, and let [(IF) = C a E F y U If .c is a scalar and T : V + W ' is a continuous linear operator, we have
c
cx, = cs(ff),
USE
1
(xu
+ ya) = s(F) + [(F),
U € E
C Tx, = Ts(lF). atF
From continuity, of T and of the algebraic operations in it follows that the nets {cs(F)), {s([F) + t(IF)}, { Ts([F)}converge to cx, x + y , Tx, respectively. Thus the families {cx,}, {x, + y u } , { Tx,} are summable, and
CTX,= T
0 CX,
If { x , : u ~ Ais} a family of vectors in a linear topological space Y; the corresponding net {s((F)) of finite sums is a Cauchy net if and only if the following condition is satisfied: given any neighborhood V o f 0, there is a finite subset [Fo of A such that s( IF,) - s( IF2) E V whenever IF1, [F2 are finite subsets of A and Fo E IF1 n [F2. Now SV,)
- s ( f f 2 ) = S([Fl\[FO) - s(F2\[Fo),
and given any neighborhood V 1of 0, there is a neighborhood V , of 0 such that V 2 - V2 G V , . From this, it follows easily that {s([F)) is a Cauchy net if and only if it satisfies the following "Cauchy criterion": given any neighborhood V of 0, there is a finite subset F[, of A such that s([F) E Vfor every finite subset IF of A\[F". If 9- is a locally convex space and r is a separating family of semi-norms that determines its topology, then {s(IF)} is a Cauchy net if and only if, given anyp in r and any positive E , there is a finite subset (Fo of A such thatp(s(IF)) < c for every finite subset [F of A\[Fo.
27
1.2. LINEAR TOPOLOGICAL SPACES
1.2.19. PROPOSITION. Suppose that Y’ is a linear topological space, X c $; and X is complete (in its relatire uniform structure). Let {xd:d e D} be a sumniablejumily of elements of Y{ and suppose that,for eachfinite subset F of D, the sum s(Q = E d s 5 x dlies in X . (i) If A E D, the family { x d :d e A} is summable to an element of X . (ii) I f D is a disjoint union, D = ( ) { A h: b e 5 } ,and Yb
=
(bE
&i
J t Ah
then the family { y h : b E B }is summable and x,,,eyh
= xdpDxd.
Proof. (i) Since { x d :d e D} is summable, the corresponding net of finite sums converges, and is therefore a Cauchy net. Given any neighborhood V of 0, let lFO be a finite subset of D such that s(F) E Vwhenever IF is a finite subset of D\IF,.If[F, = IFOnA,wehaveA\5,c_ D\F,,andthuss(F)E Vforeveryfinite subset IF of A\lF,. Accordingly, the net of finite sums s(IF), with IF c A,is a Cauchy net in the complete set X,and therefore converges (and { x d : dE A} is summable) to an element of X . (ii) Let x = CdtDxd.Given any neighborhood V of 0, let V , be a neighborhood of 0 such that V , + V , E V , and let IFo be a finite subset of D such that xd
- X E V1
Jt 5
whenever IF is a finite subset of D, and IF0 5 lF. Note that IFo
=
u
&n[F0=
u
&n[FO,
h t B,
baB
whereIF, isthefiniteset { b E B : A b n f f o # @ } . It now suffices to prove that X Y h - S E
v
be F
whenever [F is a finite set satisfying IF, c IF G B. To this end, suppose that IF has m members, and choose a neighborhood V2 of 0 for which V , 2 V 2 + V 2 + . . . + V 2 ( m terms). For each b in IF, there is a finite set [Fb such that Abn(Foc[FhcAh,
yh-
C-XdE d e Ijh
Since UFb? bs t
u AbnFo=ffo, heF,
vz.
28
1. LINEAR SPACES
we have
1 hsF
Yb
1 xd) + ( 1 1 - .) + + . . . + + v, E + v, c v.
- x = x(Yb -
xd
d s Fb
hsF
E v2
h s b deFl
v2
v2
v1
A family { z , : a ~ A }of complex numbers is summable if and only if { Iz,I :U E A} is summable. To prove this, we use the Cauchy criterion set out in
the paragraph preceding Proposition 1.2.19. For every finite subset IF of A,if z, = x, + iy,, lzF z a l
G zFlzal
1 Ixat + cF IYal a€
aEF
= Cxa+ aEF,
4max{
1
-xa+
C
CYa+ a e F,
asF2
z,}
G 4maXl2 , z a
-Ya
scF,
[
9
where IF1,. . . , [F4 are suitable subsets of F. It follows easily that, if either of the families { z , } , {lzal} satisfies the Cauchy criterion (which is necessary and sufficient for summability, since C is complete), then so does the other. If {x,: a E A} is a family of non-negative real numbers, the corresponding net {s(F)} of finite sums is monotonic increasing; so the family is summable if and only if the net is bounded above. When this is so, CaE x, is the least upper bound of {s(IF)}, and the set of non-zero terms x, is at most countably infinite (otherwise, for some positive integer n, infinitely many of the terms x, exceed l/n). 1.3. Weak topologies
Suppose that Y is a linear space with scalar field K ( = Iw or C),and 9 is a family of linear functionals on *y; which separates the points of 9'- in the following sense: if x is a non-zero vector in V then, for some p in R p ( x ) # 0. When p E g the equation p,(x) = Ip(x)l defines a semi-norm pp on 9': The separating family { pp: p E F }of semi-norms gives rise, as in Theorem 1.2.6, to a locally convex topology on ^v; the weak topology induced on V by 9 In this topology, which we denote'by cr(V;F), each point xo of V has a base of neighborhoods that consists of all sets of the form V ( x o : p ,( . . . ,p m ; & ) = { x E " Y - : ( p j ( x )-pj(xo)l
where E > 0 and p , , . . . ,pmE 2
<&
( j = I )...,m)),
1.3. WEAK TOPOLOGIES
29
Since Jp(.u)- p(xo)J< E when X E V ( x o :p ; E ) , each of the linear functionals 9 is continuous relative to the topology a(V;,F). However, if T is a topology on $ , and each linear functional in 9 is r-continuous, then all the sets V ( x o :p l , . . . ,p m ;e ) are r-open; from this, a ( Y i 9 ) is coarser (weaker) than z. Accordingly, a( K 9 ) is the coarsest (weakest) topology on Y relative to which each element of 9 is a continuous mapping from V into K. It is apparent that a ( K 9 ) is coarser than a(K;Y),when 9 c 99. Suppose that v E 3‘ and {u,) is a net of elements of f From the discussion if and following Theorem 1.2.18, { L J , } converges to [I, in the topology 0(<9), only if, given any p in 9 and any positive E , there is an indexj, such that pP(i1,- I‘) < E (that is, Ip(u,) - p(u)l < E ) wheneverj a j o .Thus { v , } is a(%$)convergent to L’ if and only if, for each p in %the net {p(u,)} of scalars converges to p(c). Similarly, { t i , ) is a Cauchy net, in the uniform structure associated with a( $’; 9), if and only if { p (u,)} is a Cauchy (and hence, convergent) net of scalars for each p in 5? p in
1.3.1. THEOREM.Suppose that Y is a linear space with scalarfield K (= 17% or C ) , 9 is a separating family of linear functionals on K and 9is the set of all finite linear combinations of elements of F.Then a ( K 9 ) coincides with a ( K 9 ) , and Y is the set of all a(K9)-continuous linear functionals on f a ( V i 9 ) is coarser than a(KY). Every linear Proof. Since 9 E 9, functional p in Y is a linear combination a l p l + . . . + amprnof elements p l , . . . ,pm of 9; moreover, each pj is a( KY;)-continuous, so the same is true of p. Since a($<;U) is the coarsest topology that makes every element of 9 continuous, it now follows that a ( K 9 ) is coarser than a(KF);so these two topologies coincide. It remains to prove that each a($’,’.F)-continuous linear functional po lies in 97. Given such p o , we can apply Proposition 1.2.8(iii), bearing in mind the form of the semi-norms p p (with p in 9), which give rise to the topology a(3’;9). It follows that there are elements p l , . . . ,pm of 9and a positive real number C such that
for each x in 3: In particular, p 0 ( x ) = 0 if p j ( x ) = 0 for j = 1 , . . . ,m. From Proposition 1.1.1, po is a linear combination of p 1 ,..., p m , and thus P0E.Y.
Suppose that Y is a linear space with scalar field K 1.3.2. PROPOSITION. ( = R or C ) , 9 is a separating family of linear functionals on K and cp is a mapping.from a topologicalspace 9’ into f Then cp is continuous, relative to the topology a ( Y i 9 ) on 9’; if and only if each of the composite mappings p o cp :9 4 K ( p E 9) is continuous.
30
I . LINEAR SPACES
Proof. Since each p in 9 is o(<.F)-continuous, it is evident that continuity of cp entails continuity of the composite mapping p cp. Conversely, suppose that p q~ is continuous for each p in .E In order to establish the continuity of cp, it suffices to show that cp- '( V ) is an open subset of Y whenever V is one of the basic o(V, 9)-open sets V(xo:p l , . . . ,pm ;E ) . Of course, p l , . . . ,,omE g so pj o cp :Y -P K is continuous for j = 1,. . . ,m. From this, and since 0
0
cp-'(V) = {sEY:cp(S)E V } = ~ s ~ ~ : : p ~ ( c p ( s ) ) - p<~ E ( x( ~j =) )1, . . . , m ) ) ,
cp-'( V ) is open, as required.
Suppose that Y is a locally convex space. The set of all continuous linear functionals on Y is a subspace of the algebraic dual space of Y ;it is denoted by Ya,and is described as the continuous dual space of Y? By Corollary 1.2.11, it separates the point!: of $
1.3.4. THEOREM. A convex subset Z of a locally convex space Y has the same closures, relative to the initial and weak topologies on 9: Proof. Since the weak topology on Y is coarser than the initial topology suffices to prove the following assertion: if Y E Y and y is not in the 7closure Z - of Z , then y is not in the weak closure of Z . Now Z - is convex, and it follows from Corollary 1.2.12 that there is a continuous linear functional p on Y and a real number b such that T, it
Rep(y) > b,
Rep(z) < b
(zEZ-).
31
1.4. EXTREME POINTS
The set S = { X E P': Rep(x) < b } is weakly closed since p is weakly continuous; moreover, y $ S, and Scontains Z a n d so contains the weak closure of
z. H When '1 is a locally convex space and x E 4 p ) = p(x)
(PE
the equation
0
defines a linear functional 1 on the continuous dual space V ' . The set 9-={2:xEP-)
is a linear subspace of the algebraic dual space of 9"'. It separates the points of Y' since, if p E 3 -* and 2 ( p ) = 0 for all x in K then p(x) = 0 ( X E Y ) ,and thus p = 0. We often write a(Y', V ) ,rather than a(Y', T ) ,for the weak topology induced on 3" by ?; and we refer to a(P",Y) as the weak* topology (sometimes called the w*-topology) on V'.Note that each p o in Y' has a base of neighborhoods consisting of sets of the form { P E Y ' :Ip(xj) - po(xj)I < E
(i= 1,. . . , m ) ) ,
where E > 0 and x l , . . . ,x, E 9: From Theorem 1.3.1 (with Y replaced by V', and 9 = Y = +.), the weak* continuous linear functionals on Y' are precisely the elements of 9; so we have the following result. 1.3.5. PROPOSITION. A linearfunctionalo, on the continuous dualspace Y"' of a locally convex space -V; is weak* continuous ifand only ifthere is an element x of $'such that w(p) = p ( x ) for each p in V'. 1.4. Extreme points
Suppose that Y is a locally convex space. By the closed convex hull of a subset Y of V" we mean the closure CO Y of the convex hull co Y ; it is clear that this is the smallest closed convex set that contains Y. An element xo of a convex set Xin ./''is described as an extremepoint of Xif the only way in which it can be expressed as a convex combination xo = ( 1 - a ) x l + a x 2 , with 0 < a < 1 and x l , x2 in X , is by taking x1 = x2 = x0. We shall prove (Theorem 1.4.3) that every compact convex subset of P-has extreme points and is the closed convex hull of the set of all its extreme points. In the locally convex space R2 (that is, in the plane), a closed triangle is a convex set that has just three extreme points, its vertices. For a closed disk in R2, the extreme points are precisely the boundary points. In each of these examples, it is apparent that the set specified is the convex hull of its extreme points. In the case of a triangle, one might expect that the sides, as well as the vertices, have some significance in terms of convexity structure; in fact, each side is a "face," in a sense now to be defined.
32
I . LINEAR SPACES
By a face of a convex set X in Y we mean a non-empty convex subset F of X,such that the conditions O
(I - a ) x l + a x , ~ F ,
x,,x,~X,
imply that x, ,x2E F. Note that xo is an extreme point of X if and only if the one-point set {x,,} is a face of X. It is apparent that, if a family of faces of Xhas non-empty intersection, then this intersection is itself a face of X . Moreover, if Y is a face of X and 2 is a face of Y, then Z is a face of X. In particular, therefore, an extreme point of a face of X is also an extreme point of X. The following lemma provides a slight strengthening of the defining property of a face; and upon specializing to “one-point” faces, it reduces to a similar assertion about extreme points. 1.4.1. LEMMA.I f F is a face of a convex set X and a l x l + . ’ . + a,x, E F, where x,, . . . ,x, E X and a , , . . .,a, are non-negative real numbers with sum 1, thenxjEFwhen 1 < j < n a n d a j > O . Proof. It suffices to show that x 1 E F if a, > 0. If a, = 1, then a2 = a 3 = . * . = a , = O and x1 = a l x l + . . * + a , x , ~ F . If 0 < a , < 1, let a = 1 - a , and let y be the convex combination a - l ( a Z x 2+ . * * + a,,.~,,)of x 2 ,..., x , , . T h e n x l , y ~ X , O < a < l , a n d
( I - a)xl + ay = a l x l
+ a2xz + . . . + U,X,E
F
Since F is a face of X , it follows that x , E F. I The results that follow are formulated so as to apply to both real and complex locally convex spaces, the notation Re being redundant in the real case. 1.4.2. LEMMA.If X is a non-empty compact convex set in a locally convex space ^y; p is a continuous linear functional on K and c = sup{Rep(x):xEX}, then the set F = { X E X: Re p(x) = c ) is a compact face of X . Proof. Since a continuous real-valued function on a compact set attains its supremum, Fis not empty; and it is evident that Fis compact and convex. If x , , x 2 e X , 0 < a < 1, and (1 - a)xl axzEF, we have R e p ( x , ) < c, Re p(xJ < c, and
+
(1 - a) Re p(xl)
+ a Re p(x2) = Rep(( 1 - a ) x l + a x z ) = c;
so Re p ( x l ) = Re p(x2) = c, and x, ,x2E F. Thus F is a face of X. 1.4.3. THEOREM(Krein-Milman). If X is a non-empty compact convex set in a locally convex space K then X has an exrremepoinr. Moreover, X = E6 E, where E is the set of all extreme points of X .
1.4. EXTREME POINTS
33
Proof: The family 9 of all compact faces of Xis non-empty since XE 8 and is partially ordered by the inclusion relation G . Let 9, be a subfamily of 9 that is totally ordered by inclusion. It is evident that 9,, has the finiteintersection property, so by compactness the set Fo = n { F : F E ~ , is} nonempty. Thus Fo is a compact face of X , and is a lower bound of 9,, in $? Since every totally ordered subset of 9 has a lower bound in it follows from Zorn’s lemma that 9 has an element F that is minimal with respect to inclusion. We shall show that F consists of a single point x, and since F is a (compact) face of X , it then follows that x is an extreme point of X . To this end, suppose the contrary, and let x,, x 2 be distinct elements of F. By the Hahn-Banach theorem, there is a continuous linear functional p on Y such that Re p(.ul) # Re p ( x 2 ) . From Lemma 1.4.2 we can choose a real number c so that the set
Fo = {xEF:Rep(x)= c)
is a compact face of F. Accordingly, Fois a compact face of X ; that is, F, E 9? Since Rep(xl) # Rep(x2), at least one of .xl, x2 lies outside F,; so F, is a proper subset of F, contrary to our minimality assumption. Hence Fconsists of a single point. So far, we have shown that each non-empty compact convex subset of V has an extreme point. If E denotes the set of all extreme points of X , it is clear that= E c X , and we have to show that equality occurs. Suppose thecontrary, and let xo E x\cO E ; we shall obtain a contradiction. From the Hahn-Banach theorem, we can find a continuous linear functional p on Y and a real number a such that Rep(x,) > a >, Rep(y)
(1)
If
c 1 = sup{Rep(x):xEX},then
(~ECOE).
cl > a , and the set
F, = { x E X :Re@)
= cl}
is a compact face of X by Lemma 1.4.2. In particular, F1 is a non-empty compact convex subset of -U; and so has an extreme point xl.Since x1 is an extreme point of a face of X , it is an extreme point of X ; that is, x, E E. However, Rep(x,) = c1 > a, contradicting (1). 1.4.4. COROLLARY. If X is a non-empty compact convex set in a locally concex space and p is a continuous linear junctional on Vi there is an extreme point so o j X such that Re p(x) < Re p(xo)for each x in X . Proof: Let c = sup{Rep(.x):.xEX}.
By Lemma 1.4.2, the set { x E X :Rep(x) = c} is a compact face of X . In
34
I . LINEAR SPACES
particular, it is a non-empty compact convex set in and so has an extreme point xo. Since xo is an extreme point of a face of X , it is an extreme point of X ; and Re p(x,)
=c
2 Re p ( x )
( x X~) .
1.4.5. THEOREM.IfXisanon-empty compact convexset in a locally convex space V and Y is a closed subset of X such that CO Y = X , then Y contains the extreme points of X . Proof. Suppose that xo is an extreme point of X . In order to show that it suffices to prove that xo + V meets Y whenever V is a balanced convex neighborhood of 0 in .Y ; for Y is closed, and sets of the form xo + V constitute a base of neighborhoods of xo. Given V as above, the family { y + + V :Y E Y } is an open covering of the compact set Y , and so has a finite subcovering { y j + : V : j = 1,. . . ,n } , with y , , . . . ,y , in Y. Let V - denote the closure of V , and f o r j = 1 , . . , ,n, let X j be the non-empty compact convex set ( y j i V - ) n X . Then X ~ Y, E
+
Y= YnXc
u
1+
(yj+iV) nXG
[j:,
u xj.
j:,
Let S be the set of all vectors of the form a l x l . . . + anxn,where xi€ Xi . . , n ) and the coefficients a , , . . . , a , are non-negative real numbers with sum 1. Then Scontains each X j , and so contains Y ; from the convexity of X I , . . . ,X,, it is readily verified that S is convex. We assert also that S is compact. To prove this, let A be the compact subset ( j = 1,.
{ ( a l , .. . , a , ) : a l 2 0 , . . . , a , 2 0, a ,
+ . . . + a, = 1 )
of R", and write a for the element ( a , , .. . ,a,) of A . By Tychonoff's theorem, the set A x X1 x * ' . x X , is compact in the product topology; and hence S is compact, since it is the image of the product set under the continuous mapping ( a , x l ,. . . ,x,)
+alxl
+ . . . + a,x,.
From the preceding paragraph, S is a compact convex set containing Y . Thus S contains the closed convex hull X of Y ; in particular, X,ES. Accordingly, xocan be expressed as a convex combination a l x l + . * . + anxn, where xi€ X j ( s X ) . Since xo is an extreme point of X , xo = x j for somej in { l , . . . , n } (take a n y j with a j > 0 ) , and xo = x j E
xj G yj + iv-.
:
Thus .xo - y j lies in theclosure of i V ; so the neighborhood xo - y j + Vmeets i V . Since V is balanced and convex, it now follows that y j e x o + V , whence xo + V meets Y.
35
1.5. NORMED SPACES
1.5. Normed spaces
In the discussion preceding Proposition 1.1.5, we introduced the concepts of “semi-norm” and “norm” on a (real or complex) linear space. In Theorem 1.2.6, we showed how a separating family f of semi-norms on such a space gives rise to a locally convex topology. The present section is concerned with the case in which f consists of a single norm. By a normedspace we mean a pair (X,p)in which X is a linear space whose scalar field K is either R or C andp is a norm on X. When x E X,we usually write llxll rather than p ( x ) , and refer to llxll as “the norm of x.” With this notation, the defining properties of a norm can be set out as follows: whenever x, y e X and U E K, (a) llxll 2 0, with equality only when x = 0; (b) llaxll = la1 Ilxll; (c) IIx + yl( Q llxll + llyll (the triangle inequality). We recall also another property, lllxll - llylll d IIx - yll, which is an easy consequence of the triangle inequality. Suppose that (X, 11 11) is a normed space. From the properties (a), (b), and (c), just noted, it is apparent that the equation
YE X) d(x,y) = IIx - YII defines a metric d on X. Also, X has a locally convex topology, the norm topology, derived as in Theorem 1.2.6 from the family f consisting of the single norm 11 11 on X; and this topology gives rise to a uniform structure on X, described in the discussion preceding Proposition 1.2.1. In the norm topology, each xo in X has a base of neighborhoods consisting of the sets V(xo: 11 )I;E ) ( E > 0), where V(x0:II ( ( ; E ) = { x E X : ( I X - X ~ ~ ~ < E } = { X E X : ~ ( ~ , X ~ ) < E } ,
the “open ball” with center xo and radius structure has a base, consisting of the sets {(X,y)EX x X:.u - y E V ( O : I l
I[;&)}
E.
The corresponding uniform
= {(?C,Y)EX x X:llx-yll = {(X,Y)€X x
<E}
X : d ( x , y )< E } .
Accordingly, the norm topology, and the associated uniform structure, are precisely the topology and uniform structure derived in the usual way [K : pp. 119, 1841 from the metric d o n X. Since X, with the norm topology, is a linear topological space, the mappings (x, y) 4 x + y and (a, x) + ax are continuous; and so is the mapping x -, IIxII, by Theorem 1.2.6. Of course, these continuity assertions can easily be verified directly, using the defining properties (a), (b), and (c) of the norm.
36
I . LINEAR SPACES
A normed space (X,II 11) is described as a Banach space if it is complete relative to the uniform structure just mentioned; that is, if it becomes a complete metric space, with the metric d defined above. The scalar field K is a one-dimensional Banach space, the modulus function being a norm which gives rise to the usual topology on K. With either of the normsp, ,p , , defined in the discussion preceding Proposition 1.1.5, K” becomes a Banach space (in either case, the norm topology being the usual product topology). When X is a normed space and r > 0, we denote by (X), the closed ball {xE X: JJxJJ < r } ;we refer to (X), as the unit ball of X. Since (X), is convex and is closed in the initial (that is, the norm) topology, it is weakly closed, by Theorem 1.3.4. A subset 9 of X is bounded if %? c_ (X), for some positive real number r. A linear mapping T from a normed space X into another such space Y is said to be norm preserving if I)Txll = llxll for each x in X. Such a mapping is necessarily isometric (that is, distance preserving) and hence one-to-one, since
d(Tx1, Tx2) = llTx1 - Txzll = IIT(X1 - x2)ll = 11x1 - x2Il
= d(x,,x2);
and conversely, an isometric linear mapping is norm preserving. A normpreserving linear mapping from a normed space X onto another such space 9is sometimes described as an isometric isomorphism from X onto Y. A subspace X of a normed space Y is itself a normed space since the norm on OY restricts to a norm on X.The following theorem shows that every normed space can be viewed as an everywhere-dense subspace of an (essentially unique) Banach space. 1.5.1. THEOREM.If X is a normed space, there is a Banach space OY that contains X as an eiierywhere-dense subspace (and such that the norm on X is the restriction of the norm on YY).IfYl is another Banach space with theseproperties, the identity mapping on X extends to an isometric isomorphismfrom 9 onto W l . Proof. Let d be the metric on X derived from the norm; let 2 be the completion of the metric space X so obtained, and let adenote the metric on .%. Thus .% is a complete metric space, X is an everywhere-dense subset of 3, and
J(u, U ) = d(u,V ) = IIu
-
uII
(u,U E3).
We shall show that P can be made into a Banach space, with addition, scalar multiplication, and norm, extending those of X. When u, u’, 11, u’ E X and a E K (the scalar field), Il(U
+
2))
+ 0’)ll = I((u - u’) + (u - d)((< ((u- U’((+ ( ( u - cy,
- (U’
llau - au’ll = la1 IIU -
4
1
7
I llull - IIU’III < Ilu - 41.
Accordingly, the equations
f(u, 4 = u + 0,
g,(u) = au,
h(u) = IIuII
37
1.5. NORMED SPACES
define uniformly continuous mappings
f:X x X + X ( d ) ,
g,:X+X(sf),
h:X+R.
Since%. and R are complete, and X is everywhere dense in 2 (so that X x X is everywhere dense in f x A), it follows that go, h extend by continuity to uniformly continuous mappings
f :%.
x
f -+ .%, &: .% -+ .%, h: R + R,
respectively. The addition and scalar multiplication, already defined for elements of X, can now be extended to f by the equations u+v=3(u,c),
au=@,(u)
(U,UE.%,
UEK);
and we assert that, in this way, 2 becomes a linear space over K. To prove this, it suffices to verify the relations
+ 1‘ = L’ + u, u + ( - l ) =~ 0, u
+ ( 0 + w ) = (u + 0 ) + U’, u + 0 = u, (a + b ) = ~ au + bu, U ( U + U) = uu + U U , u
lu = u,
a(bu) = (ab)u,
for all u, P, M’in 2 and a, b in K. Of course, all these relations are satisfied when u, c, W E X, and simple continuity arguments show that they remain valid for elements of 2. For example, the relation a(u + ti) = au + au can be rewritten in the form
@,(f(4 11)) - f ( @ h h
40([3))
= 0,
and is satisfied when u, C E X. The left-hand side of this last equation is a continuous function of (u, 11) on .% x .%, which vanishes on the everywheredense subset X x X, and so vanishes throughout %. x $ (as required). Similar arguments establish the other relations; so $ is a linear space over 06. We prove next that h is a norm on .%.The relations h(u - 1’) - h(u, 0) = 0,
h(au) = la)h(u)
( a €K)
are satisfied when u, u E X, since h extends the norm on X. By continuity, they remain valid for all u, ti in .%.Accordingly,
h(u + 1‘) = h(u + 0,O) < h(u + 11, D )
+ &,O)
= h(u)
+ h(U),
and h(u) = d(u, 0) 3 0 (with equality only when u = 0), when u, u E 2. It now follows that h is a norm on f , which extends the norm on X,and gives rise to the complete metric a on 2. Hence $ is a Banach space and contains X as an everywhere-dense subspace. Finally, suppose that ?/ and OTY, are Banach spaces, each containing X as an everywhere-dense subspace and each with its norm extending the norm on 3.
38
I . LINEAR SPACES
The identity mapping on X can be viewed as a continuous linear operator, from an everywhere-dense subspace of the Banach space into the Banach space By Corollary 1.2.3, it extends to a continuous linear operator T from Y into ?V1. The equation f ( u ) = IITull - llull defines a continuous mapping f:9 -,R, and f vanishes on the everywhere-dense subset X of 9.Hence f vanishes throughout 9, and T is norm preserving, and therefore isometric. Since 4Y is complete and Tis isometric, the range TCY) of Tis complete, and is therefore closed in @YI. Thus T ( g )= g1,since T ( Y )contains the everywheredense subset X of Y l ; and T is an isometric isomorphism from !Y onto ?Yl, which extends the identity mapping on X. The Banach space ?2/ occurring in the statement of Theorem 1.5.1 is called the completion of the normed space X. The following lemma provides a useful criterion for the completeness of a normed space. It is couched in terms of the convergence of certain infinite series of vectors in the space; by the convergence of such a series, 1; x,, we mean convergence of the sequence of partial sums s, = x1 + . . . + x,. The result could easily be reformulated in terms of unordered sums of the type considered at the end of section 1.2. 1S . 2 . LEMMA. equioalent ;
if
X is a normed space, the ,following
two
conditions are
(i) X is a Banach space. (ii) /fxI,x2,. . .~XaandCIIx,,lI< oo,theseriesEx,conoeryes, in themetric of 3, to an element of X. Proof. Suppose first that X is a Banach space. Let {x,) be a sequence of elements of X, such that Cllx,ll < a,and write s, for the partial sum x 1 + . . . + x,. Given any positive E , there is a positive integer N such that ))x,)J< E (and hence Its, - s,)) = ))I;+ x,)) < E ) whenever m > n 2 N . Hence {sn} is a Cauchy sequence, in the complete space X, and so converges (that is, Ex,, converges). Conversely, suppose that condition (ii) is satisfied. If {y,} is a Cauchy sequence in X,there is a strictly increasing sequence {n(I), n(2),. . .) of positive integers such that
xr+
- Yflll < r k (m> n 2 n ( W . In particular, IIy,(k+ - yn(k)l(< 2 - k , and therefore llYm
0s
Ikn(1,II
+ 1 IIYn(k+l)
-Yn(k)ll
<
k= 1
From condition (ii), the series y , , ( ] + ) ZT(y,(k+ I , - y,,,,) converges to an element y of X; that is, the sequence {y,,(k))converges to y (because yn(k)is the
39
1.5. NORMED SPACES
kth partial sum of the series). Finally, when n > n(k), lbn
- Yll
6
lbn
- yn(k)ll
+ l b n ( k ) - yll < 2 - k + 11Yn(k) - yll;
and since the right-hand side tends to 0 when k + 00, {y,} converges to y. Hence X is a Banach space. 1.5.3. THEOREM.If J is a closed suhspace of a normed space X, the equation 1I.x
defines a norm 1) quotien t mapping
+ "Yllo = inf{llx + y ( 1 : y ~ J Y )
(~€3)
[lo on the quotient space XlQ. With this norm on X/"Y, the Q : x - + x + ? V : .X+X,+?/
isa continuous linear operator, and llQxllo 6 llxll; and X/.Y is a Banach space i f X is a Banach space. Proqt
Suppose that x, xl, x2E X and a is a scalar. Since
+
inf{((x y ( I : y ~ u Y=) inf{l)u)l:U E X
+ @Y},
the definition of IIx + Yllo is unambiguous (that is, it dependsonly on thecoset .Y + 9, not on the choice of x within that coset); moreover, IIx + 9YlIo 2 0. For all y , y l , and y 2 in J , 11x1
+ .y2 + Y l + Y2ll G I b 1 + Y l l l + 11x2 + Y21L
llax
+ ayll = la1 llx + Yll.
Thus
+ x2 + +Y?'l(Io < llxl + 9(lo+ JIx2+ Yyllo, lax + tVl0 = la1 I(x + ??/l o. If I(x + 9110= 0, there is a sequence {y,} in ??/ such that IIx + y,JI < l / n ( n = 1,2,. . .); since 9Y is closed, while - y , E I and lim( - y,) = x, it follows that ~ € 9 whence , x + "Y is the zero vector in X/J.Hence )I (lo is a norm on (Ixl
X1.Y. The quotient mapping Q is a linear operator, and is continuous since
IIQ-YI - QxzIlo = 11x1 - x2 = inf{llxl
+ Jllo
- x2 + r l l : y ~ t V 6 } 11x1 - x ~ l l
for all x, and x2 in X. Suppose that X is a Banach space. If {x, + tV) is a sequence of elements of X / I , and CIlx,, + ?Vllo < 00, we can choose y , , y 2 , .. . in 9Y so that ((x, + y,(( < ((x, 9lIo 2 - " . Thus C((x, + y,(( < 00, and by Lemma 1.5.2, the series C(x, + y,) converges to an element z of X. Since Q is a continuous
+
+
40
1. LINEAR SPACES
linear operator, and Q(x,, + y,,) = x,
n= 1
+ y , + ?Y = x, + Y, it follows that \n=
1
as rn + 00 ;that is, C(x, + "Y) converges to Qz (E X/?/). Again by Lemma 1.5.2, X/?q is a Banach space. 1
V Y and 3 are subspaces of a normedspace 3, with Y 1.5.4. COROLLARY. closed and 2Tfinite-dimensiona1, then (?7 + 2'is closed in X. Proof. The quotient mapping Q : X + X/oY is a continuous linear operator; the subspace Q ( 3 )o f X/"Y is finite-dimensional and is therefore closed in Xkq (Theorem 1.2.17); and (?7 + 2T is the inverse image Q - l ( Q ( 9 ) ) . 1 We now consider some elementary properties of linear operators acting on normed spaces. 1.5.5. THEOREM.I f X and Y are normed spaces and T :X + 3Y is a linear operator, the following four conditions are equivalent.
(i) T is continuous. (ii) There is a non-negative real number C such that IITxIJ< Cllxll for each x in X. (iii) sup{)lTxll/llxll:x~X, x # 0) < co. (iv) sup{llTxll:x~X, llxll = I } < co. When these conditions are satisfied, the suprema occurring in (iii) and (iv) are both equal to the smallest real number C with the property set out in (ii). Proof. The equivalence of (i) and (ii) is a special case of Proposition 1.2.8(ii), with both rl and Tz consisting of a single norm. A real number C has the property set out in (ii) if and only if IITxll/llxll < C ( X E X, zc # 0). Upon taking x, = [lxIl-'x, it follows that this last condition is satisfied if and only if IlTXlll < c (x1 E X , IlXlll = 1). This proves the equivalence of (ii), (iii), and (iv), and shows that the suprema in (iii) and (iv) coincide with the smallest possible value for C in (ii). 1 When X,? arel normed spaces and T :X + Y is a linear operator, we denote by llTll the (equal, and possibly infinite) suprema occurring in parts (iii) and (iv) of Theorem 1.5.5; we refer to IlTll as the (operator) bound of T. Thus T is continuous if and only if llTll < co; then IITxll
IlTll llxll
(XEX),
41
1.5. NORMED SPACES
and llTll is the smallest real number with this property. It is clear that ((TI1= 0 only when T = 0. Since continuity is equivalent to the existence of a finite bound, continuous linear operators between normed spaces are often described as bounded linear operators. The set d(X,Y) of all bounded linear operators from X into "Y is a linear space (with the same scalar field I6 as X and c!V). If S, TE&?(X,"Y)and a € 06, Il(S +
m-11= llSx + Txll 6 IlS4l + IITxll d IlSll + llTll9 Il(aT)-4l = Il4Tx)ll = la1 IITXII
whenever S E X and llxll = 1. By taking suprema, as x varies subject to the conditions just stated, we obtain
11s + TI1
IISII + IlTll?
IlaTIl = la1 IlTll.
Since, also, llTll 2 0, with equality only when T = 0, it follows that &(X, 9) is a normed space, with the operator bound as norm. When Y = 3, we usually write B(X)rather than a(X,X). If X, Y, Y are normed spaces, and S : 9 + I ,T :X-+ "Y are continuous linear operators, then S T : X -+ Y is continuous. Since IlST.UIl 6 IISII IITXll d IlSll IlTll
( x E x
llxll =
it follows that IlSTll 6 IlSll IlTll. This applies, in particular, when X = 9 = Y. The set d ( X ) is an associative linear algebra with a unit element I(the identity mapping on 3 ) ;it is also a norrned space, and its norm (the operator bound) satisfies 11111 = 1, JISTII< IlSll IlTll. These properties of d ( X ) are characteristic of Banach algebras, which are studied in Chapter 3.
I .5.6. THEOREM.If X is a normed space and tV is a Banach space, both hariny the same scalar field K, then the set d?(X,"Y) of all bounded linear operators from X into 'Y is a Banach space oijer K,with the operator bound as norm. Proof. We have already seen that d ( X , 'Y) is a normed space, so it remains to prove that it is complete. Let { T,,)be a Cauchy sequence in 9 ( X , "Y). Given any positive E , there is a positive integer N(E)such that IlT,,, - T1.1 d
E
(m > n 2 N(E)).
When X E X , (1)
(m > n 2 N(E)).
IIT,,,.u - T,,.xll d /IT,,, - T1.1 IIxlI 6 cllxll
Thus { Tnx)is a Cauchy sequence in the Banach space c!Y, and so converges to an element of d.Accordingly, the equation
Tx = lim T,,x n-
T
(XE
X)
42
I . LINEAR SPACES
defines a mapping T: X + '9, and it is apparent that T is a linear operator. Upon taking limits as m + c;o in (I), we obtain llTx - T,,xll < ~llxll
( n 2 N(E), XEX).
This shows that T - T, is a bounded linear operator (whence, so is T )and that IIT - T1.1 < E whenever n N(E).It follows that { T,) converges to the element T of .%?(X,Yq), and hence B(X,Y) is complete. is a Banach space, both I f ' X is a normed space and 1 S . 7 . THEOREM. having the same scalar field, then every bounded linear operator T: X + 9 extends uniquely to a bounded linear operator p: 2 -+ 9,where %. is the completion of X. The mapping T + $is an isometric isomorphismfrom g ( X , #) onto a(%, Y). Proof. By Corollary 1.2.3, each continuous linear operator T: X -+ 9 extends uniquely to a continuous linear operator T : .% -+ 9. The inequality IlTll llxll - IIFxll 2 0 is satisfied when X E X , and by continuity it remains valid for all x in 2. Thus 11T11 < IlTll; the reverse inequality is evident since Textends T, SO ( ( ~ 1 1= IITII. It is apparent that the norm-preserving mapping T + F : g ( X , Y) -+ B(.%,3 ) is linear. Its range is the whole of B(2,fq) since, when S 0 € g ( . % ,Y),we have So = Po, where To (EB(X,?Y)) is the restriction SolX. H 1 S . 8 . THEOREM. Suppose that X and ?iY are normed spaces, T: X + 'Y is a bounded linear operator, Xo is a closed subspace of X such that T(Xo) = { 0 } ,and Q: X -+ X/Xo is the quotient mapping. Then there is a bounded linear operator To:X/X0-+ 9such that T = Toe; moreover, llToll = JJTJI, and To is one-to-one if Xo is the null space of T.
Proof. From purely algebraic considerations T has a factorization TOP, where To:X/Xo + 9 is a linear operator, and is one-to-one if .X0 is the null space of T. When X E X, llQxll < llxll by Theorem 1.5.3; so llQll < 1. Moreover, IIToQxll = IITxll = IIVx + xo)ll
IlTlllI~+ xoll
(XOEXOL
so
IIToQxll < llTllinf{llx+ x o l l : x o ~ X o )= IlTll IlQxll. Since Q(X)
= X/Xo,
it now follows that To is bounded, and
IlToll < IlTll = IlToQll < IlToll IlQll so IlTOll = IlTll.
< IlToll;
1.6. LINEAR FUNCTIONALS O N N O R M E D SPACES
43
1.5.9. LEMMA. IfX and 9/ are normed spaces, S : X -+ 'Y and T :9 -+ X are linear operators such that S T = I (the identity operator on "Y), and a = inf{llSxll:xE T(JY), llxll = I},
then IlTll = a - ' (where0-' is to he interpretedas m).Inparticular, Tishounded i f and only i f a > 0. Proof. Since S T = I, T can be viewed as a one-to-one linear mapping from ,Y onto T(!Y),and as such, it has an inverse mapping, the restriction SI T(iY). Also, when 0 # Z E T(CiY),11z11-'z is a unit vector x in T(9/). Thus
=
sup{
: Z E T(?Y),z # 0) IlSZll
1.5.10. COROLLARY. Suppose that X and 9 are normed spaces, S is a oneto-one linear operator from X onto 'I, T :9/ + X is its inuerse operator, and
a
= inf{IJSxll:xEX, llxll =
I]
(i) IlTll = a-' (where 0 - ' is to he interpreted as 03); in particular, T is bounded i f and only i f a > 0. (ii) I f X is a Banach space, S is hounded, and a > 0, then 9 is a Banach space. Proof: (i) This follows from Lemma 1.5.9, since T ( g )= X. (ii) Since a > 0, T (as well as S) is bounded; so both S : X -+ 9/ and its inverse T : I -+ X are uniformly continuous. Hence the completeness of X entails completeness of 'Y. 1.6. Linear functionals on normed spaces
In this section we shall be concerned with the continuous dual space X uof a normed space X and with the properties of the weak topology o(X, Xu)on X and the weak* topology o(X',X) on X'. I t turns out that, in a natural way, X u becomes a Banach space and X is isometrically isomorphic to a subspace of the second dual space Xu' ( = (3')').We describe a necessary and sufficient condition for this subspace to be the whole of X". In Section 1.5 we considered linear operators from a normed space X into another such space ,!Y and introduced the normed space @(X, 3 )ofcontinuous linear operators. By taking, for
44
1 . LINEAR SPACES
concerning linear functionals. From Theorem 1S . 5 , a linear functional p on X is continuous if and only if it is bounded, in the sense that there is a nonnegative real number C such that Ip(x)l G
Cllxll
(XEXt’).
When p is bounded, the least possible value of C is the bound llpll of p, defined by
The continuous dual space X xof X,defined (as in Section 1.3)as the linear space of all continuous linear functionals on X, coincides with B ( X , W);by Theorem 1S.6, it is a Banach space, its norm being given by (1). We refer to X’,with this norm, as the Banach dual space of X. For normed spaces, we have the following Hahn-Banach extension theorem.
If Xo is a subspace of a normed space X and po is a 1.6.1, THEOREM. bounded linearfunctional on Xo, there is a bounded linear functional p on .t‘ such that llpll = llpoll and p ( x ) = po(x) when X E Xo. ProoJ
This follows at once from Theorem 1.1.7, with
P(4
=
llpoll llxll
(xe
1.6.2. COROLLARY. Ifx0 is a non-zero tlector in a normedspace X, there i s a bounded linear functional p on X such that llpll = 1 and p(xo) = Ilxoll.
Proof. The equation po(cxo)= cllxoll defines a bounded linear functional po on the one-dimensional subspace Xo generated by xo ; moreover, po(xo)= IIxoll,and llpoll = I . By Theorem 1.6.1, po extends (still with norm 1 ) to a bounded linear functional p on X. R
1.6.3. COROLLARY. If I/y is a closed subspace of a normed space X and there is a bounded linearfunctional p on X such that llpll = 1, p ( y ) = 0 for each y in 9,and p ( x o ) = d ( > 0), where .yo E X\’Y,
d=inf{(lx, + y ( ( : y ~ q } , the distance from xo to +Y. Proof. The quotient mapping Q : X + X/.Y is a bounded linear operator, and d = IIQxoll > 0. By Corollary 1.6.2, there is a bounded linear functional p o on X/.Y such that llpoll = 1 and po(Qxo) = d. The equation p ( x ) = po(Qx) defines a bounded linear functional p on X; p ( x 0 ) = d, and p ( y ) = 0 for each y
1.6. LINEAR FUNCTlONALS ON NORMED SPACES
45
in #. Since p has the factorization p0Q through XIY, it follows from Theorem 1.5.8 that 1 1 ~ ) )= ) ) ~ o = ) l 1. H 1.6.4. THEOREM.tf X is a normed space and x E X, the equation 2(P) = p ( x )
(PEX')
defines a bounded linearfunctional 2 on the Banach dual space Xu. The mapping x + i is an isometric isomorphismfrom X onto the subspace f = {a:x E X} of the second dual space .Xu'. Proof. It is evident that 2, as defined in the theorem, is a linear functional on X', and that the mapping x + 2 is a linear operator from X into the algebraic dual space of X'. When x0eX, I-$o(P)I = IP(X0)l
< IlPll llxoll
( P E - 0
When p is chosen as in Corollary I .6.2, M P ) I = ll-yoll = llpll IIxoll.
Thus ,tois a bounded linear functional, and Il2,,ll = ~ ~ x so o /the ~ ;mapping x is an isometric isomorphism from X onto a subspace f of 3". H
+2
When X is a normed space, the mapping x -+ ?, occurring in Theorem 1.6.4 is called the natural isometric isomorphismfrom X into X", and 2 is described as the natural image of X in X". The weak* topology o(X', X) (as defined, in the discussion preceding Proposition 1.3.5, for locally convex spaces) is the weak topology induced on X' by .%. If .% = X", the normed space X is said to be reflexive. A reflexive normed space X is necessarily a Banach space since it is isometrically isomorphic to the Banach dual space 3". However, many Banach spaces are not reflexive (see, for example, Exercise 1.9.24). Part (i) of the following result is known as the Alaoglu-Bourbaki theorem. 1.6.5. THEOREM.Suppose that X is a normed space and f is the natural image ofX in X". (i) (ii)
The unit ball (X')), is compact in the weak* topology g(X', 3 ) on X'. The weak* closure in Xu' of the unit ball (f), off is the unit ball (X"), of
XU'. Proof. (i) For each x in X, let D, denote the compact subset { a :la1 < Ilxll} of the scalar field K. The product topological space
P= n D x r;EX
is compact, by Tychonoff's theorem. It consists of all functionsp: X + tt6 such
46
1. LINEAR SPACES
that P ( X ) E D, (xEX); each element p o of P has a base of neighborhoods consisting of all sets of the form { p P :~Ip(xj) - po(xj)l < E ( j = 1 , . . . , m ) } ,
where&> 0 and x l , . . . , x , E X . For each x i n X, the mappingp + p ( x ) : P - t K is continuous. The unit ball (X‘)),consists of all linear mappings p : X + K such that P ( X ) E D, (that is, Ip(x)l < Ilxll) for each x in X. Thus
(X’)),= { p ~ P : p ( a x + b y ) - a p ( x ) -b p ( y ) = O ( x , y E X ; a , b E K ) } . From the final sentence of the preceding paragraph, it now follows that (X’)), is a closed subset of P and is therefore compact in the relative topology. In view of the form of the basic neighborhoods of points in P (as described above) and the definition and discussion of the weak* topology (preceding as a subset of P Proposition 1 . 3 3 , it is clear that the relative topology on (X’)), coincides with its relative weak* topology as a subset of Xu. Thus is compact in the latter topology. (ii) From (i), (X”)), is compact in the weak* topology a(X’’, X’), and so contains the weak* closure %? of its convex subset (g)),; we have to show that % = (fig*)),.Suppose the contrary, and choose a. in (X”)),\%?;we shall obtain a contradiction. By the Hahn-Banach theorem, there is a weak* continuous linear functional wo on X” and a real number a such that Rew,(a,) > a,
Rew,(o)
(oE%?).
By Proposition 1.3.5,there is an element po of X‘such that wo(a) = a(po)for all a in Xu‘. When X E X and llxll = 1, we can choose a scalar b such that Ibl = 1 and Ipo(x)l = bpo(x) = b f ( p o )= w o ( b i ) = Rewo(bf);
it follows that Ipo(x)l < a. Thus llpoll and since b f E ( g ) l E %?, since llooll < 1, we have
< a. However,
llp~ll2 I ~ o ( P o ) ~ 2 Re ao(p0)= Re wo(ao) > a, contradicting the previous inequality.
1.6.6. COROLLARY. If9 is a bounded weak* closed subset of the Banach dual space Xuof a normedspace X, then .Yis weak* compact. in addition, Y is contiex, it is the weak* closed convex hull of its extreme points. Proof. For some positive r, 9 is a (weak* closed) subset of the ball (X’)), ( = r(X‘)l),and this ball is weak* compact by Theorem 1.6.5(i); so Y is weak*
compact. The final assertion in the corollary now follows from the Krein-Milman theorem (1.4.3), since Xu,with the weak* topology, is a locally convex space.
47
1.6. LINEAR FUNCTIONALS ON NORMED SPACES
1.6.7. THEOREM. A normed space X is reflexive ifand only if its unit ball (X), is compact in the weak topology. Proof. It is clear that X is reflexive (that is, .% = 3") if and only if (Xs%)l.By Theorem 1.6.5, (X"), is weak* compact, and is the weak* closure of (f)l.Thus (R), = (X"), if and only if (g), is weak* compact. The natural isometric isomorph is^ x -+ 2 : X + f { c 3'') carries (X), onto When x 0 E X, y , ,. . . ,pmE XP,and c > 0, it carries the basic neighborhood (.%)l =
( x € X : ( p j ( x ) - p j ( s , , ) l < & ( j =1, ..., m)]
of x,, (in the weak topology on X) onto the basic neighborhood
{a&:
[a(pj) - a,(pj)l < & ( j = 1,.
. . ,m)}
(in the relative weak* topology on f , as a subset of 3"). It is therefore a of io homeomorphism between X and f , with the topologies just mentioned. Thus (.%),is weak* compact if and only if (X), is weakly compact. Suppose that X and 9are normed spaces and T :X .+ 9is a bounded linear operator. If p is a continuous linear functional on 9, the composite mapping p~ T is a continuous linear functional on X. Accordingly, we can define a -+ X' by mapping T': 9%
T'p = p o T
(p~%*).
We assert that T' is a bounded linear operator and that [[T'ljl= Il7'll. The linearity of TPfollows from the fact that (alp,
+ a2p2)
T = a l b l a 7-1 + az(P2 O T ) ,
when p , , p2 E 9'and a t , a2 are scalars. For each p in g',
IIPII 1 1 ~ ~ s1 1 IIPII IlTlI IIXII (XEX), and thus IIT'plI < llTll IIpII; so T s is bounded, and ((?"'I[< IITJI.To prove the reverse inequality, it suffices to show that IlTxll < IIT'II JIxIJfor each x in 3. ltT'p)(x)/ = lp(Tx)l d
Given such x, it follows from Corollary 1.6.2that we can choose p in @so that llpll = I and p(Tx) = /l7'xll; and
IITxll = Ip(Tx)l = I(z-'p)(-4 d IITPPllIlxll < lITPllMI IlXJl = IIT'll IIXlL as required. When T , , T2 : X -3 $9 are bounded linear operators and p&@, it results from the linearity of p that (UlT,
+ a2T2Yp = P " h T 1 + a2T2) =U ~ ( P O Ti)
+ az(po T2) = a1T:p +
~ 2 7 ' : ~
48
I . LINEAR SPACES
for all scalars a , , a 2 . Thus (al T I
+ a2T2)' = al T : + a2T i ,
and the mapping T -+ T' is a norm-preserving linear operator from g(X,?Y) into g(@,3'). If X, g,3 are normed spaces, and S E ~ ( . Y , Y ) ,T E ~ ( X , then ~ ) ,
( S T ) $ = ~ ~ ( S T ) = ~ . ( S =. T( )~ . s ) . T = ~'(s'p) for each p in 2'; so (ST)' = T'S'. The operator T': g' + X' is called the (Banach) adjoint of the bounded linear operator T : X -+ @. When X and Y are Hilbert spaces, there is another (and, in that case, more important) adjoint operator, the (Hilbert) adjoint T* :??/ X, which will be described in Section 2.4. -+
If T is a bounded linear operator from a normed space 1.6.8. PROPOSITION. X into another such space Y, then T' is continuous relative to the weak* topologies on 9'and X'. Proof. We use the criterion set out in Proposition 1.3.2. The weak* topology on X' is cr(X',,), where 2 is the natural image of X in 3". Accordingly, it suffices to show that the linear functionals i o T' ( x e X) on [Y' are weak* continuous. Suppose XEX, and let y = T x ; for each p in g', (2" T')(p)= 2(T'p) = (T'p)(x) =
P(W
= P(Y) = Bb).
Thus 20T' = BE @, and therefore 2 0T' is continuous in the weak* topology f?(a',,).
1.7. Some examples of Banach spaces In this section we describe some of the Banach spaces that will be used in later chapters. In some cases, we shall first indicate a general process by which, from a given Banach space X, another such space can be constructed; we then obtain specific examples by taking X = DB or @. Whenf, g are mappings from a set A into a Banach space X, and c is a scalar, f + g and cf will always denote the mappings defined by
( f + s ) ( a )= f ( a ) + g(a),
(cf)(a)= c f ( 4
A).
1.7.1. EXAMPLE.1, spaces. If A is a set and X is a Banach space with scalar field K, we denote by I,(& 3 ) the set of all functionsf: A X such that sup(I1f ( a ) l l : a E A }< co. Given two such functions, f and g, and a scalar c, -+
1.7. SOME EXAMPLES OF BANACH SPACES
49
f + g and d a r e functions of the same type. Thus I x ( A ,X) is a linear space over H,and it has a norm defined by llfll = suP{llf(a)ll:aE A}. We shall show that, with this norm, Im(A,X) is a Banach space. To this end, suppose that Ifn} is a Cauchy sequence in Im(A,3 ) ;we have to show that it converges to some elementfof I , (A,X).Given any positive E , there is a positive integer N(E)such that llfm -fn" Q E whenever m > n 2 N ( E ) . Hence
Ilfm(a) Am11Q l l f m -hll Q & ( a E 4 m > n 2 W E ) ) . Accordingly, for each a in A, { f , ( a ) }is a Cauchy sequence in the Banach space X, and so converges; we can define a mapping f : A + X by fTa) = limf,,(a). When m + x, in (l), we obtain (1)
(2)
I f l a ) -f(a)Il
6
A, n 2 ME)).
6
+
Thus Ilf(a)ll 6 E Ilfn(a)ll < c + llhll, when U E A and n 3 N ( E ) ; so sup{llfla)ll:aEA} < oo,andf€I,(A,X).Sincel)f-f,,ll 6 ~ w h e n n2 N ( ~ ) , b y (2), {fn} converges tof; and Im(A,X) is a Banach space. If a sequence (or net) {fn) converges in I x ( A ,X ) , with limit f , then supllfia) -.L(a)Il a6
=
Ilf-fnll
0;
A
that is, f n ( a )+ f l u ) uniformly on A. Convergence in the Banach space I , (A,X) is uniform convergence on A. By taking X to be R or @, the construction just described gives rise to Banach spaces I , (A, R) and I, (A, @); the latter is usually denoted by In@).
1.7.2. EXAMPLE.Spaces of continuous functions. If S is a topological space and X is a Banach space with scalar field H,we denote by C(S,X) the set of all continuous functions f : S + X such that sup{llf(s)ll:s~S}< a.It is apparent that C(S,X) is a subspace of the Banach space la(& X) defined in Example 1.7.1; we shall show that it is a closed subspace. From this it follows that C(S,X) becomes a Banach space when the norm is defined by
Ilf'll
= suP{llf(s)ll:s~Sl.
Suppose, then, thatf'E I=(S,X). andfis the limit of a net {f,,} of elements of the subspace C(S,3 ) .Thenji(s) +As) uniformly on S , and sincefis a uniform limit of continuous functions, it too is continuous on S. ThusfE C(S,X), and C ( S ,X) is closed in 3). We shall usually be interested in the case in which Sis a compact Hausdorff space. In this case, C(S,X) consists of all continuous functions f:S + X.
50
I . LINEAR SPACES
Indeed, given any suchJ the mappings + Ilf(s)ll: S 4 R is continuous, and is therefore bounded on S ; so sup{IIf(s)ll:s~S}< co. By taking X = R and C,we obtain Banach spaces C(S, R) and C(S,C);the latter is usually denoted by C ( S ) .
In discussing the next example, we shall make use of Minkowski’s inequality [R: p. 62, Theorem 3.51: if 1 < p < co, and x1, . . . , xn,yI,...,yneCrthen (3)
{
Ixj j= 1
+ yjl.)“p < { i
j = 1 lxjlp}l’p
+
{i
j = 1 lyjlp}l’p.
The inequality extends at once to infinite sums; iff and g are complex-valued functions defined on a set A and the sums 2 If(u)lP, C Ig(a)lP converge, then so does 1 IAu) g(u)lP,and
+
For this, it suffices to observe that (by Minkowski’s inequality for finite sums), the net of finite subsums of Ifla) + g(u)lp is bounded above by
1.7.3. EXAMPLE.1, spaces. If A is a set, X is a Banach space with scalar field [M, and 1 < p < 00, we denote by Ip(A,X) the set of all functionsf: A 4 X such that CaEAIlf(~)llP < co.Given two such functions,fand g, it follows from Minkowski’s inequality that
so
f+ g
E
lp(A,X).
Also, c f ~Ip(A,X), and
{c aE
A
llc~u)ll~}l’p= ICI
{ c llAu~llp}l~p a€A
for each scalar c. Accordingly, /JA, X) is a linear space over K, and has a norm
1.7. SOME EXAMPLES OF BANACH SPACES
51
defined by
We shall show that, with this norm, I,(& X) is a Banach space. To this end, suppose that if,)is a Cauchy sequence in I,(& 3). Given any positive E , there is a positive integer N(E)such that Ilfm -frill < E whenever rn > n 2 N ( E )that ; is,
1 IIfmta)
(5)
<
-fn(a)IIP
(m> n 3 N(E)).
QEA
It follows that Ilfm(a) - fn(a)ll < E when m > n 2 N(E)and a~ A;so, for each a in A, { f , ( a ) }is a Cauchy sequence in the Banach space X, and therefore converges. Accordingly, we can define a mappingf: A + X byf(a) = limfn(a). For each finite subset iF of A,it results from ( 5 ) that
C IIfm(a)
< cp
-fn(a)IIP
(m > n 2 N(E))*
asli
When rn 4 co, we obtain
1 IMa) -fn(a)IIp
tp
tn 2 N ( E ) ) ;
aeF
and since this last inequality is satisfied for every finite subset iF of A,
1 IlA4 -fn(a)llP
(6)
G EP
( n k N(4).
ae A
Thus
f-.L
E
lp(&X)
when n 2 N(E),and thereforef= (f-fn) + f n ~ l ~ ( A , X By ) . (6), !If-All < E whenever n 3 N(E),so { f n } converges toJ This proves that IJA, 3)is a Banach space. The construction just described gives rise, in particular, to Banach spaces Ip(A,R) and l J A , 42);the latter is usually denoted by Ip(A). H By taking for A the set { 1,2,. . . ,n ) in Examples 1.7.I and 1.7.3, it follows that there are norms I / ( I p ( I < p < 00) on the linear space 116” (where 06 is 88 or C),defined by the equations
Il(r1,. lI(c1
. . ,(.n)(lp = ElCll”
,+..,CJIlx
+
*
. . + Icnlp]l’p
= max(lc,I,. .
(1 G p < a),
. ,ICnl).
(In the case of 11 /Il and // [ I x , this has already been noted near the beginning of Section 1.5, the two norms being denoted there by p I , p a .) It is easily verified
52
I . LINEAR SPACES
that each of these norms gives rise to the usual product topology on H" (necessarily so, by Proposition 1.2.16). Our next two examples are drawn from measure theory, and we refer to [H, R] as standard sources of information on this subject. For the sake of simplicity we confine attention to a-finite measures, which suffice for our purposes. Accordingly, we shall assume throughout the remainder of this section that m is a a-finite measure defined on a a-algebra Y of subsets of a set S. m ) ) the set 1.7.4. E X A M P L E . L , spaces. We denote by L , (= L,(S, 9, of all measurable complex-valued functions f on S that are essentially bounded in the following sense: there is a positive real number c such that If(s)l Q c for almost all s in S. It is evident that L , is a complex vector space,cdntaining as a subspace the set N of all null functions (those that vanish almost everywhere) on S.We shall observe that, with a suitable norm (defined in (7) below), the quotient space L,/N is a Banach space. It is easily verified that, when f E L , , there is a smallest constant c such that 1fls)l < c almost everywhere on S [R: p. 641. It is denoted by ess sup{(f ( s ) l :SES},the essential supremum of I f [ ; and it is 0 only when f is a null function. There is an equivalence relation on L , , in which f g if and only iffls) = g(s) for almost all s in S;and the equivalence class [fl off is the coset f + N . The equation
-
-
(7) IlCflll = esssupIlAs)l:s~Sl ( f E L , ) defines a norm on L,/N. It is a straightforward result in measure theory that, with this norm, L,/N is complete, and is therefore a Banach space [R : p. 66, Theorem 3.1 I]. There is a common convention, which we shall usually follow, that the Banach space just defined is denoted by L , (rather than L , / N ) , and that its elements are described as functions (although, strictly speaking, they are equivalence classes of functions, modulo null functions). This convention is convenient, and should not lead to confusion, provided it is remembered that two essentially bounded functions must be regarded as the same member of L , if they are equal almost everywhere. It is not difficult to verify that a sequence {h}in L , converges, in the norm topology, if and only if there is a null set Z such that the functions { h ( s ) } converge uniformly on S\Z [R: p. 671. If m is a regular Bore1 measure on a compact Hausdorff space S andfE L , , there is a sequence {h}of continuous functions on S , such that f(s) = limf,(s) almost everywhere on S and If,(s)l < llf'll for all s in S and all n = I , 2 , . . . [R: p. 54, Corollary]. H 1.7.5.
EXAMPLE.
L, spaces. Suppose that 1 Q p <
and denote by L,
GO,
( = L,(S, 9, m ) ) the set of all measurable complex-valued functionsf' on S for
which
Ss
If(s)lp
dm(s) is finite. When f , g E L,, it follows from Minkowski's
1.7. SOME EXAMPLES OF BANACH SPACES
53
inequality for integrals [R: p. 62, Theorem 3.53 t h a t f + Y E L, and
From this, it is evident that L, is a complex vector space, which contains as a subspace the set N of all null functions. The quotient space L,/N has a norm defined by
where [f] denotes the cosetf’+ N. It is a result in measure theory [R: p. 66, Theorem 3.1 I] that, with this norm, L,JN is complete and is therefore a Banach space. Just as in the preceding example, we shall adopt a convenient (though not strictly accurate) convention, by referring to the Banach space L, (rather than L d N ) and describing its elements as functions (rather than equivalence classes of functions). We note three further results from measure theory [R: pp. 67, 68; Theorems 3.12, 3.13, 3.141 concerning properties of L, spaces. First, a sequence {f,)in L, that converges in the norm topology to an elementfhas a subsequence that converges almost everywhere to f. Second, the set of all simple functions in L, is an everywhere-dense subset of L,. Finally, if m is a regular Borel measure on a compact Hausdorff space S, the continuous functions form an everywhere-dense subset of L,. 1.7.6. R E M A R K . Several of the Banach spaces described above will be used frequently in later chapters. Our primary concern in this book is with a certain class of Banach spaces (namely, Hilbert spaces), and with certain algebras (C*-algebras) that can be represented as algebras of bounded linear operators acting on Hilbert spaces. Now l2 spaces and L2 spaces are the simplest examples of Hilbert spaces (see Examples 2.1.12 and 2.1.14). Moreover, Banach spaces of the types I, L , , C ( S ) can be provided with additional algebraic structure (multiplication and involution), and then become abelian C*-algebras. As one would expect, our use of L2 and L , spaces involves occasional appeal to results from measure theory. In fact, measure theory will sometimes be needed also in connection with C ( S ) ,where S i s a compact Hausdorff space. One of the main tools in the study of C*-algebras is the theory of positive linear functionals; and the Riesz representation theorem [R: p. 40, Theorem 2.143 asserts that there is a one-to-one correspondence between positive linear functionals p on C ( S ) and regular Borel measures m on S , determined by
54
I . LINEAR SPACES
the equation P(f) = [A S s)dm(s)
(f€C(S)).
(We shall always assume, as one of the defining properties of a regular Bore1 measure m on a topological space S, that m(K)< co for each compact subset K of S . In the present case, S itself is compact, and thus m ( S ) < 00.) H Our next objective, achieved in Theorem 1.7.8 below, is to show that L,,, can be identified with the Banach dual space of L , . For this purpose we shall require the following simple result concerning a-finite measures.
1.7.7. LEMMA.Suppose that c > 0 and g is a measurable complex-valued function on S. for every measurable set X ( E S ) of finite measure, g is integrable over X and
u,
then Ig(s)( < c for almost all s in S . Proof. We have to show that the set Y = {sES:Jg(s)l> c} is null. Let {z, ,z2,. . .} be a countable everywhere-dense subset of the unit circle
{ z € @ :IZI = l}, and note that n
y=
u Yjk, j.k = 1
where Yjk = {sES:Rezjg(s) 2 c + l/k}. Thus it suffices to show that each of the sets Y j k is null. If Yjk is not null, it has a measurable subset X such that 0 < m(X)< 00 (since m is a-finite). Then,
a contradiction; so each Yjk is a null set. H
1.7. SOME EXAMPLES OF BANACH SPACES
55
1.7.8. THEOREM. Suppose that m is a a-finite measure defined on a oalgebra Y of subsets of a set S. For each g in L , , the equalion
P,W
=
i
(fg L 1 )
f(s)g(s) dm(4
defines a bounded linearfunctional pe on the Banach space L1, and the mapping g -+ pg is an isometric isomorphismfrom L , onto the Banach dual space (L,)'. Proof. The usual norms on LI and L , will be denoted by 11 Ill and 11 Ilm respectively. When f~L1 and g E L,, the function f g is measurable, and If(s)g(s)l G l l g l l * l ~ ~ ) l for almost all s in S. ThusfgE L1 and
From this, it follows easily that p , , as defined in the theorem, is a bounded linear functional on L , ,with llpgll < llgllou.When X ( c S ) is a measurable set of finite measure, its characteristic function xX is an element of L 1 ,and
G
lP,II
IIXXlll
= IIPellm(X).
From Lemma 1.7.7, I&)[ G IIp,II for almost all s in S ; so Ilgll, ,< llp,ll, and therefore IlPglI = lI91lm. The mapping g pe is linear, and from the preceding paragraph it is an isometric isomorphism from L , onto a subspace of (Ll)'. It remains to prove that its range is the whole of (L,)'. Suppose that p E ( L l ) ' ;we shall show in due course that p = p e for some g in L , . Observe first that, if { Y,) is an increasing sequence of measurable sets whose union Y has finite measure, then --f
IIXY - XY"ll1 = m(Y\ Y")-b 0; and, since p is continuous, it follows that
P(XY)
=
lim
dXYJ
n-r co
We now choose (and, for the moment, fix) a measurable set X of finite measure, and we define a complex-valued function 1.1 on Y by (9) P( r) = P f X X n Y ) ( Y E 9). It is apparent that p is a finitely additive set function, and (10)
p(Y)=p(YnX),
p(Z)=O
( Y , Z E Y , m(Z)=O).
56
I . LINEAR SPACES
Moreover, if { Y,,} is an increasing sequence of measurable sets with union Y , it results from the preceding paragraph that P ( Y ) = P ( X X n Y ) = lim n-
P(XX,Y,)
=
lim P( Y n ) . n- ou
00
Thus p is a complex measure on 9. Since it vanishes on the null sets of m , it follows from the Radon-Nikodym theorem that there is an element h of L , such that h(s)dm(s)
(11)
( Y E9).
Now
G llpll I l X X n Y I I I = IIpllm(Xn Y ) G IIpllm(Y)
(YE9).
By Lemma 1.7.7, Ih(s)l < llpll for almost all s i n S ; moreover, jYh(s)dm(s)= 0 whenever Y is a measurable subset of S\X, and thus h(s) = 0 almost everywhere on S\X. When the values of h are suitably adjusted on a null set, (1 1) remains valid, and in addition (12)
hEh,
Ih(s)l < llpll
(SES),
h(t) = 0 ( f E S \ X ) .
Since m is a-finite, S is the disjoint union of a sequence {XI, ,'A ,. . .} of measurable sets of finite measure. For each X,, we can use the process described in the preceding paragraph to obtain a complex measure p,,and the corresponding Radon-Nikodym derivative h,, ; and X,,, h,, p, satisfy conditions analogous to (9), ( 1 l), and (12). Each s in S lies in exactly one of the sets X,, ; so the sequence {h,,(s)}has at most one non-zero term, and
The equation a
g(s) =
c hflW
n= 1
defines a bounded measurable function g on S (so g E Lou). We shall prove that pe = p. To this end, it suffices to show that p , ( f ) = p ( f ) whenever f is the characteristic function of a measurable set Y (cS ) of finite measure; for p and pe are continuous linear functionals, and linear combinations of such functionsfform an everywhere-dense subset of LI
1.7. SOME EXAMPLES OF BANACH SPACES
(the integrable simple functions). Now Y =
u
57
Y,, where ( n = 1,2 ,...).
Yn= Y n ( X , u X 2 u . . . u X n )
Thus P ( x ~= ) lim n-
P(xY,)
=
n
=
=
x1
i j =
XYnX,
(J:1
)
T.
C
lim n-
2
lim P n- Ix
D
P(XYnX,)
=
I
C PJ(Y) j =
]rhj(s)dm(s)=
I
I
JY
g(s)dm(s);
the last step follows from the dominated convergence theorem, since x.Hence p ( x Y ) = pg(xy),as required. W
m( Y ) <
A topological space is said to be separable if it has a countable everywheredense subset. It is not difficult to verify that a metric space is separable if and only if its topology has a countable base [K: p. 120, Theorem 113; and from this, it follows that a subset of a separable metric space is itself separable in the rela ti ve topology. In proving the two following results, we shall use the term rational complex number to describe a complex number whose real and imaginary parts are both rational.
1.7.9. PROPOSITION.IfA is a set and 1 < p < a,the Banach space lp(A)is separable i’and only if A is countable. Proof. If A is countable, we may enumerate it as a (possibly finite) sequence { a I a, z , . . .}. For each positive integer r, let Yr be the set of all functions y on A such that y(a,) is a rational complex number when 1 < n < r and y(a,) = 0 if n > r. Since each Yr is a countable subset of lp(A),so is the set Y = Y,. If ,YEI,(A) and E > 0, we can choose first a positive integer r and then rational complex numbers c1, . . . ,c, so that
u
r
C
lx(an)- cnIp < it”. , n= 1 If y(an)= cn ( 1 < n < r ) and y(an)= 0 (n r), then y e Yr ( G Y ) and 1I.y - y11 < c. Hence Y is an everywhere-dense countable subset of lp(A). Conversely, if IJA) is separable, the same is true of its subset X = {,xu: U E A},where xu denotes the function whose value is 1 at a and 0 elsewhere on A.Thus Xhas an everywhere-dense countable subset, which must
C Is(a,,)lP<
I,
~ E P ,
>
=-
58
1. LINEAR SPACES
be the whole of X , since Ilx, - xbll = 2l’, when a # 6 . In other words, X (and therefore, also, A) is countable. Suppose that 1 < p < co and m is a a-jinnite measure 1.7.10. PROPOSITION. m) defined on a a-algebra Y of subsets of a set S. Then the Banach space L,(S, 9, is separable i f and only i f there is a sequence { X , ,X 2 , . , .} of measurable sets of finite measure with the following property: given any measurable set X offinite measure and any positioe E , there is an integer j for which
rn((Xj\X) u (fix,))< E . Proof: Let .4”0 be the family of all measurable sets of finite measure. When
X , Y E Y ~the , characteristic functions x x , xy lie in L,, and Ilxx - xvllP = m ( ( x \ Y ) u ( Y \ X ) ) .
Accordingly, the existence of a sequence { X , ,X 2 , . . .} with the properties set out in the proposition is equivalent to the existence of a countable everywheredense subset of the set C = { x x : X € Y 0 } ( G L,). If L, is separable, so is C. Conversely, suppose that C has a countable everywhere-dense subset { g , , g 2 , . . .}. Let R be the countable subset of L, that consists of all finite linear combinations z l g , + . * . + zngn,in which the coeflicients z l , .. . ,z, are rational complex numbers. Given anyfin L, and any positive E , there is a simple functionf, in L, such that )If - f,(1 < $8. Sincef, is a finite linear combination (with complex coefficients) of elements of C, there is a finite linear combination f 2 (with rational complex coefficients) of elements of the everywhere-dense subset { g l , g 2 . . . .} of C such that [Ifl - j i l l < ic. Then f2E R and 1) f - f i l l < E ; so R is a countable everywhere-dense subset of L,. 1.7.1I . REMARK.It follows from Proposition
1
1.7.10 that, when
< p < GO, the L, space, associated with Lebesgue measure on a measurable
subset E of R”, is separable. To prove this, we consider the set of all “rational cells” {(x,, . . . ,x,): a j < x j
< b j ( j = 1,. . . , n ) }
in R“, where the aj’s and bj’s are specified rational numbers. We can list, as a sequence { Y , , Y 2 , .. .}, the set of all finite unions of rational cells, and take X j = E n Y j .The sequence { X j } then has the properties set out in Proposition 1.7.10. We shall show, in Exercise 2.8.7, that certain 0-finite measures give rise to inseparable L2 spaces. We shall see later that C ( S )is separable when S is a compact metric space (see Remark 3.4.15), while infinite-dimensional I,, and L,, spaces are not separable (Exercises I .9.31, 1.9.32).
1.8. LINEAR OPERATORS ACTING ON BANACH SPACES
59
1.8. Linear operators acting on Banach spaces In this section we prove four basic results concerning linear operators acting on Banach spaces, namely, the open mapping theorem, the Banach inversion theorem, the closed graph theorem, and the principle of uniform boundedness. The first three are so closely related as to be more or less equivalent, and the fourth is easily deduced from the third. We recall that a mapping cp from a topological space X into another such space Y is said to be open if q(G)is open in Y whenever G is an open subset of X. 1.8.1. PROPOSITION. Suppose that X, CY are normedspaces and T :X -+ 9 is a linear operator. Then Tis open ifandonly ifthe image { T x :x E (X),} of the unit ball (X), contains the ball (9),, for some r ( > 0). If T is open, T(X) = ”Y. Proof: If T is open, it carries the open unit ball { x E X : llxll < l } onto an open subset of Y that contains 0 and so contains (Y), for some r ( > 0). Thus (Y),
G
{ T x :X E X , llxll < 1)
G
T((X)I).
Moreover, the subspace T(X) of 3 contains (”Y),, and is therefore the whole of “Y. Conversely, suppose that r > 0 and (CY), G T((X)l).We have to show that T(G)is open in Y when G is an open subset of X. If xoE G, then G contains a ball xo + c(X), (where c > 0), and
T ( G ) 2 TxO
+ c T ( ( 3 ) l )2 TXO+
C(9’)r.
Hence each point Txo of T(G) is an interior point, and T ( G ) is open, as required. W 1.8.2. PROPOSITION. If T is a bounded linear operatorfrom a Banach space
X inlo a normed space CY, and the closure C - of the set C = { T x :X E ( X ) ~ } contains the ball (“Y),, for some r ( > 0), then T is open. Proof. From Proposition 1.8.1, it suffices to show that Ccontains the ball (Y)r,2.To thisend, suppose t h a t y e g a n d llyll < ir. Since2yE(”Y), G C - , we can choose y1 in C so that IVY - Y l l l < 9.
Since 22y - 2y, ~ ( G 9C)- ,~ we can choose y 2 in C so that
1P2Y - 9,- Y211 < ir. Since z 3 y - 2 2 y , - 2y2e(g), E C - , we can choose y 3 in C so that 1 1 2-~ 22y, ~ - 2y2 - Y,I
< tr.
60
I . LINEAR SPACES
By continuing in this way, we obtain a sequence {yl , y 2 , .. .} in C such that
11Yy - 2"-'y1
-T
( n = I , 2 , . . .),
2 y , - . . . - y,l( < +r
Thus n
1Iy-
C 2-jyjll<2-"-'r
( n = 1 , 2 , ...),
j= I
and y = 2-Jyj. Now y j € C, so y j = Tx, for some x j in (X), . Since X is a Banach space and
c 2-j UJ
(r,
l12-jxj[l<
= 1,
j= 1
j= 1
the series C;= 2 - J x jconverges to an element x of ( X ) l . Moreover
c 2-jTxj c 2-'y, ou
aJ
TX=
=
j= 1
= y,
j= 1
and since x ~ ( f i )it ~follows , that Y E C, as required. H When X is reflexive, Proposition 1.8.2 reduces immediately to Proposition 1.8.1, since in this case the set C is closed (see Exercise 1.9.17). 1.8.3. LEMMA. If T is a linear operator from a normed space X onto a Banach space Y, there exist positive real numbers r, s, and an element yo of "Y, such that the closure in "Y of the set { T x :X E (X),} contains the ball y o ("Y),.
+
Proof. For each positive integer n, let
c, = { T x : x E ( X ) , } . When y ~ 9 Yand r > 0, let B(y, r ) denote the closed ball y + ("Y),. It suffices to show that, for some n, the closure C,- of C, contains such a ball. We assume the contrary, and in due course obtain a contradiction. (There is a short cut available here for the reader who is familiar with the following result, known as the Baire category theorem: a complete metric space X cannot be expressed as the union of a sequence { X , ) of subsets, each of which is nowhere dense in X . Indeed,=;J( C,, is the complete metric space Y since T has range "Y, and the category theorem implies the required conclusion that at least one of the sets C, is not nowhere dense in Y (that is, for at least one value of n, the closure C,- has non-empty interior [ K :p. 2011).The argument that follows is in fact a proof of the category theorem, within the particular context now under consideration, but in a form easily adapted so as to apply to the general case.) From our assumption, C ; does not contain the unit ball ("Y), ; so we can choose y , (EY) and rl ( > 0) so that Yl€("Y)l\c;,
B ( y 1 , r l ) n C l = 0.
1.8. LINEAR OPERATORS ACTING ON BANACH SPACES
61
Since C; does not contain the ball B ( y l ,+rl),we can choose y , ( ~ g and ) r2 so that
( > 0)
yzEB(yl.:rl)\C;, B ( y 2 , r 2 ) n C 2= 0, r2 < $rl. Since C j does not contain the ball B ( y 2 ,$r2),we can choose y 3 (E@?) and r3 ( > 0) so that . v ~ E B ( ~ z , $ ~ ~ \ CB;( ,y 3 , r 3 ) n C 3= 0, r3 < $r2. By continuing in this way, we obtain sequences {y,,}in OY and {r,} in R such that, for n = 2 , 3 , . . ., ynEB(yn-l,:r,-l)\Cn-, These conditions imply that
BO?,,r,)nC,
=0, O <
B(yn.m)c B ( y n - l , r n - l ) ( n 3 2),
lim r, fl-
r, <+r,-,.
= 0,
' I
and from this, ll.Yrn-ynll < r ,
(1 < n < m ) . It follows that { y n } is a Cauchy sequence in the Banach space 'J,and so converges to an element y of 9. The closed ball BQ,, r,) contains the sequence ( y, , y n + l r y n +.... 2 J,SoyEBCY,,r,)for a l l n = 1,2,... . Since T(X) = 3, we have y = Tx for some x in X. I f n > IIxII, then YrnEB(Yrn,rrn)~BQfl,rfl),
vEBCy,,rfl)nCn. contradicting the earlier assertion that B b , , r,) n C, = $3. W 1 . 8 . 4 . THEOREM (Open mapping theorem). A bounded linear operator T from a Banach space X onto a Banach space 9 is open.
Proof. By Lemma 1.8.3 we can choose positive real numbers r, s, and an element y o of 9 so that the closure C - of the set C = { T x :X E (X),} contains the ball y o + ($),. Upon replacing y o by s- ' y o and r by s- lr, we may assume that s = 1. Suppose that ~ E ( L ' Y ) , . Since C - is balanced and convex, and yo f y
E
yo
+ (qrE c-,
it follows that
Y
Hence
G
= +[(Yo
+ Y>- ( Y o -
E
c-.
C - , and T is open by Proposition 1.8.2. W
1 . 8 . 5 . THEOREM (Banach inversion theorem). If T is a one-to-one hounded linear operator from a Banach spuce X onto a Banach space Y, the inverse T - I : 9 -+X is a bounded linear operator.
62
1. LINEAR SPACES
Proof. It is apparent that T - ' is a linear operator. We have to show that it is continuous; that is, we must prove that the inverse image, under T - I , of any open subset G of X, is open in Y. Since this inverse image is T(G),the result follows at once from Theorem 1.8.4. H
Suppose that X and Y are normed spaces with the same scalar field K. The product set X x Y is a normed space when the algebraic structure and norm are defined as follows: (x1 l Y l )
+ (XZ9YZ) = (x1 + XZ7YI + YZ)?
a(x9y) = (ax?aY),
Il(X~Y)II=
llxll + Ilvll.
When X and Y are both Banach spaces, so is X x 9. If T :X Y is a linear operator, the graph of T is the subspace Y ( T ) of X x 9 defined by %(T)= {(x, T x ) : x E X } . As a linear subspace of a normed space, %(T) is itself a normed space. Note that Y(T) is closed (in X x Y) if and only if the following condition is satisfied: if a sequence {x,} in X converges to an element x of X, while { Tx,} converges to an element y of Y, then Tx = y. It is clear that a bounded linear
operator has a closed graph; for operators acting on Banach spaces, there is a converse. 1.8.6. THEOREM(Closed graph theorem). If'X and 9 are Banach spaces, and T is a linear operator from X into g,then the graph of T is closed if and only if T is bounded. Proof. In view of the preceding discussion, it suffices to show that T is bounded when its graph % (T )is closed. In this case, Y( T ) is complete, and is therefore a Banach space, since it is a closed subset of the complete metric space X x ?Y. The equation H(x, Tx) = x (X E X) defines a one-to-one linear operator H from 9(T ) onto X; and H is bounded with IlHll 6 1, since
IIWx, Tx)ll = llxll 6 llxll + lIT4l = IKX, Tx)ll. By the Banach inversion theorem, H - is bounded; and the same is true of T since IITxll 6 Il(x,Tx)ll = IIH-'xll 6 l l ~ - ' l l l l ~ l l ( X E X ) . If E is an idempotent bounded linear operator acting on a normed space X, the corresponding complementary subspaces
Y={X€
x : Ex = x},
z = { X E X : Ex = 0)
1.8. LINEAR OPERATORS ACTING ON BANACH SPACES
63
of X are closed, being the null spaces of the continuous linear operators I - E and E, respectively. For Banach spaces, there is a converse. 1.8.7. THEOREM. If Y and 2 are closed complementary subspaces of a Banach space X, then the projection E from X onto Y parallel to Z is bounded,
Proof. The graph of E can be expressed in the form {(X,Y)€X x X:y€ Y , x
-y€z}
and is therefore closed in X x X; so the result follows from the closed graph theorem. W 1.8.8. COROLLARY. If Y and Z are closed subspaces of a Banach space X and Y n Z = { 0 } ,then Y Z is closed in X ifand only ifthere is a positive real number C such that
+
llvll G CllY + zll
(vE y ,
z E Z).
Proof. Suppose that there is such a constant C. If a sequence {x,} in Y + Z converges to an element x of X, let x, = y , + z,, where y , E Y and z, E Z . Since IlYm - Ynll G Cll(Ym - Yn)
+ ( z m - z,)ll
= C l b m - xnll,
{y,} is a Cauchy sequence in the closed (and hence complete) subspace Y of X, and so converges to an element y of Y. Since 2 is closed, and x
-y
= lim(xn - y,) = lim z,,
it follows that x-y
E
z,
x=y+(x-y)
E
Y+Z.
Hence Y + Z is closed in X. Conversely, if Y + Z is closed in X,it is a Banach space Xocontaining Y and Z as complementary subspaces. From Theorem 1.8.7, the projection E from Xo onto Y parallel to 2 is bounded; and the stated condition is satisfied, with c = IIEJJ.W If y and z are unit vectors in a Banach space X, it is reasonable to consider the “angle” between y and z to be large, or small, according as Jly- zll is large or small. Accordingly, when Y and Z are closed subspaces of X, the lower bound
inf{lly - z l l : y ~Y , Z E Z ,llyll = llzll = 11 can be regarded as indicating the (minimum) angle between Y and Z . It is not difficult to show that this lower bound is strictly positive if and only if there is a constant C with the property set out in Corollary 1.8.8 (see Exercise 1.9.5).
64
I . LINEAR SPACES
Thus the corollary can be interpreted in the following geometrical form: if Y n Z = {O}, then Y + Z is closed if and only if the angle between Y and Z is strictly positive. We conclude this section with various forms of the principle of uniform boundedness. 1.8.9. THEOREM.Suppose that { T,: aE A} is a family of bounded linear operators from a Banach space X into a normed space 9 and sup{llT,xll:aEA} < cc for each x in X. Then sup{llT.II:a~A)< m. Proof. Since T, can be viewed as a bounded linear operator from X into the completion @ of 9, we may suppose that UY is a Banach space. For each x in X, the equation (Sx)(a)= Tax
( a € A)
defines a mapping Sx: A -+ ??/,and S x is an element of the Banach space I z ( A , 9 )(Example 1.7.1). It is evident that the mapping x-+ S x is a linear operator S from X into Im(A,CiY). We assert that the graph of S is closed. For this, suppose that a sequence {x,} in X converges to x (EX), while {Sx,} converges to an element f of l % ( AY); , we have to show that S x =f. For each a in A,
Ilm - (Sxfl)(a)lld I l f - Sxnll
-+
0.
From this, and since T, is continuous, f ( a ) = lim(Sx,)(a)
so f
= lim
Tax, = Tax = ( S x ) ( a ) ;
= Sx, and the graph of S is closed. From the closed graph theorem, S is bounded. For each x in X and a in A,
IITaxll = II(Sx)(a)ll d llsxll Q SO
IlTall
llsll
llsll llxll;
A). R
1.8.10. THEOREM.Suppose that { p a :a € A} is a family of bounded linear functionals on a Banach space X and sup{lp,(x)( : a E A) < co for each x in X. Then sup{llp,ll:aEA} < m.
Proof. This follows from Theorem 1.8.9, with +Y the scalar field. R 1.8.1 1. COROLLARY. Suppose that {x,: a E A} is a family of elements of a normedspace X andsup{ lp(x,,)I :a E A} < mfor each bounded linearjunctionalp on X. Then sup{(lx,ll:aEA}< 00.
65
1.9. EXERCISES
Proof. For each p in the Banach dual space X’,
sup{li,(p)l: a € A} = sup{Ip(x,)l: a € A} < cc (where x 4 iis the natural isometric isomorphism from X into X”). From Theorem 1.8.10 (with X‘ in place of X and i, in place of pa), and since ~ ~= iIIx,,II, J we have sup{llx,ll: a E A} < ca. 1.8.12. THEOREM.Suppose that {To:a € A} is a family of bounded linear operators from a Banach space X into a normed space t!Y and sup{(p(T,x)l: a € A } < m for each x in X and p in CV‘. Then sup{11 Tall: aE A} < cc. Proof. By Corollary 1 . 8 . 1 1 , s u p { ~ J T , x J ~ : a<~cc A }foreachxinX;sothe result follows from Theorem 1.8.9. H 1.8.13. REMARK.In the context of complex Banach spaces, the open mapping, Banach inversion, and closed graph theorems, and the various forms of the principle of uniform boundedness, apply also to conjugate-linear operators; for such operators can be viewed as linear mappings between the corresponding real Banach spaces. H
1.9. Exercises
1.9.1. Show that, if y is a sublinear functional on a (real or complex) vector space Y< and V is the convex set { X E V :4(x) < l}, then the support functional p of V is given by p(x) = max{y(x),O)
(XE
Y).
1.9.2. Suppose that 4 , and 42 are semi-norms on a (real or complex) vector space ‘%,’ p is a linear functional on U; and Ip(x)l 6 4i(.x) + qz(x)
(Xe
Y).
Show that p can be expressed in the form p1 + p 2 , where p l , p 2 are linear functionals on f ,’and lp,(x)l 6 yj(x)
(.YE
?7
j = I , 2).
[Hint. Define a semi-norm p on the vector space P” x Y and a linear functional go on the “diagonal” subspace {(x,x): .YE U - } of V” x Y by P((x9-v))= 4i(x) -k 42(Y).
go((x,x)) = P(X1.1
66
I . LINEAR SPACES
1.9.3. Suppose that Y is a (real or complex) vector space on which two locally convex topologies, r1 and t2, are specified. Let fj be the set of all sj-continuous semi-norms on for j = 1,2; and let
<
l- = I41 + 42:41 E
r 1 9
42 E f21.
Prove that r is a separating family of semi-norms on K and that the corresponding locally convex topology r is the coarsest locally convex topology on Y that is finer than both T , and r 2 . Show also that a linear functional p on f is r-continuous if and only if it has the form p1 + p 2 , where p j is a Tj-continuous linear functional on Y for j = I , 2. 1.9.4. Show that, if {xl,. . . ,x,} are elements of a normed space X such that 0 is in the closure of
alxl
+ . . . + a,x,:aj scalars,
n
n ( a j - 1) = 0 j= 1
then {xl,. . . ,x,} are linearly dependent. 1.9.5. Suppose that Y and Z are closed subspaces of a Banach space X such that Yn 2 = {O}, and let
k = inf{lly - z l l : y ~Y, ZEZ,llyll
=
llzll = I } .
Show that the subspace Y + 2 of X is closed if and only if k > 0. [See Corollary 1.8.8 and the discussion following it.]
1.9.6. Show that, if X is a real Banach space, then X x X becomes a complex Banach space X, when its linear structure and norm are defined by (x, y ) + (u, 0) = (x (a
+ 4y + u),
+ ib)(x,y ) = (ax - by, bx + ay), Il(x,y)ll = sup{ll(cos0)x + (sin 0)yll:0 < U d 27c}
for all x , y , u, u in X and a, b in R. Prove also that the set {(x,O): X E X} is a closed real-linear subspace X, of X,, that X, = { h ik: h , k E X R } , and that the mapping x -+ (x,O) is an isometric isomorphism from X onto (the real Banach space) XR.
+
1.9.7. Suppose that X is an infinite-dimensional normed space, and V i s a neighborhood of 0 in the weak topology on X. Show that V contains a closed subspace of finite codimension in X. Deduce that the weak topology on X is strictly coarser than the norm topology.
67
I .9. EXERCISES
1.9.8. Show that, if X is a separable Banach space, each bounded sequence X' has a subsequence that is weak* convergent to an element of X'.
{ p , } in
1.9.9. Prove that, if X and f9are normed spaces, X # {0}, and W(X, tY) is complete, then 9 is complete. 1.9.10. Suppose that Xo is a closed subspace of a Banach space X, Q :X + X/Xois the quotient mapping, and X t (c3') is the closed subspace consisting of all bounded linear functionals on X that vanish on Xo. (i) Show that the (Banach) adjoint operator Q* is an isometric linear mapping from (X/Xo)' onto Xt. (ii) Show that the mapping T : p + X,l+ p I X o is an isometric isomorphism from X*/X,I onto Xi.
1.9.11. Show that, if Xo is a closed subspace of a Banach space X,then X is reflexive if and only if both Xo and X/Xoare reflexive. 1.9.12. Show that a Banach space X is reflexive if and only if its dual space 3' is reflexive. 1.9.13. Suppose that X is a Banach space with the following property : given any positive real number E , there is a positive real number 8 ( ~such ) that IIx - y(I < E whenever x and y lie in the unit ball (X),and 1 1 % ~ + y)ll 1 -6(~) (such a Banach space is said to be uniformly convex). Prove that X is reflexive. [Hint.Suppose that sZo E X" and llsZoll = 1. Choose po in X' so that ( ( p o (=( 1 and (sZo(po)- 11 < B(E); and let
=-
x
= {.?:xE(X)1, Ipo(x) -
11 < 8 ( E ) } ,
where I + ,i is the natural isometric isomorphism from X into 3". Prove that sZo lies in the weak* closure of X, and that 112 - jll < E whenever 2 , j e . X Deduce that JJQ0- i l l d E for each .? in X.] 1.9.14. Suppose that X is a normed space, .Iis a linear subspace of the dual space X', and .,M separates the points of X. Let p be a linear functional on X.
(i) Suppose that the restriction p((X), of p to the unit ball (X)] is continuous in the weak topology a(X, A!).Show that p e 3'. Prove also that, if E > 0, there is a finite subset {q,. . . , m n } of . X such that n
lp(x)J < c
whenever ~ E ( X ) and ~
loj(x)l j= 1
< 1.
68
1. LINEAR SPACES
Deduce that n
IP(x)I 6
EIIxII + IbII C
IWj(x)I
(~€3).
j= 1
From this inequality, together with the result of Exercise 1.9.2, deduce that p has the form p1 + p 2 , where p1 EX’, llplll 6 E , and p2 EM. (ii) Prove that pI(X), is o(X,d)-continuous if and only if p lies in the norm closure .I= of A in X’. 1.9.15. Suppose that X is a Banach space, x + .t is the natural isometric isomorphism from X into X“, and W E X“. Show that, if the restrictiod wl(.X’), of w to the unit ball of Xgis continuous in the weak* topology o(X‘, X), then Q = 1 for some x in X. [Hint. Use the result of Exercise 1.9.14(ii).] 1.9.16. Suppose that X and 3 are Banach spaces, and S E ~ ( O ~ ’ , X ’ ) . (i) Prove that, if S is continuous relative to the weak* topologies on gg and X’, then S = T’ for some T i n g ( X , 3 ) . (ii) By using the result of Exercise 1.9.15, show that (i) remains valid when the weak* continuity of S is replaced by weak* continuity of S[(@)l.
1.9.17. Suppose that X, Oy are Banach spaces and T E ~ ( X 9). , Prove that (i) the set { T ’ p : p ~ ( O y ‘ is ) ~weak* } compact, and hence norm closed in 3’; (ii) if X is reflexive, the set { T X : X E ( X is ) ~weakly } compact, and hence norm closed in 9. 1.9.18. Suppose that X and Oy are Banach spaces, TEYB(.X,1v), and the image T ( 9 )( = { Tx : X E X})is a closed subspace of ‘3.Prove that T‘(9’) is the closed subspace of X’ consisting of all bounded linear functionals on X. that vanish on the null space of T. [Hint.If p E X‘ and p vanishes on the null space of T, the equation wO(Tx) = p(x) defines (unambiguously) a linear functional w0 on T(X).By applying the open mapping theorem to T, as an operator from 9 onto T(X),prove that w0 is bounded. Deduce that p = T’u, for some w in ?Yo.] 1.9.19. Let I, denote the Banach space I,( N, C) of Example 1.7.I , where N is the set of positive integers; an element of I, is a bounded complex sequence{xl,x2,...), and Il{x,,}II= sup{lx,,l:n~N). Let candc,, bethelinear subspaces of I, defined by
i
I
c = {x,,}€1, : lim x, exists , n-r
69
1.9. EXERCISES
Prove that (i) (ii) I,, and (iii)
c and co are closed subspaces of I , ; the sequence { 1, I , I , . . .} is an extreme point in theclosed unit ball of also in the closed unit ball of c; the closed unit ball of co has no extreme point.
Deduce that co is isometrically isomorphic neither to c, nor to any Banach dual space. 1.9.20. With the notation of Exercise 1.9.19, let U be the element { 1, 1 , 1 , . . .) of c; and, for k = 1,2,. . . , let Ek(in co) be the sequence that has 1
in the kth position and zeros elsewhere.
(i) Provethat,ifX = {x,} E C ~thenX , =Z =; I SkEk, theseriesconverging -,0 as m --* cc). in the norm topology of co (that is, JIX- I:=, x,, then X - X U E Cand ~ (ii) Prove that, if X = {x,} E C and x = X = .xu + C;= (xk - x)&, the series converging in the norm topology of c. 1.9.21. Adopt the notation of Exercises 1.9.I9 and I .9.20; in addition, let
Il denote the Banach space ll(N, C ) of Example 1.7.3, so that an element of f l is a complex sequence Y = {yl, y 2 , .. .) such that (11 Y ( (=) C."= (y,l < m . (i) Show that, if Y = { y l ,y,, . . .} € I I , the equation x
c
P(X) =
( X = { x n ) E co)
Yn-Y,
n= 1
defines a bounded linear functional p on co, and llpll = IIYII. Prove also that Y n = P(E,).
(ii) Show that each bounded linear functional on co arises, as in (i), from an element Y = {yl, y 2 , .. .) of Il . (iii) Deduce that the Banach dual space c$ is isometrically isomorphic to I , . 1.9.22. Let c and lI be the Banach spaces defined in Exercises 1.9.19 and 1.9.21. (i)
Show that, if { y o , y , , y 2.,. . } is a complex sequence such that
C,"=ol.vnl < x,the equation a
p ( X ) = y o lim x, fl+
7c
+ C ynxn
( X = {x,}
EC)
n= 1
defines a bounded linear functional p on c, and llpll = Iy,,l. (ii) Prove that every bounded linear functional on c arises, as in (i), from such a sequence {yo,yl, y 2 , .. .}. [Hint. Use the results of Exercises 1.9.2O(ii) and 1.9.21(ii).]
70
I . LINEAR SPACES
(iii) Deduce that the Banach dual space c' is isometrically isomorphic to
11.
(iv) Deduce that ch and c' are isometrically isomorphic, while c,, and c are not. 1.9.23. Let I, and l1 be the Banach spaces defined in Exercises 1.9.19 and 1.9.21, and, for each positive integer k,let e, (in 11)be the sequence that has 1 in the kth position and zeros elsewhere. Without using Theorem 1.7.8: (i) Prove that, if Y = { y l , y z , .. .} € I I , then Y = C ; = l y k e k ,the series converging in the norm topology on II . (ii) Show that, if X = {xl, x z , . . .} el,, the equation
c OU
P(Y)
=
( Y = Lhl}€11)
XnYn
n= 1
defines a bounded linear functional p on 11,and llpll = IlXll. (iii) Prove that each bounded linear functional on II arises, as in (ii), from an element X of I,. (iv) Deduce that the Banach dual space 1; is isometrically isomorphic to I,. 1.9.24. By using the results of Exercises 1.9.21, 1.9.22, and 1.9.23, show that neither of the Banach spaces co, c is reflexive. Deduce that neither of the Banach spaces II , I, is reflexive. 1.9.25. Give a second proof that none of the Banach spaces co, c, I, is reflexive, by using the results of Exercises 1.9.19 and 1.9.11. : = 1,2,. . .} is a double 1.9.26. (i) Suppose that E > 0 and { x ~ , ~m,n sequence of complex numbers that satisfies the conditions CC
4E <
1 Ixm,J<
00
(rn = 1,2,. . .),
n=l
lim x ~= ,0 ~ (n = 1,2,. . .). m - ou
Show that there exist integers 0 = n(0) < n(1) < 4 2 ) < . . . and 1 = rn(1)
2
Ixm(j),nI
< 6,
IXm(j1,nI
>
IXm,j),nI
< E.
n= 1
n(j)
1
3
n = 1 + n ( j - 1) a3
1 n=l+n(j)
~
9
71
1.9. EXERCISES
Prove also that there is a sequence {y,} ofcomplex numbers of modulus 1 such that
I J, I m
ynxm(j),n
>E
( j = 1-29.. .).
(ii) Prove that, if a sequence {X,,,} of elements of the Banach space Il is weakly convergent to 0, then it converges to 0 in the norm topology. 1.9.27. Suppose that 1 < p < 03, q = p / ( p - I), and I, is the Banach space I,( N, C ) of Example 1.7.3, so that an element of I, is a complex sequence X = {xl , x 2 , .. .} such that I,"=lxnlP ( = /lXllp)< 00. (i) Show that, for each Y = { y l , y 2 , .. .} in f,, the equation z
1 ynxn
P ~ X=)
( X = {xn}
n= 1
defines a bounded linear functional py on I,, and llpyll < 11 Yll, (the norm of Y in I,). By considering the sequence X , = { r , , ~ y , , ~ ~where ~ P } , t l ,t 2 , . . . are suitable complex numbers of modulus 1, show that llpyll = IIYII,. (ii) Prove that the mapping Y -,py is an isometric isomorphism from I, onto the Banach dual space I;, and deduce that I, is reflexive (that is, that the natural isomorphism of I,, into I F is onto). 1.9.28. Suppose that p > 1, q = p / ( p - l), and Y = {yl , y 2 , . . .} is a complex sequence with the following property : whenever X = {xl, x2, . . .} el,, the sequence {y,,x.} is an element of I l . Prove that YE^,. 1.9.29. Suppose that 1 < p < 03, q = p / ( p - l), and L,, is the Banach space associated, as in Example 1.7.5, with a a-finite measure space ( S ,9, m). (i) Prove that, for eachfin L,, the equation pr(g) =
J
f(s)g(s) ~
s
)(9 E L,)
S
defines a bounded linear functional p, on L,, and llp,ll < Ilfll, (the norm offin L,). By considering a suitable function go, of the form go(s) = If(~)l~'~t(s), where t(s)f(s) = If(s)l, show that IIPJII = Ilfll,. (ii) Suppose that m ( S ) < 00, and let p be a bounded linear functional on L,. Show that, if X ( E S ) is measurable, the characteristic function x x lies in L,. Prove that there is an elementfof L1 such that p(xx) =
J
f(s)xx(s)~s) S
for every measurable subset X of S. Show thatfe L,, and p = p,.
72
I . LINEAR SPACES
(iii) In the general case (with m a-finite, but not necessarily finite) prove that the mappingf4 p , is an isometric isomorphism from L, onto the Banach dual space LL, and deduce that L, is reflexive. 1.9.30. Suppose that 1 < p < 00, q = p / ( p - I), and L,, L, are the corresponding Banach spaces associated with a o-finite measure space ( S ,,Y,m).Show that, iffis a complex-valued function on S , andfy E L 1 for each g in L,, thenfcL,. [Hint.Show that f is the limit, almost everywhere, of a sequence {f,}of functions in L, such that If,(s)I < If(s)l, and consider the corresponding sequence {p,} of bounded linear functionals on L, .] 1.9.31. Show that the Banach space f,(A) is separable if and only if the set
A is finite. Deduce that a separable Banach space may have a non-separable dual. 1.9.32. Show that the Banach space L , , associated with a o-finite measure space (S,9, m),is separable if and only if S can be expressed as the disjoint union of a finite family of "atoms." (An atom is a measurable subset So of S such that m(So)> 0 and each measurable subset of Sohas measure 0 or m(S01.) 1.9.33. Consider the Banach spaces L 1 and L , associated with a o-finite m),and the isometric isomorphism (or Theorem 1.7.8) measure space (S,9, from L , onto L : .
(i) Suppose that g , g l , g 2,...EL,, and let p , p l , p 2 ,... be the corresponding bounded linear functionals on L 1 . Show that, if SUPIllSnllm :n€NJ < ~0 and y(s) = limn+K, g,(s) for almost all s, then the sequence { p , } is weak* convergent to p. (ii) Show that, if m is Lebesgue measure on the interval [0,1], and yl, y 2 , g 3 , .. . (in L,) are defined by g,(s)=(-
I)'
( 2 - " r < s < 2 - " ( r + I),
r = 0 , 1 , ..., 2 " - I),
then the corresponding sequence { p , } of bounded linear functionals on L 1 is weak* convergent to 0. 1.9.34. Suppose that En"= x, is a series of elements of a complex Banach space X such that, for every strictly increasing sequence {n(l), n(2),. . .} of positive integers, the subseries x,,(~) converges in the weak topology to an element of X.
73
1.9. EXERCISES
,
(i) Prove that I,"=Ip(x,)l < 00 for each p in X'. (ii) Show that the equation S p = {p(xl),p(?cz),. . .} defines a bounded linear operator S from X' into the Banach space I,. (iii) Prove that, if A = { a , ,a 2 , .. .) E I,, the equation I
Q,(P)=
1 p(anxn)
(~€3')
n= 1
defines a bounded linear functional SZ, on X', and llSZAll < IJSJI llAlla. (iv) Show that SZA lies in .% (the natural image of X in 3'') whenever A (in I = ) is a sequence that takes only finitely many distinct values. Deduce that a, E .% for all A in I, . (v) Prove that, for every A = { a , , a 2 ,. . .} in I , , the series I,"=a,,x,, is weakly convergent to an element of X. (vi) Show that, if a sequence {p,,} in X'is weak* convergent to an element p of X', then {Sp,) is norm convergent to Sp. [Hint. Use (v) and the result of Exercise 1.9.26.1 , that (vii) Prove that, if E > 0, there is a positive integer n ( ~ )such It= lp(.~,,)[< cllpll for each p in X'. [Hint. Upon replacing 3E by the closed linear span of {x,,}, reduce to the case in which X is separable. If the result were false, we could choose p , , p 2 , . . . in the unit ball of X', satisfying I,"=Ipk(x,)[ 2 E . Obtain a contradiction by using Exercise 1.9.8 and (vi).] (viii) Prove that, for each bounded complex sequence { a ,, a 2 , .. .}, the ,'= a,x, converges in the norm topology on X. [The assertion, that series I weak convergence of every subseries entails norm convergence, is known as the Banach-Orlicz theorem.]
,
1.9.35. (i) Prove that, if the dual space X' of a Banach space X is separable, then X is separable. [Hint. Let { p , } be a countable dense subset of the surface EX': llpll = 1 ) of the unit ball (X')),; for each n = 1,2,. . . , choose x,, in (X), such that ~ p , , ( x , )>~ i.Show that X is the closed linear span of
{-%}.I (ii) Show that a reflexive Banach space is separable if and only if its dual space is separable. (iii) Give an example of a Banach space that is not reflexive but has a separable dual space (and is, therefore, separable).
1.9.36. Show that a bounded sequence {x,,} of elements of a reflexive Banach space X has a subsequence that is weakly convergent to an element of .X. [Hinr. Show that it is sufficient to consider the case in which X is separable, by replacing X by the closed linear span of {x,). In the separable case show, by use of Exercise 1.9.35(ii),that the required result can be deduced from Exercise 1.9.8.1
74
I . LINEAR SPACES
1.9.37. Suppose X is a separable normed space and {x,,:n E N} is a (norm-)dense subset of (X), . Define m
d(P,P’) =
c 2-“KP
- P’)(Xn)l
(P9P’EX’).
n= 1
(i) Show that d is a metric on X‘. (ii) Show that the metric topology induced by d o n (X’)),is the weak* topology on (X’)), . (iii) Use the fact that each sequence in a compact metric space has a convergent subsequence to solve Exercise 1.9.8 (and 1.9.36) again.
1.9.38. Use the Baire category theorem (see the proof of Lemma 1.8.3) to give another proof (direct) of the uniform boundedness principle (Theorem 1.8.9). 1.9.39. If { T,,} is a sequence of bounded linear transformations of one Banach space X into another Banach space Y and { T,,x}converges for each x in X, show that T, defined by Tx = limn T,,x,is a bounded linear transformation of X into Y. 1.9.40. Suppose X and $Y are Banach spaces and T is a linear transformation of 3 into Y. With q in the algebraic dual of Y, let ( T ‘ q ) ( x )be q( Tx) for each x in X; and suppose that T’pE X’ for each p in OY‘. Show that Tis bounded and that T‘lY’ = T’. [Hint. Consider the graph of T.]
CHAPTER 2 BASICS OF HILBERT SPACE AND LINEAR OPERATORS
This chapter deals with the elementary geometry of Hilbert spaces and with the simplest properties of Hilbert space operators. Section 2.1 is concerned with inner products, and the corresponding norms, on linear spaces with complex (or, occasionally, real) scalars. It introduces the concept of Hilbert space and provides a number of examples. Section 2.2 is devoted to the notion of orthogonality in a Hilbert space. In it we deal with orthogonal complements of closed subspaces, orthogonal sets, orthonormal bases, dimension, and the classification of Hilbert spaces up to isomorphism. This is followed, in Section 2.3, by Riesz’s representation theorem concerning the form of bounded linear functionals on a Hilbert space, and some corollaries concerning the weak topology of such a space. Section 2.4 is devoted to bounded linear operators acting on Hilbert spaces, with primary emphasis on elementary properties of the “Hilbert adjoint” of such an operator. Special classes of operators (normal, self-adjoint, positive, unitary) are considered briefly, and illustrative examples are given. Section 2.5 is concerned with orthogonal projections, corresponding to the decomposition of a Hilbert space as the direct sum of a closed subspace and its orthogonal complement. It includes an account of the order structure of projections, and its relation to the strong-operator topology. In Section 2.6 we deal with elementary constructions with Hilbert spaces, such as direct sums and tensor products, together with related aspects of operator theory. Section 2.7 is concerned with unbounded linear operators on Hilbert spaces. 2.1. Inner products on linear spaces
By an inner product on a complex vector space a?, we mean a mapping ( x , y ) + ( s , ~ )from , x‘ x X into the scalar field @, such that
+
z> = a ( x , z > + &Y, z > , (i) ( a x by,-~ (ii) ( Y . X > = (x,Y>, (iii) (x,x> 2 0, whenever x, y , z E a? and a, b E @. If, in addition, (iv) (x,x> = 0 only when x = 0, 15
76
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
the inner product is said to be definite (sometimes, positioe definite is used). In (ii) we adopt the convention that 2 denotes the complex conjugate of an element c of @. From (i) and (ii), an inner product satisfies the further condition (conjugate linearity in its second variable)
+
+
(v) ( z , a x by) = i i ( z , x ) 6(z,y). When ( , ) is an inner product on a complex vector space X, the pair ( X ,( , )) is called a (complex) inner product space, and we refer to the complex number ( x , y ) as the inner product of the vectors x and y in 2. For real vector spaces, the definition of inner products is the same as the one given above, except that scalars and the values ( x , y ) are required to be real, so that the "bars" denoting complex conjugation no longer appear in (ii) and (v). As regards elementary geometrical properties of inner product spaces, there is very little difference between the real and complex cases. In the main, we shall restrict attention to the complex case, making only 'occasional comments on the modifications needed to deal with real spaces. For the theory of linear operators on inner product spaces and algebras of such operators, the complex case has significant advantage over the real one. The finite-dimensional linear spaces C" and R" provide the simplest examples of inner product spaces, with the inner product defined by
+ . . + a,,&
( ( a l , .. .,an),( b l , .. . ,b,,)) = a161
*
(complex conjugation being redundant in the case of UP).Just as the real space R"can be viewed as a real-linear subspace of the complex space @", it can be shown that every real vector (or normed, or inner product) space can naturally be imbedded in a complex space of the same type (see Exercises 1.9.6 and 2.8.3). 2.1.1. PROPOSITION. Suppose that ( , ) is an inner product on a complex vector space 2. (0 I(x,y>12 < ( x , x ) ( y , y ) , f o r a11 x a n d y in 2. (ii) The set 9 = { z E ~( z: , z ) = 0) is a linear subspace of .X, and the equation (x
+9,y +9>1 =
defines a definite inner product ( ,
(x9.Y)
(X,YE*)
on the quotient space 219.
Proof. (i) When x, YE X and a, b E @, (ax
+ by,ax + b y ) = a ( x , a x + by) + b ( y , a x + b y ) = aii(x, x ) + a 6 ( x , y ) + bii(y, x) + b 6 ( y , y ) ,
so (1)
(ax
+ by,ax + b y ) = laI2(x,x) + 2 Re a 6 ( x , y ) + Ib12(y,y).
77
2.1. INNER PRODUCTS ON LINEAR SPACES
By taking a = t( y, x), where t is real, and b = 1, we obtain 0 < (ax
+ by,ax + b y )
= mx,Y)12(x,x)
+ 2tl(x,y)I2 + ( y , y >
( t E W
If ( x , x ) = 0, it follows, by considering large negative t, that I(x,y)l = 0, so I(x,y)I* = ( x , x ) ( y , y ) = 0 in this case. If ( x , x ) > 0, we can take t = - l / ( x , x ) , to obtain 0 d I(xtY>I21(x1x>- 21(x,Y)l2/(x,x) + (YtY) =(%Y)
- I(xlv>l2I(x,x),
whence I(-w9l2d ( X , ~ > ( Y , Y > . (ii) Let
Yl = {zEX: (z,y) = 0 for each y in X ) . It is evident that Y l is a linear subspace of X, contained in the set Y defined in the proposition, and that
PI= ( z E X :( y , t ) = 0 for each y in X ) . With z in 9, it follows from (i) that l(Z,V>l2 < ( z , z ) ( y , y ) = 0, (Z,Y> = 0 (YE-% so z E Yl. Hence Y = Yl, and Y is a linear subspace of A?. If x , y ~ %and z l , z z ~ (= P PI), we have ( x + Z1,Y + z t ) = ( X , Y > It follows that the equation (x
+ (x,zt) + ( Z l t Y ) + ( Z l l Z 2 )
+ P , Y + -2% = ( X , Y >
= (x,v>.
(X,YEX)
defines (unambiguously) a mapping (u, u ) ( u , u ) ~from ( X / Y )x (X/2’) into C. It isclear that ( , inherits from ( ,) the threedefining properties of an inner product. If 0 = ( x + 9,x + Y)l ( = ( x , x)), then X E 9,and x + 9 is the zero element of X/Y.Thus ( , is a definite inner product on X / Y . H The inequality stated in Proposition 2.1.1ti) is known as the CauchySchwarz inequality.
If( , ) is an inner product on a complex vector space 2.1.2. PROPOSITION. X, the equation
(2)
llxll = (x,x)”2
( X E X )
defines a semi-norm 11 11 on X. Ifthe innerproduct is definite, 11 11 is a norm on X. Proof. With 11 11 defined by (2), it is apparent that llxl} 2 0 and Ilaxl} = la1 lixil whenever x E X and a E @. Moreover, if the inner product is definite,
78
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
llxll = 0 only when x = 0. The Cauchy-Schwarz inequalitycan be written in the form I(x,y>l < ( x , x ) 1 ’ z ( Y 9 Y ) 1 ’ = 2 llxll llvll
(X9YEm.
From (l), with a = b = 1, IIX
+ Y1I2 = llXllZ + 2 Re(x,y) + llYllZ < llxl12 + 2l<x,r>l + llY1I2 < 11x112 + 2llxll llvll + llY1I2 = (Ilxll + llY11)2.
Hence JIx+ yll
< llxll + llyll for each x and y in X .
W
When referring to the norm on a (definite) inner product space, it is understood, in the absence of an explicit statement to the contrary, that the norm intended is the one constructed as in Proposition 2.1.2 from the inner product. For such spaces, we have proved the triangle inequality IIX
+ Yll < llxll + IlYll
and the Cauchy-Schwarz inequality I(X,Y>l
< llxll Ilvll.
For each of these results, we now determine the conditions under which equality occurs. 2.1.3. PROPOS~T~ON. r f ( , ) is a definite inner product on a complex vector space 2 and x, y E 2,the following three conditions are equivalent:
0) IIX + YII = llxll + llvll; (ii) = IIXII Ilvll; (iii) one of x and y is a non-negative scalar multiple of the other. (x9.Y)
Proof.
(3)
For any scalars a and 6, it follows from (1) that llaX
+ byl12 = la1211X112+ 2ReaRx9y) + 1~1211y11z.
Thus
(Ilxll + l l ~ 1 1 )-~ Ilx + y1I2 = 2(llxll Ilyll - Re(x,y)). If (i) is satisfied, the last equation and the Cauchy-Schwan inequality give Re(x,v) = llxll llvll 2 I(X,Y)I9 and therefore ( x , y ) = Re(x,y) = llxll Ilyll. Thus (i) implies (ii). If (ii) is satisfied and a, b are real, (3) gives
llax + bY1l2 = (allxll + 41ull)z.
2.1. INNER PRODUCTS ON LINEAR SPACES
79
With a = IlyJJand b = - Ilxll, it follows that llyllx - llxlly = 0. Hence either x = 0 ( = 0 . y ) or y = IIxII-lllyJIx,and so, (ii) implies (iii). If (iii) is satisfied, we may suppose that x = ay, where a 2 0. Then
so (iii) implies (i). W 2.1.4. COROLLARY. If( , ) is a definite inner product on a complex vector space I x and x, y c Ix, then I(x,y)I = IIx((J(yI(ifand only ifx and y are linearly dependent.
Proof. If ( ( x , y ) l = I/xII Ilyll, we can choose a scalar a so that la1 = 1 and a ( x , y ) = llxll Ilyll; that is, ( a x , y ) = I(axlI Ily((.By Proposition 2.1.3,one of ax
and y is a non-negative scalar multiple of the other; so x and y are linearly dependent. Conversely, suppose that x and y are linearly dependent. We may assume that x = ay for some scalar a, and then I(X,Y>I
= la(Y,Y)l
= la1 llYIl* =
llxll Ilvll.
A complex normed space &' is said to be apre-Hilbert space if its norm 11 11 can be obtained, as in Proposition 2.1.2, from a (necessarily) definite inner product on Z.If, in addition, &' is complete relative to 11 (I, then X is described as a Hilbert space (of course, one can consider real Hilbert spaces - complete real inner product spaces). Accordingly, Hilbert spaces form a particular class of Banach spaces, and the theory developed in Chapter 1 for linear topological spaces, normed spaces, and Banach spaces is available in the case of Hilbert spaces. The geometry of Hilbert spaces is in many respects analogous to elementary euclidean geometry, and is simpler and more extensive than any corresponding theory for general Banach spaces. In consequence, the analysis of Hilbert space operators is more fully developed than its Banach space counterpart. The main objects studied in this book are certain algebras of linear operators acting on Hilbert spaces.
The inner product on a pre-Hilbert space &' is a 2.1.5. PROPOSITION. continuous mapping from &' x X into @. Proof. When x, y, xo,yo E X,
+ (x - X 0 ) l Y O + (u - Y o ) ) = (X0,Yo) + (X0,Y -Yo> + (x - X0,YO) + ( x - X O - Y -Yo>.
(X,Y) =
(xo
80
2. BASICS OF HILBERT SPACE A N D LlNEAR OPERATORS
From this and the Cauchy-Schwarz inequality, (4)
l(x9.Y) - <XO,YO>l
6 llxoll IIY - Yoll + IIX - xoll llyoll + IIX - xoll IIY - YOlL and the right-hand side is small when x is close to xo and y is close to y o . W
In Theorem 1.5.1, we showed that a normed space X can be embedded, essentially uniquely, as an everywhere-dense subspace of a Banach space f, the completion of X. We now consider the case in which 3E is a pre-Hilbert space.
If 2 is a pre-Hilbert space, its completion 2.1.6. PROPOSITION. Hilbert space.
2 is a
Proof. For n = 1,2,. . . ,let
s,,= {(x,y):x,yEx, $n=
llxll < n, llvll < n>, { ( x , y ) : x , y ~ * *IMI < n , IIYII< n > ,
and note that S,, is everywhere dense in (41,
s,,in the topology on 2 x 2.From
+ nlly - Yo11 + Ilx - xoll IIY - Y O I L when ( x ,y), (xo,y o )E S,,. From this, the mapping fn : S,, + @, defined by l(x,y) - (xo,yo>l < nllx - xoll
(5)
f . ( x , y ) = (X,Y>,
is uniformly continuous on S,,, and so extends uniquely to a continuous mappingx: + C. When m 2 n, the restrictionfmIsn is another continuous extension offn, SOT,I $,, =A. It follows that there is a mappingf: 2 x 2 + 43 such that fl,$ = A (n = 1,2,. . .). We assert that fis an inner product on 2, and gives rise to its norm, whence 2 is a Hilbert space. For this, we have to show that
s,,
f ( a x + by, 4 = a f k f(Y9
4 + b f ( y ,4,
x) = f ( x ,Y ) ,
f(x,x) = llX1l2,
whenever x , y, z E 2 and a, b E @. We can choose an integer n that exceeds the norm of each of the vectors x, y , z , ax + by, and by continuity off 1 ( =I,,), it then suffices to prove the three required equations under the additional assumption that x, y , z E X. However, in this case, these three equations follow at once from (9,since f l S,, = f..
s,,
Next, we prove two identities, both of which are frequently useful, concerning vectors in a pre-Hilbert space. The first of these is known as the parallelogram law.
2.1. INNER PRODUCT’S ON LINEAR SPACES
81
2.1.7. PROPOSITION. Ifu, u, x, and y are uectors in a pre-Hilbert space, IIx
+ Yl12 + IIx - Y1I2 = 211x1I2 + 211Y1I2,
and
+ u, x + y ) - ( u - u, x - y ) + i(u + iu,x + i y ) - i ( u - iu,x - iy).
4(u, y ) = ( u
Proof. The first identity is an immediate consequence of the equations IIx f Yl12 = llx1I2f 2 Re(x,y)
+ lly1l2,
which are particular cases of (3). For the second identity, note first that ( u f u,x f y ) = ( u , x )
+ ( 4 y ) f((u,y> + (u,x))
(with the same choice of the ambiguous sign throughout). Thus (u+u,x+y) -(u-u,x-y)=2(u,y)
+2(u,x).
Upon replacing u by iu and y by iy, we obtain (u
+ i u , x + i y ) - ( u - i v , x - i y ) = - 2 i ( u , y ) + 2i(u,x).
From the last two equations, (u
+ u , x + y ) - ( u - u , x - y ) + i ( u + iu,x + i y ) - i ( u - iu,x - i y ) =4(u,y).
We illustrate the use of the two identities just established, in obtaining the following characterization of pre-Hilbert spaces within the class of normed spaces. 2.1.8. PROPOSITION. A complex normed space 2 is a pre-Hilbert space f and only i f
IIx + y1I2 + IIx - yl12 = 211xIl2 + 211y1I2 (X,YEW. (6) When this condition is satisfied, there isa unique inner product on 2 that defines its norm, and this is given by
(7)
4 ( x , y ) = 11x
+ y1I2 - I(x - y1I2 + illx + iy1I2 - illx - iyI12.
Proof: When JV is a pre-Hilbert space, it follows from Proposition 2.1.7, with u = x and L’ = y , that the norm satisfies (6) and the inner product is determined by (7). Conversely, suppose that a? is a normed space whose norm satisfies (6); and when x , y e X , define a scalar ( x , y ) by (7). Then
+
(.u,x) = i(112~11~ ill(1
+ i)xl12
-
ill(l - +[I2)
+ ill + iI2 - ill - i I 2 ) = IIxI12,
= +11~11~(4
82
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
and
= <X,Y>.
In order to complete the proof that ( , ) is an inner product on X, which defines the norm on X , it remains only to show that, for each fixed y in i#,the equation (8)
+
f ( x ) = IIX y1I2 - IIx - yIl2 + illx + iy1l2 - illx - iy1I2
defines a linear functional f on 2.We begin by proving that
fW = is(x),
(9)
f(x1
+ x2) =f(x1) + f ( X Z ) ,
for all x, x l ,x2 in X. For this, note that
+ yII2 - llix - yllz + illi(x + y)IIz - illi(x - y)I12 = i(- illi(x - iy)1I2 + illi(x + iy)llz + IIx + yllz - IIx - y1I2)
f ( i x ) = llix
= if(x).
Since the norm satisfies the parallelogram law (6), 11x1
+ x2 + 2YllZ+11.:
+ Y1I2 + 11x2 + YIIZ = 1:1. = 21l:(x1
- X21l2
+ x2) + Y1I2 +
11 .:
- X21I2.
From this and the three similar equations obtained when y is replaced by - y, iy, - iy, it follows that (10)
f(x1) + f ( x 2 ) = 2f(t(Xl With x1 = x and x2 = 0, (10) gives
f(x) = 2f(:x)
+ x2)).
(x E A?),
since it is apparent from (8) thatf(0) = 0. Hence (10) can be rewritten in the form f(x1)
+fM
=Ax1
+ xz),
and (9) is proved. It now remains to show that f ( a x ) = af(x) whenever x E i# and a E @. Equivalently, we must prove that IF = C where IF = { a ~ @ : f ( a x=)af(x) for each x in A?}.
From (8),fis continuous, so IF is closed. It is evident that 1 E IF, and that ab, E IF whenever a, b, c ( # O ) E IF. From (9), ie IF, and a + b e ff whenever
c-
2.1. INNER PRODUCTS ON LINEAR SPACES
a, b g IF. The properties just listed imply that s + it€ rational, and so IF = @, since IF is closed.
[F
83
whenever s and t are
2.1.9. REMARK.Most of the theory developed above for complex inner product spaces is valid also in the real case. For real inner product spaces, the second relation in Proposition 2.1.7 is omitted. In Proposition 2.1.8, the relation (7) between the inner product and norm is modified by the deletion of the last two terms on the right-hand side and is easily proved by direct computation. The remaining proofs require only minor alterations. H 2.1.10. REMARK.Equation (7) gives an immediate alternative proof of the continuity of the inner product on a pre-Hilbert space. Moreover, if 2 is a normed space that satisfies the parallelogram law (6), it follows by continuity that the completion $ has the same property. This, together with Proposition 2.1.8, provides an alternative proof that the completion of a pre-Hilbert space is a Hilbert space. 2.1.1 1. EXAMPLE. With n a positive integer, the complex vector space @", consisting of all n-tuples x = ( x l , , . . ,x,,), y = (yl,. . . ,yn) of complex numbers, has a definite inner product defined by
(x, y)
= XlVl
+ . . . + x&.
The associated norm is given by
llxll = (IXl12
+ ... +
IX"12)"2.
Since @" is complete, relative to the metric d(x, y) = IIx - yll, it is a Hilbert space. In this example, the Cauchy-Schwarz and triangle inequalities reduce t o
for all complex numbers xl,. . . ,x,,,yl,. . . ,y,. In the same way, the equation (x, Y) = X l Y l
+ ... +
XnYn
(x,Y E R")
defines a definite inner product on the real vector space R". The equation
(X9Y)l
(X,YE@") defines an inner product on C"; when n 2 1, ( , ) l is not definite, for 9 (see = XlVl
Proposition 2.1.1) consists of all vectors whose first component is zero. In this case, @"/Y is a one-dimensional Hilbert space (isomorphic to C).
84
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
2.1.12. EXAMPLE.Given a set A,the Banach space 12(A)described in Example 1.7.3 consists of all complex-valued functions x on A for which the (unordered) sum CaEAIx(u)I2 is finite, and its norm is given by
When x, y E /,(A), the sum CEAx(u)y(a)converges, since
Ix(a)y(a)I G tClx(4l’ + lr(4I2),
(Ix(4l’
+ IY(4l2) < a).
acA
From this, it follows easily that /,(A) has a definite inner product, defined by aeA
which gives rise to the norm in (11). Hence /,(A) is a Hilbert space. In this example, the Cauchy-Schwarz and triangle inequalities assert that
for all x and y in 12(A). When A = { 1,2,. . . ,n}, l,(A) is the Hilbert space @”considered in the preceding example. When A is the set { 1,2,3,. . .} of all positive integers, we write 1, in place of /,(A), and sometimes denote an element of this space as a sequence {x”}. W 2.1.13. EXAMPLE.Let 1:’) be the class of all complex-valued functions defined on the set A = { 1,2,3,. . .} that take non-zero values at only finitely many points of A.Thus 17)is a linear subspace of I,, and so inherits from 12, by restriction, a definite inner product and the associated norm. Hence I:’) is a preHilbert space; we assert that it is not a Hilbert space, that is, it is incomplete. For this, we show that 1:’) is everywhere dense in 12, from which it follows (since l2 # ’”I\> that I\’) is not closed in l,, and therefore not complete. With x in 12, define x1, x 2 , x 3 , .. . in 1;’) by xj(k) =
Then
{;(k)
if k ~j if k > j .
85
2.2. ORTHOGONALITY
a s j + a .This shows that each element of f 2 is the limit of a sequence in I\’), so proves our assertion that fy)is everywhere dense in 12.
and
2.1.14. EXAMPLE.Suppose that m is a a-finite measure defined on a a-algebra Y of subsets of a set S. The Banach space L2 ( = L,(S,.Y:m)), described in Example 1.7.5, consists of all (equivalence classes modulo null functions of) complex-valued measurable functions x on S for which
with the norm defined by
~ the function x(s)y(s) is integrable, since it is measurable and its When x , y L2, absolute value is dominated by the integrable function ~lx(s>12 Iy(s)12). From this, it follows easily that L2 has a definite inner product, defined by
+
(X9Y)
= lsx(slyodm(s)7
which gives rise to the norm in (12). Hence L2 is a Hilbert space. The Cauchy-Schwarz and triangle inequalities reduce, in this example, to
for all x and y in L 2 . 2.2. Orthogonality
The theory of Hilbert spaces and Hilbert space operators is more tractable than its Banach space counterpart, largely because the presence of an inner product permits the introduction of a satisfactory concept of orthogonality. In the present section we study this concept, after obtaining some preliminary results. We show first that, in a Hilbert space, the minimal distance from a point to a closed convex set is attained.
86
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
2.2.1. PROPOSITION. lf Y is a closed convex subset of a Hilbert space X, and a unique element yo of Y such that
x ~ E ,there ~ , is
11x0 - Yoll G 11x0 - Yll
(1)
(YE
Y).
Moreover, R e ( y o , x o - Y O > 2 Re(y,xo - YO>
(2)
(YE
Y).
Proof. With d = inf{llxo - y l l : y ~ Y}, there is a sequence {y,} of elements of Y such that llxo - ynll + d. By the parallelogram law, 211x0 - ymI12
+ 211x0 - ynI12 = 112x0 - y m - ynI12 + IIyn - ymI12
for all positive integers m and n. Since :(ym + y,) 112x0 - y m - ynII = 211x0 - +
E
Y , we have
+ Y J I 2 2d,
and therefore
IIY, - ymI12 = 211x0 - YmI12 G 211x0 - y,ll2
+ 211x0 - Y~II’- 112x0 - y m - ynI12
+ 211x0 - y,I12 - 4d2
+
0
as min(m, n) + 00. Hence {y,} is a Cauchy sequence, and so converges to an element yo of X. Moreover, yo E Y, since Y is closed ;and yo satisfies ( l ) , since JIxo- yolJ= lim llxo - ynll = d = inf{llxo - y l l : y ~Y}. n-+ m
If yb is another element of Y that satisfies ( l ) , then llxo- ybll = I(xo - y o [ [= d. We can apply the preceding reasoning, with yo and yb in place of y, and y,, to obtain
llvb - yO1l2= 211xo - Y
~ + ~2 1 I 1-~~&li2 ~ - 4 b 0 - $(yo + yb)li2
< 2d2 + 2d2 - 4d2 = 0. Hence yb = yo, and yo is uniquely determined by (I). For each y in Y and t in (0, l), yo + t ( y - yo)€ Y , and (1) gives 11x0 - YO1l2
< 11x0 - Y o - t(Y - y0)1l2 = 11x0 - Yoll
2
- 2 t R e ( y - Y0,XO - Yo> + t211Y - Yol12.
Hence
- 2 W y - yo,xo - y o ) and this gives (2) when t
-+
0.
+ tlly - yO1l23 0
(0< t < 11,
87
2.2. ORTHOGONALITY
2.2.2. REMARK.We have proved, in Theorem 1.3.4, that a closed convex subset Y of a locally convex space is closed also in the weak topology. For a Hilbert space X, Proposition 2.2.1 permits an alternative proof of this result. For this, suppose that xoE X \ Y , and let yo be the element of Y that satisfies (1) and (2). Then llxo - yell > 0, and , -YO) W y , xo - yo> < R ~ ( Y oxo = Re(x0, xo
- yo)
- (xo - Yo, xo - yo)
= Re(x0,xo - yo) -
11x0
- YO1l2
for each y in Y. Thus X \ Y 2 V, where V = {xEX Re(x,xo : - y o ) > Re(xo,xo - y o ) - llxo - yO1l2}.
The equation p(x) = (x, xo - y o ) defines a linear functional p on 2 ;and from continuity of the inner product, p is bounded, and is therefore weakly continuous. Since V = { x E XRep(x) : > Rep(xo) - llxo - y o J I z } ,
it follows that V ( c X \ Y ) is a neighborhood of xo in the weak topology on X. Hence X \ Y is weakly open, and Y is weakly closed. Suppose that X is a Hilbert space, u, u E X, and X,Yare subsets of X. We say that u is orthogonal to u if ( u , v ) = 0, that u is orthogonal to Y if (u, y) = 0 for each y in Y , and that X is orthogonal to Y if (x, y ) = 0 whenever x E X and y E Y . The set of all vectors, in X and orthogonal to Y, is denoted by Y '. When Y is a closed subspace of X, we sometimes write X 0 Y in place of Y I. If u is orthogonal to u, then also u is orthogonal to u, and by expanding the inner product (u + u,u + u) we obtain IIU
+ uIl2 = llU1l2 + lIuIIZ.
From continuity and linearity of the inner product in its first variable, Y' is a closed subspace of H. It is apparent that X c Y if and only if Y G X I , and that X' c Y' if X 2 Y . If U E Y', then Y G {u}'. Moreover, since {u}' is a closed subspace containing Y, it contains the closed subspace [ Y] generated by Y , and so u E [ Y ]I.This shows that Y I c .[ Y ] * , and the reverse inclusion is apparent since Y E [ Y ] ; so
Y' = [ Y ] ' . If Y E Y n Y', then ( y , ~ = ) 0, whence y = 0; so Y n Y' = (0).
In particular, X' = X n X' = (0).
88
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
The following theorem includes the assertion that, if Y is a closed subspace of a Hilbert space X, then Y and Y l are complementary subspaces in the sense discussed in Section 1.1 (preceding Theorem 1.1.8). For this reason, Y ' is called the orthogonal complement of Y.
2.2.3. THEOREM.If Y is a closed subspace of a Hilbert space X, each element xo of X can be expressed uniquely in the form yo + zo, with yo in Y and zo in Y l. Moreover, yo is the unique point in Y that is closest to xo. Proof. Since Y is a closed convex subset of X, we can choose yo as in Proposition 2.2.1, and define zo = xo - yo. From (1) and (2), yo is the (unique) point in Y that is closest to xo, and Re(y, zo) ,< Re(yo, zo) for each y in Y. By writing ay in place of y , we obtain
< Re(yo,zo)
Rea(y,zo)
(YE
Y, a€@).
Hence ( y , zo) = 0 for each y in Y, and zo E Y l. This proves the existence of a decomposition xo = yo zo, with yo in Y and zo in Y l . If, also, xo = y , + zl, with y , in Y and z1 in Yl, then
+
yo
+ zo = y1 + z , ,
yo - y1 = z1 - Z
~ Y E
nYl
=
{O};
and so yo = y , , zo = zl. H
2.2.4. COROLLARY. If Y is a closed subspace of a Hilbert space H' and
X _c X, then (Y1)l = Y,
(Xl)' = [XI
Moreover Y = X ifand only if Y l = (0). Proof. Since [ X I is a closed subspace of 2, and (Xl)' = ( [ X I ' ) ' , it suffices to prove only the results concerning Y. If y E Y, then y is orthogonal to each element of Y I,and so y E ( Y l)l. This shows that Y E (Yl)', and we have to prove the reverse inclusion. With xo in ( Y ')', we can choose yo in Y and zo in Y so that xo = yo + zo, by Theorem 2.2.3. Then xo E ( Yl)', yo E Y G ( Y l)l, and therefore zo = xo - yo E ( Y.')l Hence
z o ~ y l n ( Y 1 ) ' = {O}, and xo = yo E Y. This gives the required inclusion ( Y *)l E Y , so ( Y ')' = Y. If Y = 2,then Y' = #1 = {0};conversely, if Y ' = { O j , then Y = (Y ')' = {O}'= X. A subset Y of a Hilbert space X is described as an orthogonalset if any two distinct elements of Yare mutually orthogonal. By an orthonormal set we mean an orthogonal set of unit vectors. An orthonormal set Y is linearly independent
2.2. ORTHOGONALITY
89
(by which we mean that every finite subset of Y is linearly independent); for if y , ,. . . ,yn are distinct elements of Y , a , , . . . ,an E @, and I;=, ajyj = 0, then n
ak = ( C a j y j , y k )= 0
(k = I ,..., n).
j= 1
In developing the theory of orthogonal expansions in a Hilbert space, we make use of the concept of unordered summation introduced in Section 1.2 (following Theorem 1.2.18). I f Y is an orthogonal set in a Hilbert space X, the sum 2.2.5. PROPOSITION. CWYy converges ifand only ifCyEylly112 < 00. When this condition is satisfied,
(3) Proof. With F a finite subset of Y , expansion of the inner product y1(~ expression for l l , ~ ~ ~gives (4)
From this, and the Cauchy criterion for unordered sums, it follows that convergence of either of the sums in (3) impliesconvergence of the other. When these sumsconverge, they are limits of the finite subsums occurring in (4); since the norm is continuous, (3) is an immediate consequence of (4). W I f Y is an orthonormal set in a Hilbert space X, andf is 2.2.6. COROLLARY. a complex-valuedfunction defined on Y , the sum cWy f ( y ) y converges ifand only ifzFyIf(y)I2 < 00. When this condition is satisfied,
II Cf(Y)Yl12= C lf(Y)I2. YEY
FEY
Proof. It suffices to apply Proposition 2.2.5 to the orthogonal set { f ( Y ) Y : Y EY ) . 2.2.1. PROPOSITION. If Y is an orthonormal set in a Hilbert space X and ME&,
then
(i) CyeY I(U9Y>l2 6 Ilul12; (ii) the sum CBtY( u ,y ) y converges, and
u-
(U,Y)YE YE
w
Ib - C y E Y (U7Y)YIl2
y*;
r
= 1 1 ~ 1 1 2 - CWY I(UtY>l2.
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2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
Proof. With F a finite subset of Y and p(y) = ( u , y ) for each y in Y,
IIU -
(U9Y)Y1I2
= (u
-
YSF
c P(Y)Y,U - c
YEF
c P(Y) llUllZ - c IP(Y)l2 c ll~IlZ c
= (4u> -
YGF
=
-
YEF
=
ZEF
P(Z)Z)
reF
-
1pO(u7 z> + 1 P ( Y ) p o ( Y ?z> ZEF
lP(Z)I2
YJEF
+ c lP(Z)IZ zeF
I(U,Y>IZ.
-
Y ~ F
Hence (5)
1
l(UJJ>l2 =
llul12 - IIU -
1
(U9Y)Y1I2
G llu1I2
Y EF
YEF
for each finite subset F of Y . This proves (i), and the convergence of IFy ( u , y ) y now follows from Corollary 2.2.6. For each y o in Y , continuity and linearity of the inner product in its first variable entail (U,Y)Y,YO) = (U9Yo) -
(u YEY
1 (U~Y>(Y~YO> YEY
= (u,ro> - ( U , Y O > = 0,
so u - CyeY( u , y ) y E Y I.This proves (ii), and (iii) is an immediate consequence of ( 5 ) since the norm is continuous. The inequality in Proposition 2.2.7(i) is usually known as Bessel's inequality. 2.2.8. COROLLARY.Y is an orthonormal set in a Hilbert space JP and u E X, then CFY ( u , y ) y is the unique vector closest to u in the closed subspace [ Y ] generated by Y. Moreover, the following three conditions are equivalent:
(0 u E [ y l ; (ii) u = CyeY (u,Y>Y; (iii) lu112 = CyeY I(4Y>I2. Proof. With v ' = (u, y ) y , it is evident that U E [ Y], and Proposition 2.2.7(ii) asserts that u - U E Y l = [ Y ] l .Since, also, u = u + (u - v ) , Theorem 2.2.3 now implies that u is the unique point closest to u in [ Y ] . From the last statement, it follows that v = u if U E [ Y ] , so (i) implies (ii). The reverse implication is apparent, and the equivalence of (ii) and (iii) follows from Proposition 2.2.7(iii). W
2.2. ORTHOGONALITY
91
2.2.9. THEOREM.If Y is an orthonormal set in a Hilbert space 2, the following six conditions are equivalent:
(i) (ii) (iii) (iv) (v) (vi)
for each u in 2,u = CYEY ( u ,y ) y ; for each u and v in 2, (u, 0) = CyEY ( u ,y ) ( y , 0); for each u in 2, llu112 = CYEY I(u, y)12; Y is not contained in any strictly larger orthonormal set ;'A Y 1 = {O}; [ Y ] = S.
Proof. By the linearity and continuity of the inner product in its first variable, (i) implies (ii). It is apparent, by taking v = u, that (ii) implies (iii). If Y is contained in a strictly larger orthonormal set,'A (iii) fails since, when XEX\Y.
1 I(X,Y>I2= 0 # 1 = llX1l2. YEY
It follows that (iii) implies (iv). If Y l has a non-zero element x, Y is contained in a strictly larger orthonormal set Y u {llxll- ' x }; so (iv) implies (v). By Corollary 2.2.4, and since Y 1 = [ Y ] l ,(v) implies (vi). It follows, from the equivalence of the first two conditions stated in Corollary 2.2.8, that (vi) implies (i). H An orthonormal set Y in a Hilbert space X that satisfies (any one, and hence all six, of) the equivalent conditions set out in Theorem 2.2.9 is called an orthonormal basis of 2. When Y is an orthonormal basis, the equation in condition (ii) is known as Parseval's equation. 2.2.10. THEOREM.Each Hilbert space 2 has an orthonormal basis, and every orthonormal set in X is contained in an orthonormal basis. Moreover, all orthonormal bases of X have the same cardinality. Proof. The class of all orthonormal sets in 2 is partially ordered by inclusion. If a family { Y,} of orthonormal sets is totally ordered by inclusion, then U Y, is an orthonormal set that contains each Yo;for any two distinct elements y , z of U Y, are contained in the union Yb u Y, of two sets in the family, Ybu Y, coincides with Yb or Y, and is therefore orthonormal, and so ( y , z ) = 0. In view of this, it follows from Zorn's lemma that there is a maximal orthonormal set Y o ;since Yo is not contained in a strictly larger orthonormal set, it is an orthonormal basis. If Y is a given orthonormal set, we can repeat the above argument, restricting attention throughout to orthonormal sets containing Y . In this way, we prove that there is an orthonormal basis containing Y .
92
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
Suppose that X and Y are two orthonormal bases of X, and that the cardinal numbers corresponding to these sets are m and n, respectively. In proving that m = n, we consider separately two cases. If X is finite dimensional, the (linearly independent) sets X and Y are necessarily finite. Since both satisfy condition (i) in Theorem 2.2.9, each has linear span X, and is therefore a basis of X in the elementary algebraic sense. The basis theorem for finite-dimensional vector spaces now implies that m = n. We note also an alternative proof, that m = n when m and n are known to be finite, which is more akin to the argument needed in the infinite-dimensional case. By condition (iii) in Theorem 2.2.9
m =
c Ilx1l2
=
xcx
1c
I(X9Y>12
xsx ysY
yoY xox
YeY
If X is infinite dimensional, both X and Y are infihite sets, since [ X I = [ Y] = X. If m # n, we may assume that m c n ; we show, in due course, that this assumption leads to a contradiction. For each x in X ,
1
I(X9Y>12 =
llX1l2 = 1,
YeY
so the set Y, if y~ Y,
= {YE Y : ( x, y>
# 0} iscountable. Moreover, Y =
uxcx Y,; for,
1 I(x3r>l2= llYl12 = 1, xcx
whence ( x , y ) # 0 and Y E Y,, for at least one element x of X. The remainder of the argument consists of elementary cardinal arithmetic, amounting, essentially, to the observation that n 6 mKo = m (where, as usual, KOdenotes the cardinal of the set of natural numbers). In order to prove that n < m, and so obtain the desired contradiction, we need only show that there is a mapping from Xonto Y. For this, it sufices to prove that Xcan be expressed as a disjoint union UXEX X , of a family (indexed by X) of countably infinite subsets; for then each X , can be mapped onto the corresponding Y,, and X ( = U X,) can be mapped onto Y (= U Y,). To prove the existence of such a family (XJxeX, it is enough to show that X x Z has the same cardinality as X , when Z is a countably infinite set. Since Z x Z is countably infinite, it now suffices to prove that X has the same cardinality as A x Z, for some set A. This, in turn, amounts to showing that Xcan be expressed as the disjoint union UOsa Z, of a family (with arbitrary index set A) of countably infinite subsets of X.For this, observe that a simple argument using Zorn’s lemma proves the existence of a maximal disjoint family { Z o }of countably infinite subsets of X. The maximality of this family implies that X \ U Z , has only a finite number of
93
2.2. ORTHOGONALITY
elements; by adding these to any one Z,, we obtain the required partition x=uz,. W By the dimension of a Hilbert space .F we mean the cardinal number dim H corresponding to an orthonormal basis Y of X. From the preceding theorem, this does not depend on the choice of Y; moreover, it coincides with the elementary algebraic concept of dimension when X is finite dimensional. Suppose that ,X,and 2,are Hilbert spaces, and U is a linear mapping from 2,onto H,. By expressing inner products in terms of norms, as in Proposition 2.1.8, it follows that U preserves inner products if and only if it preserves norms. Accordingly, the concept of isomorphism, from XI onto X,, is the same whether 2,and 2,are regarded as Banach spaces or as Hilbert spaces. It is evident that isomorphic Hilbert spaces have the same dimension. 2.2.11. EXAMPLE.With A any set, the Hilbert space I,(A) has an orthonormal set Y = { y , :a E A}, in which y, is the function taking the value 1 at a and 0 elsewhere on A. Since ( x ,y , ) = x(a)for each x in I,(A) and a in A, it follows that YL = (0).Hence Y is an orthonormal basis of I2(A),and the dimension of I,(A) is the cardinal number corresponding to the set A. A necessary and sufficient condition for two spaces I,(A) and I,(B) to be isomorphic is that the sets A and B have the same cardinality. The condition is necessary because isomorphism preserves dimension; it is sufficient since, iff is a one-to-one mapping from A onto B, the equation (Ux)(a)= x(f ( a ) ) defines an isomorphism U from I,@) onto I2(A). W 2.2.12. THEOREM.Two Hilbert spaces are isomorphic fi and only have the same dimension.
if they
Proof. In view of Example 2.2.1 1, it suffices to prove that a Hilbert space 2 with an orthonormal basis Y is isomorphic to I,( Y). For each x in 2,we can define a complex-valued function Ux on Y by ( V x ) ( y )= ( x , y ) . From condition (iii) in Theorem 2.2.9,
llX1l2 =
1 I(X,Y)l2 = 1 I ( ~ X ) ( Y ) I 2 , YeY
YGY
so U is a norm-preserving linear mapping from X into I,( Y). With f in f,( Y), it follows from Corollary 2.2.6 that the sum CFrJIY)y converges to an element x of X Moreover, for each yo in Y , ( W ( Y 0 ) = (X,YO) = C f ( Y ) ( Y , Y O ) w so U x = J Hence U is an isomorphism from 2 onto I,( Y). =f(Yo)9
2.2.1 3. COROLLARY. Every Hilbert space is isomorphic to one of the form I2(A).A Hilbert space with finite dimension n is isomorphic to @”.
94
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
2 . 2 . 1 4 . REMARK. We assert that a Hilbert space X is separable if and only ifdim X 6 KO.In consequence, all separable infinite-dimensional Hilbert spaces have dimension KO,and are therefore isomorphic. In particular, 1, spaces for countably infinite sets, and L2 spaces for Lebesgue measure on measurable subsets of R", are all isomorphic. To prove the above assertion, let Y be an orthonormal basis in X . If Y is countable, X has a countable everywhere-dense subset, which consists of those finite linear combinations of elements of Y in which each coefficient has rational real and imaginary parts. If Y is uncountable, the open balls with radius +$ and centers in Y form an uncountable disjoint family, since llYl - Y2112 = IlYI1l2 + llY21I2 = 2
when y , and y , are distinct elements of Y. An everywhere-dense subset of ,# meets each of these balls, and is therefore uncountable. In proving the following result, we describe the GramSchmidt orthogonalization process, by which a linearly independent sequence of Hilbert space vectors gives rise to an orthonormal sequence. The linearly independent sequence may be finite or (countably) infinite, and the orthonormal sequence has the same number of terms. We recall that the (necessarily closed) subspace generated by a finite set x ] , . . . ,x,, of vectors is denoted by [ x ] , . . . ,x,,].
2 . 2 . 1 5 . PROPOSITION. If (xl, x2,x3, . ..) is a linearly independent sequence of vectors in a Hilbert space X, there is an orthonormal sequence (y, ,y2,y,, . . .) such that [xl,. . . ,x,] = [yl,. . . ,y,] for each n = 1 , 2 , 3 , . . . . Proof. We construct yl, y,, y,, . . . inductively, and start the process by x l . suppose that we have produced an defining y1 to be ~ ~ x l ~ ~ - lNow orthonormal set {yl,. . . ,yr- ]}, with the property that (1
[yl,...,ynl = [ x ~ , . . . , x , , I
Since xr$ [XI
9 .
* . xr9
11
=
[YI
1
-
,Yr-
11
the vector r- 1
z = xr -
C (xr,Yj>Yj j= 1
is non-zero, and [YI,* -
* 9.Y-
1
9
~
= 1 [XI9
1
xr-
1,
xrl.
Moreover, for k = 1,. . . , r - I, r- 1
-
<xr,yj)(yj,Yk) = (xr,Yk) j= 1
- <xrtYk)
=o*
95
2.2. ORTHOGONALITY
With y r = 11~11-~z, { y l , .. . , y r } is an orthonormal set and [ y l , . . . , y r ] = [ x l , .. . ,x r ] . Hence the inductive process continues (indefinitely, if there are infinitely many x j , but terminating with the construction ofy, if there are just n vectors x j ) .
2.2.16. REMARK.When an orthonormal sequence {y,} is constructed from a linearly independent sequence { x , } , as in the proof of the last proposition, we have r- 1
(Yr,xr> = ( y r , z > +
1 ( y r , y j > ( y j , x r ) = (yryz> = IIzII > O . j= 1
It is not difticult to verify that the conditions
determine the orthonormal sequence { y,} uniquely. We conclude this section with a brief discussion of certain orthogonal families of functions in L z spaces, which are encountered in classical analysis and its applications. The best known examples arise in connection with the theory of Fourier series, for functions in Lz( - A, n). With Z the set {0, k 1, f 2 , . . .} of all integers, we can define functions x, (n E Z) in L2( - n,a) by x,(s) = exp(ins)
(- n
< s < n).
By evaluating the appropriate integrals, we obtain
ItxnII =
fi,
<xrn,xn) = 0
(m z n),
so the functions (211)- Il2x, form an orthonormal set in L2( -n, n). We shall note later, as a consequence of Theorem 3.4.14 (see Remark 3.4.19, that linear combinations of these functions are everywhere dense in L2(- n,n). Accordingly, the set {(2n)-1’2x,,: n E Z} is an orthonormal basis of L2( - n,n). Each function x in L2( - n,n) has an expansion
convergent in the norm topology on L2( - n,n), in which 1 2n
c, = -(x, x , ) =
x(s)e- ins ds.
The numbers c,, are called the Fourier coeflicienfs of x, and the series 1c,x, is called the Fourier series of x. Given two functions x and y in L2( - n,n),with Fourier series 1c,x, and C d,x,, respectively, Parseval’s equation assumes the
96
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
form
in particular, (7)
n J :
1 ~x(s)lzds= -
c 211
Icnl
2 *
neZ
The preceding paragraph is concerned with the “complex form” of Fourier series; we now consider briefly the (essentially equivalent) “real form.” The functions 1, coss, sins, C O S ~ Ssin2s, , C O S ~ Ssin%, , . form an orthogonal sequence ( y o ,y l ,y z , . . .) in L2( - n,n),since evaluation of the appropriate integrals shows that Since
the linear span of they, is the same as that of the x,, and is therefore everywhere /~y~, dense in Lz( - n,n). It follows that the sequence { ( 2 ~ ) - ~ n-l”y,, n - l i z y z ,. . .} is an orthonormal basis of L2(- n,n). The corresponding orthogonal expansion of a function x in L2(- II,n) is a series of trigonometrical functions, the “real form” of the Fourier series of x. Parseval’s equation yields results, analogous (and equivalent) to ( 6 ) and (7), in which the integrals representing ( x , y ) and are expressed in terms of “real” Fourier coefficients. Certain classical sequences of polynomials form orthogonal sets in appropriate Lz spaces. For example, in L,( - 1, I), the functions xo, xl, xz,. . . ,defined by xn(s) = sn
( - 1 6 s < l),
form a linearly independent sequence. By the Gram-Schmidt process, we can construct an orthonormal sequence ( y o , y l , y z , .. .) in L 2 ( - 1, I), such that [ y o , . . .,
~ n = l
[xo,.. ., xnl
(n = 0,1329. .
The linear span of the yn is the set of all polynomials, and this is everywhere dense in Lz(- 1, l), by the Weierstrass approximation theorem (noted later, in Remark 3.4.15). Accordingly ( y o ,y l ,y , ,. . .) is an orthonormal basis of Lz(- 1,l). It is not difficult to verify that
97
2.3. THE WEAK TOPOLOGY
where P, is the Legendre polynomial, defined by Rodrigues's formula 1 d" P"(S) = -((s2 - ly}. 2"n! ds" ~
The orthogonality relations
2 [P,(s)] ds = Zn+ I '
J1
Pn(s)P~(s) ds = 0
+
(m n)
(which are equivalent to the assertion that the sequence { y n }is orthonormal) are easily deduced from (8), upon repeated integration by parts. Other classical sequencesof polynomials arise in a similar manner. Let E be a Borel subset of R, with positive Lebesgue measure, w a strictly positive Borel measurable function on E, such that IEs2nw(s)ds< oo
(n = 0,1,2,. ..).
The set of (equivalence classes modulo null functions of) Borel measurable functions x on E, such that
1.
Ix(s)12w(s) ds < 03,
is a Hilbert space X, with inner product defined by <X,Y>
= jEx(s)Gw(s)dS.
The functions 1, s,s2,s3,. , . form a linearly independent sequence in S,and by applying the Gram-Schmidt process we obtain an orthonormal sequence ( y o , y ,, y 2 , ...), in which y , is a polynomial of degree exactly n. When E is [ - 1,1] and w ( s ) = ( l -s)"(I +s)", where v , p > - 1, we obtain the Jacobi polynomials P:+"(s). The Laguerre polynomials L:(s) ( v > - 1) arise when E = [0, to) and w(s) = s'exp( -s). The Hermite polynomiais correspond to the choice E = R, w(s) = exp( - s2). These are the main three classical sequencesof polynomials, from which the others can be derived; for example, the Legendre polynomials are a particular case (v = p = 0 ) of the Jacobi polynomials.
2.3. The weak topology In this section we prove Riesz's representation theorem (Theorem 2.3.1), which describes the general continuous linear functional on a Hilbert space X.
98
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
By means of this result, we establish certain properties of the weak topology on 2 (see Section 1.3).
2.3.1. THEOREM.If X is a Hilbert space and
YE%,
the equation
cpy(x)= ( x , y ) ( ~ €defines 2 ) a continuous linear functional cpy on X, and llcpyll = Ilyll. Each continuous linear functional on X arises, in this way, from a unique element y of X.
Proof. For each y in 2, Icpy(x)l = I ( X 9 Y ) I
< llxll llYll
(XEWr
with equality when x = y. Thus cpy is a continuous linear functional on X, and IIcpyll = IlYll.
If cp is a non-zero continuous linear functional on X, the closed subspace Y = cp- '(0) is not the whole of X, so Y l # ( 0 ) .Let u be a unit vector in Y l , and note that cp(cp(u)x - cp(x)u) = cp(u)cp(x) - cp(x)cp(u) = 0
for each x in 2.It follows that cp(u)x - cp(x)u E Y , and since u E Y '-,we have 0 = = cp(u)<x,u>
- cp(x).
Hence cp(x) = cp(u)(x,u> = < X , Y >
( X E W ,
~
where y = cp(u)u. This shows that cp has the form cpy for some y in X (and the same conclusion is apparent when cp = 0). If, also, cp = cpz with z in X, then IlY - zll = Ilcpy-ZII = IIcpy - cpzll = IIcp - cpII = 0,
whence y = z; so there is only one y in X for which cpy = cp.
2.3.2. COROLLARY. If2 is a Hilbert space, the equation (JY)(X) = < X , Y >
( X , Y E W
defines a conjugate-linear norm-preserving mapping J from X onto the Banach dual space 2'. Proof. Since Jy is the continuous linear functional cpy occurring in Theorem 2.3.1, J is a norm-preserving mapping from 2 onto 2',and it is evident from the conjugate linearity of the inner product in its second variable that J also is conjugate linear.
2.3.3. COROLLARY. Every Hilbert space is reflexive. Proof. Suppose that X is a Hilbert space and @ is a continuous linear functional on its Banach dual space Xp. With J defined as in Corollary 2.3.2,
99
2.4. LINEAR OPERATORS
the equation P(Y) = @(JY) (VEX) defines a bounded linear functional cp on M.By Theorem 2.3.1, we can choose z in Ad so that ~ ( y=) ( y , z ) for each y in H. Every element of &” has the form Jy, with y in X , and
+
- -
Y ) = cp(Y) = ( Y - 2 ) = (Z.Y> = ( J Y ) ( Z ) = +(a. Since each bounded linear functional @ on %“ arises in this way from an element z of .X. it follows that X is reflexive. H @(+) = W
2.3.4. COROLLARY. Suppose that family of all sets of the form
3y; is
a Hilbert space and x o e X The
1,..., n)}, with y , , . . . ,y. ( E X ) and E ( > 0 )preassigned, is a base of neighborhoods of xo in the nveak topology on X. A net { x u }of elements of JV converges weakly to xo f and only if(xa,y ) (xo, y ) f o r each y in A? The closed unit ballof X is weakly compact . { x ~ X : J ( x - . x ~ , y< ~E ()Jl=
Proof. Since X is reflexive, its unit ball is weakly compact by Theorem 1.6.7. The remaining assertions in the corollary are simply reinterpretations of the appropriate Banach space definitions, taking into account the information in Theorem 2.3.1 concerning the form of continuous linear functionals on .?F.
2.3.5. PROPOSITION. v a net {xu}of rectors in a Hilbert space X converges weakly to an element x of X, and Ilxall
Ilxll?
then { x u }converges to x in the norm topology. Prooj: Since (xav
x> 7(-v, -v> = IIxII’,
we have llxu - XI(’ = I(X,((* - 2 Re(xa,x)
+ llxll’ y 0.
H
2.4. Linear operators We recall from Theorem 1.5.5 that a linear operator T, from a normed space 3E into another such space ?iY, is continuous if and only if it is bounded, in
100
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
the sense that there is a real number c such that IITxll 6 cllxll for each x in 3. The set .%?(3E,%) of all such bounded operators is itself a normed space, when the norm of an element T is defined to be the least such constant c; equivalently, IlTll = sup{IITxl/:xEX, llxll Q 1).
By Theorem 1.5.6, 99(3,?4’)is a Banach space when 9is a Banach space. In this section we obtain more detailed information concerning bounded linear operators acting on Hilbert spaces. General theory. Suppose that X and X are Hilbert spaces. By a conjugate-bilinear functionalon X x X, we mean a complex-valued function b on H x X that is linear in the first variable and conjugate-linear in the second. We say that such a functional b is bounded if there is a real number c such that Ib(x,y))6 c1)x11))yllfor all x in JV and y in X. When this is so, we denote by llbll the least possible value of c, which is given by
llbll = ~ ~ P I l b ( x , y ) l : x ~ ~ , y ~ 6 ~1,llvll , l l xQ l l 1). When X = Jr, we refer to a conjugate-bilinear functional “on X,” rather than “on .X x X.” 2.4.1. THEOREM. If Jr, X are Hilbert spaces and T E B ( . # ” X ) ,the equation
(1)
b,(X,y) = ( T x , ~ )
(XE
X, Y E X )
defines a bounded conjugate-bilinearfunctional bT on 2 x X, and llbTll = 11 TI[. Each bounded conjugate-bilinear functional on 2 x X arises in this way from a X). unique element of 99(
=bdx,
Tx) 6 11br11 l l ~ lIITxll, l
show that b , is bounded and llbTll = llTll. If b is a bounded conjugate-bilinear functional on # x .X and XEX, the equation ( W ( Y ) = b(x,Y )
(Y E . X )
defines a linear functional U x on .X. Since I(Ux)(y)l
llhll llxll Ilull.
U x is bounded, and I(Uxl(6 llbll IIxII. From this, together with the linearity of b in its first variable, it is evident that U is a bounded conjugate-linear mapping
2.4. LINEAR OPERATORS
101
from X' into the Banach dual space X' of X. With J the norm-preserving conjugate-linear mapping from X onto X', as defined in Corollary 2.3.2, J - ' U is a bounded linear operator T from X' into X ; moreover,
'
~ T ( .Y) x , = (Tx,V ) = ( J - U X ,y ) =
( y , J - ' W = ( W ( Y ) = b(X,Y),
so b = bT. If, also, b = bs for some S in %(X,X ) , we have
11s - TI1 = Ilbs- TI1 = llbs - b ~ l=l Ilb - bll = 0, and S = T.
2.4.2. THEOREM.Suppose that X, X, and 9 are Hilbert spaces. T E B ( X ' ,X ) , there is a unique element T* of B(X,X ) such that ( T*x, Y ) = ( x , 0)
(2)
(X
If
E X, YE*).
Moreover, (i) (aS + bT)* = cTS* + 6T*, (ii) (RS)* = S*R*, (iii) (T*)* = T, (iv) lIT*TlI = llT112, (v) IIT*ll = IlTlll whenever S, T E B ( # , X ) , R E B ( X , ~ )and , a,bE@. Proof. The equation b(x,y ) = ( x , Ty) defines a conjugate-bilinear functional b on X x X . With the notation introduced in Theorem 2.4.1,
Ib(x,y)l = I(TY,X)l = IbT(Y7X)L so b is bounded and ((b(( = ((bT(( = (IT((. B y the theorem just cited, there is a unique element T* of %(X,X ) for which ( T *x , y ) = b(x,y) = ( x , T y )
( x E X ,Y E % ) ;
and IIT*JI = llbll = (ITIJ.For each x in 2, IITx((*= ( Tx, T x ) = (T*Tx, X )
< IIT*TII llxll',
so
IITII' < llT*TlI < llT*Il IlTll = lITI12, and (iv) follows. It remains to prove (i), (ii), and (iii). These follow from the identities ((cTS*
+ ~ T * ) x , Y=) cT(S*x,y) + 6(T*x,y) = a(x, S y ) + 6(x, T y ) = ( x , (aS + bT)y),
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2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
( S * R * z , y ) = ( R * z , S y ) = (z,RSy), ( T Y , ~=) (x, TY) = ( T * x , Y ) = ( Y , T * x ) ( x E X ,EX, Z E ~ ’ ) , together with the fact (proved above) that, for each bounded linear operator T, there is only one operator T* satisfying (2).
When T E W ( S ,X ) ,the operator T* occurring in the preceding theorem is called the (Hilbert)adjoint of T. When dealing with Hilbert space operators, it is understood that the term “adjoint” refers to the Hilbert adjoint T*, rather than the Banach adjoint Tp discussed at the end of Section 1.6, unless there is an explicit indication to the contrary. While T* maps X into X, T#maps the Banach dual space X’ into X’. If J 1 : X + S’and J2 : X + X‘ are the norm-preserving conjugate-linear mappings defined as in Corollary 2.3.2, we have T* = J F 1 P J 2 ,since (x, J ; ’ T J Z Y )= (FJZYMX) = (JZY)(TX) =(T~,Y)
for all x in S and y in X. We now specialize to the case in which X = X. With T in W ( X ) ,the adjoint T* is again in W ( S ) .Accordingly, B ( X )is a complex Banach algebra with an “adjoint operation” * satisfying (i) (ii) (iii) (iv)
+
(aS bT)* = (5S* (ST)* = T*S*, (T*)* = T, IIT*TIl = IITIIZ,
+ 6T*,
for all S, Tin W ( X ) and a, b in @. These four basic properties (from which a fifth, I(T*ll = IlTll, is easily deduced) form part of the basic background material for much of our subsequent work. When T EW ( X ) and x, y E S, we have (3)
+Y> - (T(x -Y ) , X - Y> + (i T ( x + iy), x + iy) - (i T(x - iy),x - i y ) .
4(TX,Y) = ( T ( x + Y ) , X
This relation is called the polarization identity. It is a particular case of the second relation stated in Proposition 2.1.7 (and is essentially equivalent to it). When T = Z, it reduces to the expression of inner products in terms of norms, already noted in Proposition 2.1.8. 2.4.3. PROPOSITION. If S and T are bounded linear operators acting on a Hilbert space 2 and (Sx, x> = (Tx, x) for each x in S, then S = T. Proof. Since ( S X , ~ )= ( T x , ~for ) each vector x , it follows by polarization (that is, by means of the polarization identity (3)) that (Sx, y ) = ( T x , y ) for all x and y in X. Hence S and T give rise to the same conjugate-
103
2.4. LINEAR OPERATORS
bilinear functional on 2,and from the uniqueness clause in Theorem 2.4.1, S=T. 2.4.4. REMARK.For linear operators acting on real inner product spaces, the analogue of Proposition 2.4.3 is false. For example, the equation T ( x l , x 2 )= ( x 2 ,- x l ) defines a non-zero operator T acting on R2, and ( T x , ~=) 0 for each x in R2. 2.4.5. PROPOSITION. If X and X are Hilbert spaces and T E B ( X X , ), then T is an isomorphism from X onto X if and only if it is invertible, with T - ‘ = T*.
Proof. The operator Tis an isomorphism from X onto X if and only if it is both invertible and norm preserving. Accordingly, we may suppose that T has an inverse, and it suffices to show that T preserves norms if and only if T*T = I (which is equivalent to T - = T* when T is invertible). Since
( T * T x , x ) - ( x , x ) = ( T x , T x ) - ( x , x ) = IITX112 - I x I ’ , for each x in X, the required result follows from Proposition 2.4.3.
Classes of operators. A bounded linear operator T, acting on a Hilbert space 2,is said to be self-adjoint if T* = T, and unitary if TT* = T * T = I. Both these conditions imply that T is normal, by which we mean that TT* = T* T. We say that T is positive if ( T x , x) 2 0 for each x in X. For every Tin $#(X),
(T*Tx,x)=(Tx,Tx)>O
(xEX),
so T * T is positive; we shall see later (Theorem 4.2.6(iii)) that each positive operator arises in this way. A conjugate-bilinear functional b on X is said to be symmetric if b(y,x ) = b(x,y ) for all x and y in X, and positive if b(x, x ) 2 0 for each x. With T in &I(%), and bT the conjugate-bilinear functional defined by bT(x,y ) = ( T x ,y ) , it is evident that T is positive if and only if bT is positive; moreover, ~~
bT*(X,y)= ( T * x , y ) = ( x , TY) = (Ty, x> = bT(y, x), so bT = bT*(equivalently, T is self-adjoint) if and only if bT is symmetric. Suppose that Tis a bounded linear operator acting on a 2.4.6. PROPOSITION. Hilbert space 2. (i) T is self-adjoint if and only if ( T x , x > is real for each x in X. In particular, positive operators are self-adjoint. (ii) T is unitary if and only fi T is a norm-preserving (equivalently, inner product-preserving) mapping from X onto 2. (iii) T is normal ifand only if IITxll = IIT*x(lfor each x in X.
104
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
Proof. (i) Since
( T x , x ) - (T*x, x ) = ( T x , x ) - ( x , T x ) = 2iIm( Tx, x ) , it follows from Proposition 2.4.3 that T = T* if and only if ( T x , x ) is real for each vector x. (ii) An element T of A?(%) is unitary if and only if it is invertible, with inverse T * ; so the assertion (ii) is a special case of Proposition 2.4.5. (iii) Since ( T * T x , x ) - ( T T * x , x ) = ( T x , T x ) - ( T * x , T * x ) = IITxll’
-
((T*x(~’,
if follows from Proposition 2.4.3 that T*T = TT* if and only if [(TxlI= IIT*x(l for each vector x. From part (ii) of the above proposition, a “unitary operator” acting on a Hilbert space X is simply an isomorphism from X onto itself. We shall sometimes describe isomorphisms between different Hilbert spaces as unitary operators (or unitary transformations). 2.4.7. REMARK.When X is an infinite-dimensional Hilbert space, a norm-preserving linear operator Tacting on X is not necessarily unitary, since its range may fail to be the whole of X . For an example in which this occurs, consider the operator that acts on the sequence space l2 (see Example 2.1.12), and maps the vector ( x 1 , x 2 , x 3 ., ..) onto ( 0 , x 1 , x 2 ,...). H 2.4.8. LEMMA. r f T is a bounded normal operator on the Hilbert space 2 and 0 < inf{llTxll:xEX, llxll = 1) ( = a ) ,
then T has a bounded, two-sided inverse, and 11 T - ‘ ( 1
= a - l.
Proof. By Corollary 1.5.10, T is a bicontinuous linear mapping from 2 onto the range W(T ) of T, and the inverse mapping T - : W(T ) -,X satisfies llT-lll = a - l ; moreover, W(T)is complete, and is therefore closed in X. It remains to prove that W(T)= X. If W(T)# 2,there is a unit vector x in W(T)*; by Proposition 2.4.6(iii),
0 = ( x , TT*x) = ( T*x, T*x) =
IIT*xll’ = IITx11’ 2 a’,
a contradiction. Thus W ( T ) = S. The simple properties of the adjoint operation * on a(%), as set out in (i), . . . ,(iv) in the discussion preceding Proposition 2.4.3, in some respects resemble those of the process of complex conjugation for elements of the scalar field @. The analogy can usefully be pressed a good deal further. The self-
105
2.4. LINEAR OPERATORS
adjoint elements of g ( X ) (those for which T = T*) correspond to real numbers (the scalars for which a = a). Parallel to the expression of a complex number in terms of its real and imaginary parts, each Tin B ( X )can be written (uniquely) in the form H + iK, with H a n d K self-adjoint operators acting on X ; moreover
H = i ( T + T*),
K = $i(T* - T).
The operators H a n d K are sometimes called the “real” and “imaginary” parts of T, and denoted by Re T and Im T, respectively. It is easily verified that T is normal if and only if H and K commute. The classes g9, Jf and g ( i f ) + of all self-adjoint, unitary, normal, and positive operators (respectively) on X are norm-closed subsets of g(&). Moreover, 9is a multiplicative group, while .Y is a real-linear subspace (that is, aH + bKEY whenever H, K E Y and a, b E IF!). From Proposition 2.4.6(i), a(.%)+ is a subset of 9; it is apparent that aH b K E B ( H ) + whenever H , K E B ( X ) +and a, b are non-negative real numbers. If both HE^?(#)+ and - HEg ( X ) ’ , then (Hx, x) = 0 for each vector x, and H = 0 by Proposition 2.4.3; so B ( X ) +n - g(#)+= (0). In view of the properties of a(#)+just stated, there is a partial order relation < on Y: in which H < K if and only if K - HEB ( X ) +As . in the case of real numbers, one can add such inequalities between self-adjoint operators and multiply throughout by non-negative scalars. Multiplication by negative real numbers reverses the inequalities. Moreover, if H, K e g T E ~ ( % ) ,and H < K, then T*HT < T*KT; for K - HEB(.%)+,and therefore
+
(T*(K - H)Tx,x) = ( ( K - H)Tx, Tx) 2 0 ( x ~ i f ) , whence T*(K - H ) T E ~ ( # ) + .For each H in Y: the operators llHllZ f H a r e positive, since IIHll(x,x) f (Hx,x) 2
llHll llxll’
-
WxlI IIxII 2 0
(xEJ~‘).
It follows that -
IIHIII < H ,< llHlll
( H = H* E B ( X ) ) ,
and that each self-adjoint operator H can be expressed as the difference of positive operators llHllZand IIHIJI- H. Each Tin a(#)has the form H + iK, with Hand K self-adjoint, and is therefore a linear combination of at most four elements of B ( X ) + . We shall see later, in Theorem 4.1.7, Theorem 6.1.2, and Proposition 4.2.3, that each element of a(.%) is a linear combination of at most four unitary operators, and has a “polar decomposition” analogous to the expression of a complex number in terms of its modulus and argument; moreover, there is an “optimal” way of expressing a self-adjoint operator as a difference of positive operators.
106
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
2.4.9. REMARK. For the study of bounded linear operators, a Hilbert space is much more convenient than a real inner product space. This is due, in part, to the fact that, in contrast with the complex case, a non-self-adjoint operator acting on a real inner product space cannot be expressed as a linear combination (necessarily with real coefficients) of self-adjoint operators. 2.4.10. EXAMPLE.Suppose that Y is an orthonormal basis in a Hilbert space X, g is a bounded complex-valued function on Y, and
k = suP{lg(Y)l:YE y > . For each x in 2,
1 Ig(Y)(x,Y)lz 6 kZ 1 I(X,Y>lZ YCY
= k211X112;
YCY
so the equation (4) defines a vector Tx in X, and
It is apparent that Tx depends linearly on x, so T is a bounded linear operator on 2, with IlTll 6 k. From (4),
TY = d Y l Y
(6) From this,
(YE
Y).
IlTll 2 suP{llTY/l:YE Y ) = suP{lg(Y)l:YE Y ) = k, and so (7)
IlTll = suP{lg(Y)l:YE y>. By the same process, the complex-valued function defined by g ( y ) = g(y),gives rise to a bounded linear operator S on X. For all u and u in
a,
3,
(SU, u>
=
(
1 s(v)(u,v>r, u>
YSY
107
2.4. LINEAR OPERATORS
Hence S = T* and, in parallel with (4), (9,and (6), we have
(9)
IIT*xll =
(1la(Y)(x,Y)l2)
1/2 3
YEY
(10) T*y= g(ylY ( Y E r). From (5) and (9), IITxll = IIT*xJI for each vector x, so T is normal; alternatively, this can be proved by a simple direct calculation, which shows that (1 1)
TT*x = T*Tx =
1 Ig(v)12(x,y)y. YEY
Similar calculations show that sums and products of bounded complex-valued functions correspond to sums and products of the associated operators; in particular, all such operators commute. If Tis self-adjoint,it results from (6) and (lo) that g(y) = g(y)for each y in Y; the reverse implication follows from (4) and (8). Thus Tis self-adjoint if and only if g is a real-valued function. From (4) (Tx, x> =
1 g(Y)(X,Y)(Y? x) = c s(Y)l(x,v>12; YEY
YEY
in particular, (Ty,y) = g(y) (YE Y). Hence Tis positive if and only if g takes non-negative real values throughout Y. If Tisunitary, it follows from (6) that Ig(y)l = IITyll = llyll = 1 for each y in Y. Conversely, if Ig(y)I = 1 ( Y E Y), we deduce from (1 1) that TT*x = T*Tx =
1 (x,y)y
=x
(~€2).
YEY
Hence T is unitary if and only if Ig(y)l = 1 for each y in Y. 2.4.11. EXAMPLE. Suppose that m is a a-finite measure defined on a o-algebra Y of subsets of a set S, gE L , (= L,(S, Xm)),and k is the essential supremum of 191. For each x in the Hilbert space S = L2(S,,Y;’rn), the equation
(M,x)(s) = g ( M 4 (SE S) defines a measurable function M,x on S, and I(M,x)(s)l ,< klx(s)l almost everywhere. Accordingly, M , x E(= ~ L 2 ) and IIMgxll < kllxll since (12)
108
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
It is apparent that M,x depends linearly on x; so M B is a bounded linear operator on X, and llM,ll 6 k . If 0 < a < k, the measurable set { s E S : Ig(s)l > a } has positive measure, and so has a measurable subset Y such that 0 c m( Y ) < 00, since m is rr-finite. The characteristic function y of Y is a non-zero vector in 2, I(Ma)(s)l 2 aly(s)l for each s in S ; hence llM,gll 2 allyll, and so llMJ 2 a. From this, llM,ll = k ; that is,
where (1 1(, is the usual norm on L,. With g defined by g(s) = g(s) (sE S ) , we have g E L , and
I-
therefore
M,* = M i .
(14) It is apparent that (15)
M,,+~, = a ~ +,b ~ , , M ,, = M , M ,
um,,
a,b~c).
From this, M , and M , commute for all f and g in L,; in particular, M g commutes with its adjoint Ms, and is therefore normal. Since llMg
- M,*ll= llMg-ull= esssup Ids) sss
-
go(,
M Bis self-adjoint if and only if g(s) is real for almost all s in S. Moreover, <M,x, x> =
s,
s(s)lx(s)tz dm(s)
(x E 2 1,
from which it follows that MBis positive if and only if g(s) 2 0 for almost all s. With u in L , defined by u(s) = 1 (SE S),M u= I and
Thus M gis unitary if and only if Ig(s)l = 1 for almost all s in S . It is well known that a linear operator T, acting on a finite-dimensional complex vector space, has at least one eigenvector x, with corresponding
2.5.
THE LATTICE OF PROJECTIONS
109
eigenvalue A (that is, x # 0 and Tx = Ax). As indicated in (6), the operator considered in the preceding example has eigenvectors forming an orthonormal basis Y. In contrast, the present example permits the construction of selfadjoint and unitary operators that have no eigenvalue. For this purpose, note that the equation MBx= cx (with g in L,, x a non-zero vector in X (= L z ) , and c in C ) implies that g(s) = c almost everywhere on the measurable set {SE S: X ( S ) # 0 } , which has positive measure. Accordingly, MBhas no eigenvalue if g assumes each of its values only on a null set. When m is Lebesgue measure on the o-ring Y of Bore1 subsets of the interval S = [0,1] and
f ( s ) = s,
g(s) = exp(is)
(sE S),
M J is positive, M , is unitary, and neither has an eigenvalue. H 2.5. The lattice of projections
If Y is a closed subspace of a Hilbert space X, Theorem 2.2.3 asserts that each vector in Z can be expressed uniquely in the formy + z, with y in Y and z in Y'. From Theorem 1.1.8, the equation (1)
E(y
+
Z)
=y
( Y E Y,
ZE
Yl)
defines a linear operator E acting on Z,the projection onto Y, parallel to Y *. Moreover, E 2 = E, (2)
Y = { E x : x E X }=
EX: Ey = y } ,
and Y ' = { zE Z : Ez = 0).We call E the (orthogonal)projectionfrom Z onto Y. Note that I - E is the orthogonal projection from X onto Y l , because (Y')' = Y, and
(I-E)(z+y)=z
(zEY', ~ E Y ) .
Since (y, z ) = 0, when y E Y and z E Y ', we have
IIm + 4112= llYIl2 G llY1I2 + 1 1 ~ 1 1 2 = IlY + z1IZ, (E(Y + Z),Y
+ z> = ( y , y + z> = lIy1lZ2 0.
It follows that E is bounded, with IlEll ,< 1, and is positive (hence, also, selfadjoint). Since Ey = y ( y e Y), IlEll = 1 except in the case in which Y = {O} and E = 0. Moreover (3)
Y = { x € x :((Ex((= ((x((}.
Conversely, suppose that E E ~ ( Xand ) E Z = E = E*. From Theorem 1.1.8, E is the projection from X onto the closed subspace Y defined by (2),
110
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
parallel to the closed subspace 2 = {z E X :Ez = 03.Since Z = {zEX:(Ez,x)=OforeachxinX} = {zEX: ( z , E x ) = 0 for each x in X } ,
it follows from (2) that Z = Y l . Hence E is the projection onto Y, parallel to Y l ; that is, the orthogonal projection from X onto Y. In the context of Hilbert space theory, it is understood that the term “projection” refers to an orthogonal projection unless there is an explicit statement to the contrary. The following proposition summarizes the results of the preceding discussion. 2.5.1. PROPOSITION. Relations (1) and (2) establish a one-to-one correspondence between closed subspaces Y of a Hilbert space X andprojections E acting on X. A projection E is apositive operator, and IlEll = 1 unless E = 0. The projections are precisely the self-adjoint idempotents in B ( X ) . The projections acting on a Hilbert space X inherit from the set of all selfadjoint operators the partial order relation < described in the discussion preceding Remark 2.4.9. 2.5.2. PROPOSITION. I f E and Fare theprojections from a Hilbert space 2 onto closed subspaces Y and 2, respectively, the following conditions are equivalent : (i) (ii) (iii) (iv) (v)
Y E 2; FE = E ; EF = E ; IlEXlI < IlFXll E < F.
( X E Z ) ;
Proof. If Y G Z, then, for each x in X, E X EY c Z , and therefore FEx = Ex; so (i) implies (ii). If FE = E, then EF = (FE)* = E* = E, whence (ii) implies (iii). If EF = E, then llExll = llEFxlI < llFxll for each x in 2,since IlEll < 1 ; so (iii) implies (iv). Since
( E x , x ) = ( E ’ x , ~ )= ( E x , E x ) = I ~ E X I I ~ , and similarly ( F x , ~ = ) (JFx~)’, it is apparent that (iv) implies (v). If E < F, then, for each y in Y, llY11’ = (EY,Y) i(FY9Y) = llFYllZ < llYI1’;
whence IIFyII = Ilyll, and Y E Zby (3). Hence, (v) implies (i). When the five equivalent conditions in Proposition 2.5.2 are satisfied, we describe E as a subprojection of F.
111
2.5. THE LA'ITICE OF PROJECTIONS
From the equivalence of conditions (i) and (v) in Proposition 2.5.2, it follows that the partial ordering of projections (as self-adjoint operators) corresponds to the partial ordering of closed subspaces by the inclusion relation E . Given any family { Y,} of closed subspaces of a Hilbert space X , there is a greatest closed subspace A Y, that is contained in each Y, and a smallest closed subspace V Y, that contains each Y,. Specifically, A Y, is n Y,, while V Y, is the closed subspace [U Y,] generated by U Y,. From this it follows that each family { E,} of projections acting on A? has a greatest lower bound A E, and a least upper bound V E, within the set of projections (ordered as selfadjoint operators). Of course, the projections A E, and V E, correspond to the closed subspaces A E , ( X ) and V E , ( X ) , respectively. We write E A F and E v F for the lower and upper bounds (often called the intersection and union) of two projections E and F. Since the mapping E -,I - E reverses the ordering of projections, we have
V ( I - E,) = Z - A E,, A(Z - E,) = I - V E, (4) for each family {E,,} of projections. This gives corresponding relations A Y ; = ( V Y,)' V Y t = ( A Yo)', (which can easily be verified independently) for each family { Y,} of closed subspaces of X. Our next few results are concerned with commuting sets of projections.
If E and F are commuting projections acting on a 2.5.3. PROPOSITION. Hilbert space X, corresponding to closed subspaces Y and Z , respectively, then EvF=E+F-EF,
EAF=EF,
YvZ=Y+Z.
In particular, the linear subspace Y + Z of X is closed. Proof. When U E Y A Z , Eu = Fu = u, so EFu = u. For each x in Y l , Ex = 0, so EFx = FEx = 0 ; similarly, EFx = 0 for each x in Z'. Since ( Y A Z)' = Y' v Z', it follows from the linearity and continuity of EF that EFv = 0 whenever u E ( Y A Z ) l . We have now shown that EF(u + v ) = u
(UE
Y A Z , V E ( YA Z)'),
whence EF is the projection from X onto Y A 2. By applying the same result to the commuting projections I - E and I - F, we have
( I - E ) A (I - F) = ( I - E)(I - F), and (4) gives
E vF= I
- (I - E ) A
( I - F) = I
- (I - E)(I - F ) = E
For each x in Y v 2,
x = ( E v F ) x = ( E + F - EF)x = y
+ Z,
+F -
EF.
112
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
where y = ( E - EF)x€ Y and z = F x E Z . Thus Y v Z G Y + Z , and the reverse inclusion is apparent. H 2.5.4. COROLLARY. Suppose that Eand Fare theprojections from a Hilbert space X onto closed subspaces Y and Z , respectively. Then EF = 0 if and only i f Y is orthogonal to Z , and when this is so, YvZ=Y+Z.
EvF=E+F,
Proof. Since Y = E ( X ) , Z = F ( X ) , and (EFu, v ) = (Fu, E v ) for all u and v in X, it is evident that Y is orthogonal to Z if and only if EF = 0. When this is so, FE = (EF)* = 0 (= EF), and it follows from Proposition 2.5.3 that
EvF=E+F,
YvZ=Y+Z.
2.5.5. COROLLARY. I f E and F are the projections from a Hilbert space A? onto closed subspaces Y and Z , respectively, and E < F, then F - E is the projection from X onto Z A Y l . Proof. By Proposition 2.5.2, EF = FE = E, so the projections F and I - E commute. From Proposition 2.5.3, F(Z - E ) (= F - E ) is the projection F A (I- E ) from X onto Z A Y l . H
Projections E and F, from X onto closed subspaces Y and Z , commute if and only if Y A ( Y A Z)* and Z A ( Y A Z)* are orthogonal (loosely, if and only if the spaces Y and 2 are “perpendicular”); for these spaces are orthogonal if and only if 0 =(E- E and EF = E
A
A
F)(F- E
A
F) = E F - E
A
F,
F if and only if (see Proposition 2.5.3) EF=EAF=(EAF)*=FE.
2.5.6. PROPOSITION. Zf{E,,} is an increasing net ofprojections acting on a Hilbert space X, and if E = V E,,, then Ex = lim, Eaxfor each x in X.
is an increasing net of closed subspaces of u,, and isSince a linear subspace of and has norm closure Suppose > Since we can choose an element in one of the
Proof. {E,,(X)} EJX) X E(X). XEX E 0. Ex E E ( X ) , y subspaces E , , ( X ) so that IJEx- yll < E . When b 2 a, we have En 6 Eb
< E,
y E E J X ) G &(A?)
G
E(X),
and thus llEx - Ebxll = IIE(Ex - Y ) - ‘%(Ex - Y)II
< IIE - Ebll IJEx- YII < &.
A?,
2.5. THE LATTICE OF PROJECTIONS
113
2.5.7. COROLLARY. I f {E,} is a decreasing net of projections acting on a Hilbert space X, and if E = A E,, then Ex = lim, E,xfor each x in 2. ProoJ In view of (4), it sufices to apply Proposition 2.5.6 to the increasing net { I - E,}.
By an orthogonal family of projections we mean a family (Ea)aeAof projections such that E,E, = 0 (equivalently, E , ( 2 ) is orthogonal to &(A?)) whenever a and b are distinct elements of A. 2.5.8. PROPOSITION. If(Ea)aEEn is an orthogonalfamily ofprojections acting on a Hilbert space X, E = V E,, and x E X, then Ex = 1E,x; the sum converges in the norm topology on X. Proof. When A is a finite set, it follows from Corollary 2.5.4, together with a straightforward argument by induction on the number of elements in A, that E = CaEaE,. When A is an infinite set, let 9denote the class of all finite subsets of A ;for each ff in Edefine G, = CEFE,. By the preceding paragraph, G , = VaEF E,, so (GF,F E E 2 ) is an increasing net of projections, and VG, = V{ FE3F
v E,: ad
IFEF
1v =
E, = E.
aEA
By Proposition 2.5.6, Ex is the limit, in norm, of the net (G,x,F E 4 s ) ;that is (since G,x = CEFE,x), CasAE,x converges in norm to Ex. H When H is a Hilbert space and x E X, the equation p,( 7') = I ITxll defines a semi-normp, on B ( X ) .The family of all such semi-norms separates the points of B ( X ) ,in the sense of Theorem 1.2.6, and so gives rise to a locally convex topology on B ( X ) ,the strong-operator topology. In this topology, an element To of B ( 2 )has a base of neighborhoods consisting of all sets of the type V ( T o : x ,..., l x , ; E ) = { T E B ( X ) : I I ( T -To)xjII c ~ ( j 1=,..., m)},
where xl,. . . ,X,EXand E > 0. In fact, it suffices (and is sometimes convenient) to take E = 1, since
...)X , ; & )
V(T0:Xl)
= V ( T O : E - l X I ) . . .E, - l X m ; l ) .
In a similar way, one can introduce the strong-operator topology on B(X,X ) ; the semi-norms and basic neighborhoods are defined as above, but the vectors Tx, ( T - To)xjlie in X . The strong-operator topology can be described as the restriction to W ( X ) of the point-open topology on mappings from X into X, with X in its norm topology. It is apparent that V ( T o : x , ,. . .,x,,,;E)contains the open ball with center To and radius 6 , provided that bllxjll < E for each j = 1,. . ., m .
114
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
Accordingly, the strong-operator topology is coarser than the norm topology in fact, it is strictly coarser when % is infinite-dimensional (see on a(&'); Exercise 2.8.32). In working with the strong-operator topology, it is often possible (and useful) to confine attention to those basic neighborhoods V(To:x l , . . . ,x , ;E ) in which x l , . . .,x,,, are drawn from a suitable preassigned subset 9 'of %. We mention two cases in which this occurs. First, when 9'has (algebraic) linear span %, the sets V(To:y l , . . . ,y , ;S), where 6 > 0 and yl ,. . . ,y, E 9, already form a base of neighborhoods of To. Second, if Y has closed linear span 2, while 93 is a bounded subset of 93(&') and ToE 93, the sets @>O,
Wo:y1,...,yn;6)ng
yl ,...,y , E Y )
form a base of neighborhoods of To in the (relative) strong-operator topology on 8 .We shall prove the second of these assertions (the first follows from a similar, but simpler, argument). Wemay assume that IJTJI < Mfor each T i n a . Given any positive E , and x l , . . . ,x , in S,each xi can be approximated within c/4M by a finite linear combination of elements of 9 Hence we can choose y , ,. . . ,y , in Y and scalars a j k (some of which may be 0) such that n
Let 6 be a positive real number such that 26(G= lajkl) c
j = 1,. . . ,m. It now suffices to show that
E
for each
V( To:yl ,. . . ,y, ;6 ) n 9 G V(To:xl, .. . ,x,,,;E ) ;
and this results from the fact that, for J = 1,. . . ,m, n
n
-
TO>xjll
<
-
TO)(xj -
1
ajkYk>ll
+ 11
n
< IIT -
llxj
-
1 k= 1
ajk(T -
TO)ykll
k=l
k= 1
n
ajkykll
+
bjkl
ll(T - TO)ykll
k= 1
when T (as well as To)lies in &and I ll(T - To)ykll < 6 (k = 1,. . . ,n). We can summarize the results of the preceding paragraph as follows: a set of vectors with algebraic linear span % suffices to determine the strongoperator topology on O ( X ) ; a set of vectors with closed linear span % suffices to determine the strong-operator topology on bounded subsets of
gw ).
2.5. THE LATTICE OF PROJECTIONS
115
2.5.9. REMARK.From the discussion following Theorem 1.2.18, a net { T j } of elements of A?(&) is strong-operator convergent to To ( E W ( Xif) and only if, given any x in X and any positive E , there is an indexjo such that
<E IlTjx - Toxll ( = p x ( T j - TO)) whenever j 2 j o . In other words, { T j } is strong-operator convergent to To if and only if { Tjx}converges to Tox (in the norm topology on &') for each x in 2.Similarly, { T i }is a Cauchy net, in the uniform structure associated with the strong-operator topology, if and only if { T j x } is a Cauchy (and hence convergent) net of elements of X for each x in X. Of course, these characterizations of convergent and Cauchy nets can be verified by direct reference to the basic neighborhoods that determine the strong-operator topology. By considering the appropriate nets of finite subsums, it follows that a sum C T, of elements o fg ( X )is strong-operator convergent to To if and only if C Taxconverges in norm to Tox for each x in X. Similar comments apply to
gw,w.
The results of Proposition 2.5.6, Corollary 2.5.7, and Proposition 2.5.8 can now be interpreted in terms of strong-operator convergence. An increasing net {E,} of projections is strong-operator convergent to V E,; a decreasing net {En} of projections is strong-operator convergent to A E,; for an orthogonalfamily {,To} of projections, C En is strong-operator convergent to V En. H 2.5.10. REMARK.In observing that the strong-operator topology on W ( X ) is locally convex, we have (by implication) noted that the linear space operations ( S , 7') + S + T : g ( X ) x a(#)--* W(&), (a, 7') + u T : C x
a(#)
4g(3Ep)
are strong-operator continuous. From the form of the basic neighborhoods of To, and since Il(ST - STo)xll < llsll ll(T - To)xll,
II(Ts - ToS)xlI = INT- To)Sxll, it follows that the mappings
T 4 ST,
T 4 TS : B(%) + a(&)
(of left and right multiplication by a fixed operator S) are strong-operator continuous. Since
[ [ ( S T - SoT0)xlI = Ils(T- To)x + ( S - So)Toxll G IlSll ll(T - T0)Xll + INS - ~0)TOXll
< kll(T - To)xlI + IKS - So)Toxll
116
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
when So, T, Toea(%) and S E ( B ( X )=) ~{ A E ~ ( % )llAll : that the mapping ( S , T ) + S T : (a(X))kx
< k}, it follows
a(#) a(%) -P
is strong-operator continuous. We can express these results by saying that multiplication is separately continuous in the strong-operator topology, and jointly continuous provided that the first variable is restricted to a bounded set. Similar comments apply to sums and products of operators acting between different Hilbert spaces. When X is an infinite-dimensional Hilbert space, neither of the mappings
(S, T ) + ST : W ( X ) x
T + T* :
a(%) a(#), +
a(%)+ B ( S )
is strong-operator continuous (see Exercises 2.8.32 and 2.8.33). However, the latter mapping is strong-operator continuous on the set JV of normal elements. For this, it suffices (in view of the nature of the basic neighborhoods) to note that, by Proposition 2.4.6(iii), "T*x - T,*x(l2
+ = IITx1I2 + IITOXI~~ - 2Re(T,*x, Ttx) - 2Re(T*x - T,*x,T ~ X ) = I I T * X ~ ~IIT,*xllZ ~ - 2 Re( T*x, T,*x)
=
IITxllz - IIT,X~~~ - 2Re(x,(T- To)T,*x)
< (IITxll - Il~oxll)(ll~xll + II~oxll)+ 211x11 ll(T - To)7-,*xll d ll(T - To)xll(ll(T- T0)Xll + 211~oxll)+ 211x11 ll(T - To)T,*xll, when EX and T, T o e X H In connection with the result that follows, we recall the convention of the discussion preceding Proposition 1.2.I, concerning uniform structures and completeness. The completeness of the closed balls, alluded to in its statement, is understood relative to the (linear topological) uniform structure associated with the strong-operator topology. 2.5.1 1. PROPOSITION. With X a Hilbert space, closed balls in a(%') are complete in the strong-operator topology. If{ T,} is a Cauchy sequence in a(%), relative to this topology, it is strong-operator convergent to an element of a(*). Proof. If { TJ is a Cauchy net, in the strong-operator topology on 9?(H), and X E X , then {Tax}converges in norm to some element of X (Remark 2.5.9), and thus { IIT,xll} converges. In the case of a Cauchy sequence { Tn}, the convergent sequence { 11 T,xll} is bounded (note, however, that convergent nets of real numbers need not be bounded). The principle of uniform boundedness
117
2.5. THE LAlTICE OF PROJECTIONS
(Theorem 1.8.9) now shows that a sequence in B ( X ) is bounded if it is a Cauchy sequence relative to the strong-operator topology. Hence the second assertion in the proposition is a consequence of the first. To prove the first statement, it suffices to consider only the closed unit ball ( B ( X))1 since every closed ball can be mapped onto (93(~?))~ by a (uniformly bicontinuous) afine transformation T .+ b(T - To).If { T,} is a Cauchy net in (LB(X))~, we can define a mapping T: Z? + X by Tx = lim, Tax. Since T, is linear and IIT,xll < llxll for each x in 2,the same is true of T; so T lies in ( B ( X ) ) land , is the strong-operator limit of {T,,}. H 2.5.12. EXAMPLE.We illustrate some of the results obtained in this section by continuing the discussion of Example 2.4.1 1. With i W the Hilbert space L2 (= L,(S, gm))associated with a cr-finite measure, each g in Lm gives rise to the bounded linear operator M , of multiplication by g. If M , is a projection, the equations M i = M , = M,* imply that [g(s)]' = g(s) = g(s) for almost all s in S. Thus g(s) is 0 or 1 almost everywhere on S. By changing the values of g on a null set, which does not alter M,, we may suppose that g is the characteristic function of a measurable set Y. Conversely, when g is the characteristic function of a measurable set Y, M , is a projection P( Y), since the above argument reverses. We assert that
uj"=
whenever Y1,Y,, Y , , . . . €9For this, let Y = Y j , and denote by y,yl , y 2 ,. . . the characteristic functions of the sets Y, Y 1 ,Y , , . . .,respectively. Since y y j = y j for each j , we have P( Y)P(Y j ) = P( Y j ) ;so P( Y j ) < P( Y ) , and therefore V= ,' P( Y j ) < P( Y). From Corollary 2.5.5, P( Y) - V =; P( Y j )is a projection G, and each vector x (E L,) in the range of G satisfies P(Y ) x = x , P( Y j ) x = 0 ( j = 1,2, . . .). This implies that y(s)x(s) = x(s) almost everywhere, and yj(s)x(s)= 0 almost everywhere, for eachj. Accordingly, each of the sets Zo = { S E S\ Y :x(s) # 0 } ,
Z j = { sE Y j :x(s) # 0)
( j = 1,2, . . .)
is null, and therefore so is {sES:X ( S ) # 0 } (= Uj"&Zj).Thus each x in the range of G is a null function (that is, the zero vector in L,); so G = 0, and P(Y) = Vy= P( Y j ) .This proves the first of the two equations in (5). Upon replacing Y jby S\ Yj in that equation and noting that P(S\ Y j ) = I - P( Y j ) ,we obtain
v ( I - P(Y j ) ) m
=P
j= 1
118
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
Hence m
m
( I - P(Yj)) =
A P( Yj), j= 1
and the proof of (5) is complete. When { Yn}is a pairwise disjoint sequence of measurable sets, {P(Y,)) is an orthogonal family of projections, and in view of Remark 2.5.9, the first relation in (5) becomes
When { Y j } is an increasing sequence of measurable sets, the sequence {P( Y j ) }of projections is increasing. In this case, from Remark 2.5.9 the first equation in ( 5 ) becomes
with the convergence in the strong-operator topology. This is a special case of the following result: if a sequence {A} of functions in Lm satisfies supllfj1) < co,and converges almost everywhere to a functionf(necessari1y in Lm), then { M J j }is strong-operator convergent to M J . Indeed, for each x in Lz, we have lim If($)
-fi(s)121x(s)lz
)@fI
= 0,
-fj<s)I21x(s)l2
< 4K2lx(s)I2
j- m
almost everywhere on S, where K = supllfjll; so lim II(Mr - MJj)xllz= lim j- m
j- m
s.
If($)
-&(S)~~IX(S)~~
dm(s) = 0,
by the dominated convergence theorem. When f, fj are the characteristic functions of Y, and Y j , respectively, we obtain (6). H
u;=l
With each bounded linear operator T acting on a Hilbert space &?, we associate two closed subspaces of 9, the null space { X E A ? : Tx = 0) and the range space, which is the closure [ q X ) ] of the range T ( X ) = { Tx:X E &?} of T. The corresponding projections are called the null projection, denoted by N( T), and the range projection, R(T). When E is a projection, R ( E ) = E and N ( E ) = I - E.
2.5.13. PROPOSITION. r f T is a bounded linear operator acting on a Hilbert space X, then (7)
(8)
R( T ) = I - N( T*), R( T* T ) = R( T*),
N( T ) = I
- R( T*),
N(T*T) = N(T).
119
2.5. THE LATTICE OF PROJECTIONS
Proof. Since { x ~ a ?Tx : = 0}= {
x E ~ ( T: x
, ~= ) 0 for each y in a?}
= { x E . ~ ?(:x , T*y) = 0
=
for each y in a?)
T*(a?)' = [T*(a?)]l,
it follows that N(7') = I - R(T*). Upon replacing T by T* we obtain N(T*) = I - R(T), which completes the proof of (7). For each x in a?, I I T X ~=( (~T x , T x ) = ( T * T x , x ) , so Tx = 0 if (and, obviously, only if) T*Tx = 0; that is, N ( T ) = N(T*T). From this, together with the first equation in (7) (applied to both T*T and T*),
R(T*T) = I
- N(T*T) = I - N ( T ) = R(T*).
Note that, for a self-adjoint element Kof B(H'),(7) gives R(K) = I - N ( K ) .
2.5.14. PROPOSITION. If E and F are projections acting on a Hilbert space a?,
R(E + F ) = E v F,
R(EF) = E - E
A
( I - F).
Proof. Since
+
llExllZ (IFxll' = ( E x , x )
+ (Fx,x) = ( ( E + F)x,x),
for each vector x , it follows that ( E + F)x = 0 if and onlyif Ex = Fx = 0. Thus N(E + F ) = N(E) A N(F) = ( I - E ) A ( I - F), and (7) and (4) yield
R ( E + F ) = I - ( I - E ) A ( I - F ) = E v F. The projections I - E and E and FEx = 0, we have
A
( I - F) are mutually orthogonal. If x E a?
EX = ( I - F)ExE(E A ( I - F))(a?), whence x = ( I - E)x + EX
+ E A ( I - F))(a?). Conversely, each x in the range of I - E + E A ( I - F) can be expressed as y + z, where y = ( I - E ) y and z = Ez = ( I - F)z; and FEx = FEY + FEZ = FE(I - E)y + F(I - F)z = 0. The preceding argument shows that N(FE) = I - E + E A ( I - F); this, E
(I - E
together with (7), gives
R(EF) = I - N(FE) = E - E
A
( I - F). W
120
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
2.5.15. REMARK.If E is a projection acting on a Hilbert space X, the operator U = I - 2E satisfies U = U*, U2 = I, and is therefore both selfadjoint and unitary. Each self-adjoint unitary operator U arises in this way, since it is easily verified that the conditions U = U* = U - imply that :(I - U ) (= E ) is a projection. We now give a simple description of the projection from a Hilbert space 2 onto a closed subspace Y, in terms of an orthonormal basis of Y (of course, Y itself is a Hilbert space). 2.5.16. PROPOSITION. If E is a projection from a Hilbert space S onto a closed subspace Y , and ( Y , , ) , ~is an orthonormal basis of Y , then EX=
C (x,Ya)Ya
( x ~ x ) .
aEA
Proof: Since E x € Y, we have
EX =
1 (Ex,Ya)Ya aeA
(x,EYa)ya =
= aeA
(x,Ya)Yw ae A
2.5.17. REMARK.If (Ea} is an orthogonal family of projections on the Hilbert space 2,then CaEa = E is a projection on X and Ex = CaEax for each x in X (as noted in Remark 2.5.9). Thus CIIEaxl12 = C ( E a x , x ) = < C E a x , x ) = ( E x , x ) a
a
< llx1I2*
a
(This is really an alternative version of Bessel’s inequality - Proposition 2.2.7(i).) If x is in the range of E (in particular, if E = I), then
1IIEaxII’
= ( E x , x> = IIxII’.
a
(This is an alternative version of Parseval’s equation -Theorem 2.2.9(ii).) 2.6. Constructions with Hilbert spaces
In this section we consider subspaces, direct sums, and tensor products of Hilbert spaces, together with related operator-theoretic constructions. Subspaces. Suppose that E is the projection from a Hilbert space A? onto a closed subspace Y. By restriction, the inner product on X gives rise to an inner product on Y, relative to which Y itself is a Hilbert space. With T in B(X),the restriction ETI Y (= ETEl Y )is a bounded linear operator T , acting
2.6. CONSTRUCTIONS WITH HILBERT SPACES
121
on Y, the compression of T to Y. For all y and z in Y, (TYY,Z) = (ETEY,Z) = (Y,ET*EZ), so the adjoint of T, is ET*EIY, the compression of T* to Y. We say that Y is invariant under T, or that T leaves Y invariant (invariant is sometimes replaced by stable), if T Y EY whenever Y E Y. Since Y = { E x : x E X }= {
z E ~EZ : = z},
it is apparent that T( Y ) E Y if and only if TEx = ETEx for each x in X ; that is, T leaves Y invariant if and only if TE = ETE. Since T*(I - E ) - ( I - E)T*(I - E ) = ET*(I - E ) = (TE - ETE)*, it follows that the orthogonal complement Y is invariant under T* if and only if Y is invariant under T. When Y and Y are both invariant under T, we say that Y reduces T ; from the preceding paragraph, this occurs if and only if Y is invariant under both T and T*. In this case ET = (T*E)* = (ET*E)* = ETE = TE. Conversely, if TE = ET, then ET* = T*E, so that ETE = ET = TE and ET*E = ET* = T*E. Thus Y reduces T if and only if T and E commute. Direct sums. When XI,.. . ,Xnare Hilbert spaces and .f is the set of all n-tuples { x l , .. . ,x,} with x j in X j ( j = 1,. . . ,n), there is a Hilbert space structure on X in which the algebraic operations, inner product, and norm are defined by
+ N y l , . . .,yn>= {axl + byl,...,axn + by,}, + (Xn,Yn), ({Xl,...rXnJ, {Yl,...,Yfl})= (X1,Yl) +
aIX1,...,Xn)
* * .
-
I I { X I , . . . r XnJII = [11x1112+ * * + 11xn1121l’’. The resulting Hilbert space X is called the (Hilbert) direct sum of Xl ,. . . ,X n , and is denoted by XI@ * * @ 2”or C; @ X j . For eachj = 1,. . . ,n, the set Xi, consisting of those n-tuples in which all but the j t h entry are zero, is a closed subspace of @ .Xn. The @ mapping U j :X j + XJ,defined by Ujx = (0,. . . ,0, x , 0 , . . .,0} (with x in the j t h position) is an isomorphism from X j onto X i . The subspaces X i , . . . ,XL are pairwise orthogonal, and Vy= X i = X. Suppose next that XI,.. . ,Xn are mutually orthogonal subspaces of a Hilbert space M,and V;= X j = X. By Proposition 2.5.8, the corresponding pairwise orthogonal projections E l , . . . ,En have sum I. The linear mapping U : X - + X , defined by Ux = { E l x ,..., Enx}, carries X j onto A?; ( j = 1, ..., n ) a n d X o n t o X ( = X l @ . . . @ X n ) , a n d i s u n i t a r y s i n c e n
n
122
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
+
+
Its inverse U - carries an element {xl,.. . ,xn}of X onto x1 . . . x,. In view of this isomorphism, we consider X as an “internal” direct sum of Sl ,. . . ,S,and ,, X as the “external” direct sum; occasionally, we identify S with X and Xj with 2;. If X j ,X j are Hilbert spaces and Tj€ X j )( j = 1, . . .,n), the equation
T{X~ . .,. ,xn}= { Tixi,. . . , Tnxn} (XI E X i , . . . ,X, E Hn) defines a linear operator T from Sl @ * * 0 Sninto Xl 0 * * . 0 Xn,the direct sum 2 0 Ti of TI,. ..,Tn. With c = sup{llTjll:j = 1, . . .,n},
we have
IIT{Xl,...,Xn)ll= [IIT1Xlll2+ . . * + IITnxnl1211’2
< [llT111211X1112+ . + ll~nl1211xn11211~2 < C[11X11l2 + . . + 11xn11211/2 * *
*
= Cll{Xl,. . . ,xn)ll;
so T is bounded, with llTil
< c. However, for each j
= 1,.
IITjxll = l l ~ ~ ~ ~ = IIT{O,. . . ,O,x,O,. . . ,O}”
< IlTll Il{O,.
. . ,0, x, 0,
Thus llTjll < IlTll ( j = 1,. . . ,n), whence c Since
* * *
1
. . ,n and x in H j ,
~
O}Il
~
= IITII Ilxll.
< IlTll, and so IlTll = c.
(T*{Yl,. . . ,Yn), {Xl,. . . ,xnl> = ({Yll.. . ,Yn>,T{Xl,. . . ,Xfl}> n
1
= ( { ~ l ~ . . . , ~ n } , { T l ~ ~ , . . . , ~=n x n(Yj,Tjxj> } > j=1 n
=
C (T?Yj,xj> = ( { T r ~ l , *
T,*YnI, Ixl
9 .
j= 1
when xj E Hj and yje X j ( j = 1 , . . . ,n), it follows that
T*{Yl,. * * ,Yn> = {TfYl, * . T,*Ynl. We have now proved that 9
n
II C OTjll=sup{llTjll:j= l , . . . , n } , j=1
i
* * 9
xn}>,
~
~
~
j
123
2.6. CONSTRUCTIONS WITH HILBERT SPACES
and it is apparent that
when Sj, Tj€93(Xj,Xj),Rj€9I(Xj,Yj), and a,be@. So far, we have considered direct sums offinite families of Hilbert spaces. With slight modifications, the same ideas apply also to infinite families. Given Hilbert spaces H, ( U E A), the direct sum 10 2,consists of all families {xa} such that x,EX,( ~ E Aand ) 1llxJ2 < 00. Given two such families {x,} and {y,}, we can apply the inequalities discussed in Example 2.1.12 to the elements { I l a l l } and {llY,lll of / 2 w * We obtain
c
(C IIx,
< Ilxall IlYall < (1 llx.lr2~1~2c~ llYal12)1/2 + Y,I12)1/2 < (C(Ilxall + IlYall)2)"2 < (1llxal12)1/2+ (1 llYA12)1/2 < I(xm.Y,>l
< 0,
Accordingly, the family {x, + yo}is in 10Ha;it followseasily that 10H a i sa pre-Hilbert space when the algebraic structure, inner product, and norm are defined by
+ {Y,}
{x,}
=
{x.
+Yo},
({x,}, { Y J > = C(Xo,Ya)r
C{XJ =
{cx,},
IHxJIl = [Cllxall211/2.
The element {x,} of 10X, is sometimes denoted by 10 x,. We assert that I@ X, is complete, relative to the norm just defined, and is therefore a Hilbert space. For this, suppose that (x("))is a Cauchy sequence in X,, so that each x(")is a family {xr)} of the type considered above. Given ) that any positive real number E , there is a positive integer n ( ~ such IIx(") - x(")II< E whenever m,n 2 n ( ~ )that ; is,
c@ (1)
C Ilxr) - xF)l12< E'
(m, n
n(~)).
aE A
From this, Ilx:") - xr'll < E (m,n2 n ( ~ ) ,U E A); so, for each fixed a, {xr):n = 1,2,. . .} is a Cauchy sequence in X,, and therefore converges to an element x, of A?,.With [F a finite subset of A, it follows from (1) that
C Ilx:"'
- x,(n) 11 2
< E'
(m,n 2 44).
< E2
(n 2 n(E)),
a€F
When m + 00, we obtain
C 11%
ad
- xb"'l12
124
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
and since the last inequality is satisfied for every finite subset IF of A, we have
This shows that the family { x , - x:)}, as well as {x:)}, is in 10i f a when n 2 n ( ~ )Accordingly, . {x,} (= { x , - x:)} + {x:)}) is an element x of Xu, and (2) asserts that JIx- x'")(I< E whenever n 2 n(e). Thus (x("))converges to x; so I@ ifais complete, and is therefore a Hilbert space. With b in A, %b is isomorphic to the closed subspace XL of COX, consisting of those families {x,} such that x, = 0 whenever a # b. We obtain a unitary transformation ub, from z b onto if;,by taking for ubx the family {x,} in which xb is x and x, = 0 (a # b). The subspaces X i (aE A) are pairwise 2,. orthogonal, and V 2;= If { X,} is a family of mutually orthogonal subspaces of a Hilbert space X, and V X, = if,the corresponding projections form an orthogonal family {E,} with (strong-operator convergent) sum I. Just as in the case of finite direct sums, the equation U x = {E,x} defines an isomorphism U from 2 onto X,, and we consider X as an "internal" direct sum of the family (2,). Moreover, U - ' { x , } = E x , when {x,} EX@ X,; the sum converges since { x , } is an orthogonal set in X, and 111xaI12< co. Suppose next that X,, X, are Hilbert spaces, and T, E a(#,,X,) for each a in A. If
c@
c@
c@
s~P{llTaIl:aEAl< a,
c@
the equation T { x a } = { Tax,} defines a bounded linear operator Tfrom Za into X,. We call T the direct sum 10 T, of the family { T,}. Just as in the case of finite direct sums, we have
c@
and R , E ~ ( X , , ~ ~ ) . when S,, T,E~J(#,,X,) When 2,is the one-dimensional Hilbert space @, for each a in A, c0 8, reduces to the Hilbert space /,(A) of Example 2.1.12.
2.6. CONSTRUCTIONS WITH HILBERT SPACES
125
Tensor products and the Hilbert-Schmidt class. The material in this subsection will be used in an essential way in later parts of the book (from Chapter 1 1 onward), but has a relatively minor role until that point. The reader who wishes may bypass it until that stage, and will have only very occasional need to refer back in the meantime. There are several ways o f defining the (Hilbert) tensor product X of two Hilbert spaces 2l and ifz,each method having advantages in particular circumstances. Our approach, set out below, emphasizes the “universal” property of the tensor product. The Hilbert space X is characterized (up to isomorphism) by the existence of a bilinear mapping p, from the Cartesian product Xl x X2 into X, with the following property: each “suitable” bilinear mapping L from $‘1 x X ; into a Hilbert space X has a unique factorization L = Tp, with T a bounded linear operator from %‘ into X. Before starting the formal development of the theory, we indicate some of the intuitive ideas that underly it. When x, E Xl and xz E X z ,we shall want to view the elementp(xl, x2)of 2 as a “product,” x, 8 x2, of x, and x2.It turns out that linear combinations of such products form an everywhere-dense subspace of X. The bilinearity of p implies that these products satisfy certain linear relations; for example, as the product notation suggests, (x1
+ Y l ) 0 (x2 + vz) - x1Ox2 - XI OYZ - Y l 0 x2 - Y , OYZ = 0,
whenever xl, y , E XI and x2, y 2 E SZ. In fact, all the linear relations satisfied by product vectors can be deduced by (possibly repeated) use of the bilinearity o f p . It turns out that the inner product on X satisfies (and is determined by) the condition (x1 O X Z 9 Y l
0 Y Z ) = (xl,Yl)(X2,Yz);
in particular, llxl @ x211= llxlII Ilxzll.There are various constructions leading to a Hilbert space X with the required properties. In the method we shall use, the elements of 2 are certain complex-valued functions defined on the product Xl x S2and conjugate-linearin both variables (and by introducing a concept of “conjugate Hilbert space,” these functions are viewed as bilinear functionals). When u1 E X ,and uz e X 2 , u1 @ uz is the function that assigns the value ( u , , x l ) ( u 2 , x z ) to the element (x1,x2) of Z1x Xz. In the formal development of the theory, we first introduce the class of bilinear mappings used in formulating the universal property mentioned above. The tensor product is then defined, and identified (up to isomorphism) with certain specific Hilbert spaces, such as the completion of the algebraic tensor product and the class of “Hilbert-Schmidt operators” from the conjugate Hilbert space into X 2 .We conclude this subsection with a discussion of tensor products of bounded linear operators. It is convenient in the initial stages to consider the tensor product of a finite family of n Hilbert spaces, specializing later to the case in which n = 2.
126
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
Suppose that Sl,. . . ,Xnare Hilbert spaces and cp is a mapping from the Cartesian product XI x . . x Xninto the scalar field C. We describe cp as a bounded multilinear functional on Xl x * . x X,, if cp is linear in each of its variables (while the other variables remain fixed), and there is a real number c such that
-
IV( X I
7 * *
9
xn)l
< cllxlll .
* *
.
(XIE XI9 .
llxnll
xn E %)*
When this is so, the least such constant c is denoted by llcpll. Then, cp is a continuous mapping from Xl x . . x X, into C, relative to the product of the norm topologies on the Hilbert spaces; the estimates required to prove this are much the same as those needed in showing that the mapping ( a l ,..., a n ) + c a l . * * a , ,C: x
x C+C
is continuous, so we omit the details. In the following proposition, we consider certain sums of positive terms, which may converge or diverge, and a divergent sum is to be interpreted as co. In part (ii) of the proposition, inequalities involving co are to be understood in the obvious sense, and we adopt the convention that 0 00 = 0. Whether or not the sums considered converge, the manipulations required in the proof are easily justified, in view of the final paragraph of Section 1.2.
+
1
2.6.1. PROPOSITION. Suppose that Xl ,. .. ,Xnare Hilbert spaces and cp is a bounded multilinear functional on X I x * . x X,,.
-
(i) The sum
1
(3)
1 IdYl,...,Y,)I2
* * *
YneY,
YlEYl
has the same uinite or infinite) valuefor all orthonormal bases Y1 of XI,. . . , Y, of 2,. (ii) If.%,, . . .,X, are Hilbert spaces, A , E @(X,,X,) (m = 1, . . . ,n), I) is a bounded multilinear functional on Xl x . . x X,, and ~p(xl, *.
.y x n )
= +(A1x1,.
*
,Anxn)
(XI
EX^, . . * , x n ~ X n ) ,
then
1 YlEYl
* *
.
1
I ~ Y I*
* . ,Yn)Iz
< IIA1112 . .
*
IIAnII'
YneYn
1 ZlEZl
..
*
1 I+(zl,
when Y, and Z , are orthonormal bases of .X, and X,, (m = 1,. . . ,n).
1
C Y&Y"
Icp(Y1,*..,yn)Iz<
1
Zl€Z,
* * .
1 ZnEZn
. ,2n)12,
respectively
Proof. In order to prove (i), it is sufficient to show that YlOYl
*.
ZnEZn
Icp(Zlr**.,Zn)I*
2.6. CONSTRUCTIONS WITH HILBERT SPACES
127
128
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
multilinear functional, and the sum (3) is finite for one (and hence each) choice of the orthonormal bases Y 1 in Xl,. . . , Y,, in X,,. 2.6.2. PROPOSITION. IfXl , . .. ,X,,are Hilbert spaces, the set X 9 9 of all Hilbert-Schmidt functionals on Xl x * * x X,,is itselfa Hilbert space when the linear structure, inner product, and norm are defined by
( a q + b$)(x,
9 * * *
3xn)
=a
d x ~* * * -4+ 3
9
(cP,$) = C *..
(4)
. . ,xJ,
q(~l,...,Yn)$(Yl,...,~n),
YnErn
Y ~ E Y ~
=[1
c
II~II~
(5)
C
9 .
YI€Yl
J'2
IV(Y1,.*.,Yn)l2
Y+Y.
9
where Y,,, is an orthonormal basis in Xm( m = 1 , . . . ,n). The sum in (4) is absolutely convergent, and the inner product and norm do not depend on the choice of the orthonormal bases Y1,.. . , Y,,. For each v( 1 ) in Xl ,, . . ,u(n) in X,,,the equation
. ., xn) = ( X I 4 1 ) ) . .
( ~ u ( 1...., ) v ( n ) ( 9~. ~
9
*
(xn,
4n)>
(XI E 2 1 9
. . ., xn E Xn)
defines an element qvcl),...,vc,,) of #99, and
....,u ( n ) , ( ~ w ( ..... 1 ) wen)) IIVu(1),...,u(nJ12
((~u(1)
=(
~ ( l 41)) ) , * * . (w(n),v(n)>,
= Il~(l)ll. . . Il4n)ll.
....,y(n): y ( 1 ) Y~l , . . . ,y(n)E Y,,} is an orthonormal basis of 299. The set There is a unitary transformation U from 999 onto 12( Y 1 x * * * x Y,,), such that U q is the restriction cpI Y1 x . x Y,, when cp E X9.E
Proof. Having chosen an orthonormal basis Y,,, in X,,,(rn = 1, . . . ,n), we can associate with each bounded multilinear functional q on Xl x . . . x X,, the complex-valued function Ucp obtained by restricting q to Y l x . x Y,,. Note that cp is a Hilbert-Schmidt functional if and only if UcpEl,( Y1 x
* *
x Y,,).
If U q = 0, then ( P O ( Y I , * * * , Y ~ ) = O (YlEY1,...,ynEYn)* Since Y,,, has closed linear span X,,, (m = 1,. . . ,n), it follows from the multilinearity and (joint) continuity of cp that cp vanishes throughout XI x
* * *
x
x,,.
If cp and I(/ are Hilbert-Schmidt functionals on XI x . . x X,,,the same is true of acp + b$ (as defined in the proposition) for all scalars a, b ; for acp + b$ is a bounded multilinear functional, Ucp, U$ E 12( Y 1 x . . . x Y,,), and therefore U(acp + btj) = aUcp bUcp E 12( Y1 x . . . x Y,,).
+
129
2.6. CONSTRUCTIONS WITH HILBERT SPACES
The summation occurring in (4) can be written in the form
1 ypY, x
”’
(Ucp)(Y)(W)(Y), x Y”
and is absolutely convergent with sum (Ucp, U+), the inner product in 12(Y1 x * . x Y,,) of Ucp and UJI. From the preceding argument, the set X9’9 of all Hilbert-Schmidt functionals on X, x * * x X,,is a complex vector space, (4)defines an inner product on 29’9,the restriction U I X ” Y 9 is a one-to-one linear mapping from 39’9 into I,( Y1 x * . x Y,,), and (Ucp, U+) = (cp, II/) when cp, E XYF. Since the inner product on I,( Y1 x . x Y,,) is definite, so is that on XOY9; for if c p ~ 2 Y 9 and (cp, cp) = 0, we have
+
C
...
1
If(.~l,..*,.~n)(xI,~1) . *
r
=
*
(xn,yn)I
YnEYn
Yl€Yl
lli2
1
11fl1(
,Y1)12)li2
*
..
(c
li2
l<xn,yn>l2)
=
1 ~ 1 lIx1II 1 . . Ilxnll. *
Y+Yn
YlEY1
From this, the equation ~
(
~
1
* 3 * 7.
xn) =
C YlEYl
..
C
f ( ~ l 9 .
.
*
9
. Y n ) < x l , . ~ l >*
* *
(xn,Yn>
YnEY.
defines a bounded multilinear functional cp on XI x IIcpII < Ilfll. From orthonormality of the sets Y 1 , . . , Y,,,
*..
x 2,, with
E ( U V ) ( Y ~ . 7 ~ n )= ~ Y * * I* ~.YJ = f ( ~* l* ?r ~ n ) (YI E Y1,. . . * Y ~ Yn)? so U
3
130
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
When u(1)eif1,. . . ,u(n)eif,,,(pUcl) ,...,),( (as defined in Proposition 2.6.2) is a multilinear functional on iflx * * . x if,,, and is bounded since IVu(1) .....u(n)(XlY * * xn)l G I lv(1)ll . . . II4n)ll llxlll * . . llxnll by the Cauchy-Schwarz inequality. Moreover, Parseval’s equation gives 9
1
* *
YlEYl
1 I%l)
.....v ( n ) ( Y 1 9
,Yn)12
* *
YnEYn
c
=
*
=
1 I(Yl9 41))12
*
YlEYl
*
. . KYm 4n))l2
Y.EY.
(1
1(y1
9
u(l))12)
* *
.
Ylerl
= Ilu(l)l12
(1
l
YnEY.
* * *
IlWl12. *
Hence ( ~ u ( 1 ) , . . . , u ( n ) e H and yp II(Pu(1) ,...,u(n)ll2 = Ilv(1)ll Parseval’s equation and absolute convergence,
Il4n)ll. Again, by
....,v ( n ) , (Pw(1) ,...,w(n))
( r ~ u (1)
=
1 YlEYl
=
c
1 (~”(1),...,u(n)(~l, ,yn)(Pw(l) .....w d Y 1 9 . . . ,yn) * . . 1 (rl,u(l))...(yn,u(n))(w(l),yl) *.*(w(n),Yn> * * *
Yl€Yl
=
* * *
Y.EY.
Y&Y.
( 1 (w(l)yYl)(Y,y
U(W)
YIEYI
a)). .
* *
- ( 1 (w(n)9Yn)(Yn,
a(nD)
Y&Yn
(w(n), u(n)>. When y ( 1 ) ~Y1,. .. ,y(n)e Yn,the orthonormality of Y1,. . . , Y,,implies is the function that takes the value 1 at (y(l), .. . ,y(n)) and 0 that U(P,,(~),,,,,~(,,) elsewhere on Y1 x . . x Y,.Thus = (WU),
*
*...,,, n):Y(l)e y1,. . . ,y(n)e Y n l is an orthonormal basis of 12( Y1 x . . . x Y,,),and therefore {U(Py(l)
{(Py,l)
....,y(n):y(l)E
y13 *
* *
Ynl
is such a basis of if9’9. H In order to simplify the treatment of conjugate-linearmappings, which will be used extensivelyin this subsection and in Chapter 9, we introduce the notion of the “conjugate” of a Hilbert space if.The algebraic structure and inner product on H are defined by the mappings
+y :
if x H + H, ( a , x ) + a x : a= x if+%, (x,y)+(x,y): if x i f + c .
(x,y) +x
131
2.6. CONSTRUCTIONS WITH HILBERT SPACES
The conjugate Hilbert space 2' is the same set X , with the algebraic structure and inner product defined by the mappings (x,y) +x
+y :
(a,x)+arx:
X xX
+X,
@ x 2-X,
( x , y ) -+ ( x , y ) - : X x X + @,
where and ( x , y ) - = ( y , x ) .
a - x = dx
Of course, the conjugate Hilbert space of 2' is X. A subset of a Hilbert space is linearly independent, or orthogonal, or orthonormal, or an orthonormal basis of that space, if and only if it has the same property relative to theconjugate Hilbert space. If Xl and X z are Hilbert spaces and T is a mapping from the set Xl into the set X z , linearity of T: Xl + XZis equivalent to linearity of T: 9l+ 2'z,and corresponds to conjugate-linearityof T: Xl + sz and of T: 2,+ X 2 .Of course, continuity of Tis the same in all four situations (and when Tis linear the operators have the same bound), since the norm on X j is the same as that on J P ~ .
2.6.3. DEFINITION. Suppose that Xl ,. . . ,H,, and X are Hilbert spaces and L is a mapping from Xl x . * x X,,into X. We describe L as a bounded multilinear mapping if it is linear in each of its variables (while the other variables remain fixed), and there is a real number c such that IIL(x1, . . ., xJll
< C I b 1 II
* * *
Ilxnl I
(x1 E XI
9
* * *
9
xn E %)-
In these circumstances, the least such constant c is denoted by IlLll. By a weak Hilbert-Schmidt mapping from Xl x . . . x X,,into X, we mean a bounded multilinear mapping L with the following properties : (i) for each u in X, the mapping L, defined by Lu(xl9.
-
* 9
xn)
= (L(x1
9
. . ., Xn), U >
is a Hilbert-Schmidt functional on Xl x * x Xn; (ii) there is a real number d such that llLul12< dllull for each u in X. When these conditions are satisfied, the least possible value of the constant din (ii) is denoted by IILl12. H As in the case of multilinear functionals, a bounded multilinear mapping L: Xl x . . . x X,,+ X is (jointly) continuous relative to the norm topologies on the Hilbert spaces. Condition (ii) is in fact redundant, since it follows from (i), by an application of the closed graph theorem to the mapping u + L,:X + X Y F (Exercise 2.8.36). We shall not make use of this implication, and have incorporated (ii) in the definition for convenience.
132
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
2.6.4. THEOREM. Suppose that Xl,. . . ,X,, are Hilbert spaces. (i) There is a Hilbert space & and a weak Hilbert-Schmidt mapping p : HI x x X,,+ X with the following property: given any weak Hilbertinto a Hilbert space X, there is a Schmidt mapping L from Xl x * . . x Z,, unique bounded linear mapping Tfrom 2 into X, such that L = Tp ; moreover, IITII = llLl12. (ii) If&' andp' have the properties attributed in (i) to X andp, there is a unitary transformation U from &Y onto Z'such that p' = Up. (iii) I f u,, w, E X, and Y, is an orthonormal basis of 2 ,(m = 1 , , . . ,n), then
( P ( U ~ * uAP(w1,* . ., wn)> = 9 . .
9
wl> . . *
( ~ 1 3
(unr
wn>,
the set { p ( y l, . . . ,y,,):y , E Y , , . . .,y,, E Y,,} is an orthonormal basis of X, and llPll2 =
1.
Proof. With 2, the conjugate Hilbert space of 2,,let X be the set of all with the Hilbert space Hilbert-Schmidt functionals on P1x . . . x 2,, structure described in Proposition 2.6.2. When v( 1) E XI,. . . ,v(n)E X,,, let p(u(l),. . . ,v(n)) be the Hilbert-Schmidt functional (pvcl,,...,",) defined on
2,x . . '
x
2,,
by (~"(1)
XI
7
. , x n ) = ( X I ~ ( 1 ) ) .- . * ( x n ,v(n)>9
=
< v ( l ) , x l ) * . ( W ,xn>.
Since Y j is an orthonormal basis of X j ( j = 1 , . . . ,n), it follows from Proposition 2.6.2 that the set { p ( y l , .. . ,y,,):y l E Y1,. . . ,y , , Y,,} ~ is an orthonormal basis of A?,and that (P(~I,...,U~),P(WI,...,W~)> = ( ~ 1 y u 1 ) .- . * ( W n , u n > -
= (01 IlP(vl9..
-
9
9
un)112 = llulll
Wl> *
. . (4l, wn), *
. . IIvnll.
From the preceding paragraph, p : XI x . * x X,,+ .# is a bounded multilinear mapping: we prove next that it is a weak Hilbert-Schmidt mapping. For this, suppose that cp E X, and consider the bounded multilinear functional p v : X1x . * 'x X,, + C defined by 1
P J ~ I ., xn) = 9 . .
3
*
., -4, ~p>.
With y(1) in Y1,. . . ,y(n) in Y,, orthonormality of the bases implies that (pvC1, ,.__, ),,, takes the value 1 at (y(1),..., y(n)) and 0 elsewhere on
2.6. CONSTRUCTIONS WITH HILBERT SPACES
= cp(Y(l), * .
1
1
...
Y(~)EYI
133
. 3 Y W 7
lP,(Y(l), . . . ,An))I2 = l cpll;.
y(n)EYn
From this, pq is a Hilbert-Schmidt functional on Xl x x Xn and llpIll~= llrpllz; sop: Xl x . . . x Xn+ X is a weak Hilbert-Schmidt mapping with llpllz = I . Suppose next that L is a weak Hilbert-Schmidt mapping from Xl x . * . x Xninto another Hilbert space X. If u E X and Luis the HilbertSchmidt functional occurring in Definition 2.6.3, while cp E X and [F is a finite subset of Y1 x * . . x Yn, we have
I(
1
W 1 3 . .
*
,Yn)UYl,
* * *
,Yn),
u>I
( Y r . . .. . A H
6
1
(Yl.. . , .Y"kF
I d y l9 *
* *
,Yn)I ILu(Y19.. * ,Yn)I
134
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
converges to anelement T q of X, and IlTqll < llL11211q112.Thus Tis a bounded linear operator from &' into X, and IlTll < IILl12. When y ( 1 ) ~Y1, . . . ,y(n)E Y,,, we have
. . . ,y(n)) = Tqy(1),....y(n) TP(Y(~),
C
=
C
* * *
YlEYI
qy(1),...,y(ndy1, * * * ,yn)L(ylY. . . , Y n )
Y"EY.
= L(y(l), *
,v(n)).
*
Since L and Tp are both bounded and multilinear and Y,,,has closed linear span
Xm(rn = 1,. . . ,n),it follows that L = Tp.
The condition Tp = L uniquely determines the bounded linear operator T, x Y,,) of 2. because the range ofp contains the orthonormal basisp( Y1 x For each u in X, Parseval's equation gives
I I L ~ I I ~=
1
+
*
.
=
C C
a
*
1
Y&Y"
.
YlEYI
=
I(L(y1, * * ,yn),u)12
1 I(TP(yl,***,yn),u)12
*
YlEYl
=
C Y.EY"
YlEYl
I(p(y1,.
*
*.
,yn), T*u)12
Y+Y.
llT*uIl2< lIT11211u112;
so llLll2 < IlTll, and thus llLll2 = IlTll. It remains to prove part (ii) of the theorem. For this, suppose that X ' and p': Xl x . . x &',, -+ &''(as well as &' andp) have the properties set out in (i). When X is &" and L is p', the equation L = Tp' is satisfied when T is the identity operator on X ' , and also when T is the projection from 2" onto the closed subspace [p'(X1 x x Xn)]generated by the range p'(Xl x . . . x &',,) of p'. From the uniqueness of T, [pyx1 x
* .
. x &', )I
= 2';
moreover, llP'll2 = l l U 2 = IlTll = 1141= 1. With the same choice, X = &" and L = p', it follows, from the properties of X andp set out in (i), that there is a bounded linear operator U from &' into X ' such that p' = Up and
IlUll = IILII2 = IIP'll2 = 1. The roles of X, p and A?',p' can be reversed in this argument, so there is a bounded linear operator U' from X ' into &'such thatp = U'p' and IlU'll = 1. Since U'Up(x1,. . . , X n ) = UlP'(x1,..., X n ) = p(x1,. . . ,x,,),
2.6. CONSTRUCTIONS WITH HILBERT SPACES
for all x1 in Zl, . . . ,x, in
135
x , while ..
[p(X1 x
*
x Zn)] = X,
it follows that U‘U is the identity operator on S‘; and similarly, UU‘ is the identity operator on Hi.Finally,
llxll = IIU‘Wl G IIUxll G llxll
(XEZ);
so I(Ux((= ( ( ~ ( 1 , and U is an isomorphism from X onto X ’ . By part (ii) of Theorem 2.6.4, the Hilbert space X appearing in that theorem, together with the multilinear mapping p: Xl x * x Xn-P 2, is uniquely determined (up to isomorphism) by the “universal” property set out in (i). We describe Z as the (Hilbert)tensorproduct of Zl, .. .,S,,, denoted by Zl @ . * Q X,,, and refer to p as the canonical (product) mapping from Xl x . * x #, into Xl 0 * . 6Xn. The vector p ( x l ,. . . ,x,) in Xl @ . . Q X,, is usually denoted by x1 @I . . 6x,. Finite linear combinations of these “simple tensors” form an everywhere-dense subspace of XI @ . ’ * Q Z, ; indeed, if Y, is an orthonormal basis of # , (rn = 1,. . . ,n), then +
{y l
6 . 6yn :y1 E Yl . . . ,Y n E Yfll *
9
is an orthonormal basis of X I dim(Zl O X 2 @
1
.
@ Xn.Thus
*
@ Z , ) = d i m Z l d i m X 2 ...dim#,.
-
-
As the notation suggests, the vector x1 @ . . 6x,, behaves in some respects like a formal product of xl,.. . , x n ; for example, it results from the multilinearity of p , and from Theorem 2.6.4(iii), that (7)
Oxrn-l6(ux:,+bx~)Ox,+lO
XlQ...
Qx,,,-~ @X;@X,+~
=a(x1 Q
+b(xl6 (8)
(XI@
. * *
(9)
a
*
*
OXn,y16 11x1
@
6xn * * *
OX,)
6xm-l6x~Oxm+lO...6xn), . . . O y n ) = ( ~ l , ~ l ) . . . ( x n , ~ n > ,
6 . . 0 xnll = IlXlll ’ . . IlXnll. a
In studying tensor products of Hilbert spaces, the properties just listed are usually more important than the detailed constructions employed in the proof of Theorem 2.6.4. Many of the arguments involve two stages; the first stage deals with the linear span Soof the simple tensors, and is based on the identities (7)-(9), while the second employs “extension by continuity” from Xo to its closure Xl 6 * . . 6 X,,. Since
0 ~2 Q . . 6xn) = (uxl)6x2 6 * * * 6xn, Xo consists of all finite sums of simple tensors. In dealing with X o ,it is 4x1
136
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
important to bear in mind that the simple tensors are not linearly independent. Relation (7) can be viewed as the assertion that a certain linear combination of three simple tensors is zero, and repeated application of (7) yields more complicated identities of this type. We shall look at this question in more detail in Proposition 2.6.6. In the meantime, we establish the “associativity” of the tensor product. 2.6.5. PROPOSITION. I f X l , .. . ,Xm+, are Hilbert spaces, there is a unique unitary transformation U from Xl 6 . ’ 6 X m +onto ,
( 2 1 6 . * .O - % m ) 6 ( X m + 1 6 . . . 6 X m + n ) such that (10) U ( x 1 O * * * 6 x m + n ) = (XI 6 * O x , ) 6 ( x m + 1 O Ox,+,) whenever x j €Xj ( j = 1, . . .,m + n). Proof. Since the set of all simple tensors in Xl 6 . * 6 Xm+, ( = X ) has linear span everywhere dense in X, there is at most one unitary operator U with the stated property; so it suffices to prove the existence of such an isomorphism. For this, let 1
.
.
X ‘ = (XI6 . * * 6 S m ) 6 ( X m + 1 O . . . 6 x m + n ) T and when x j ~ X( j = 1,. ..,m + n), define ~ ( ~ 1 , . . . , x m + n ) = ~ 1 6 . ” 6 ~ m + n(
P‘(XI
9 * * * 9
x m + n ) = (XI
EX)
6 * * * 6 xm) @ ( x m + 1 O * + * 6 x m + n ) ( E X ’ ) .
The ranges of p and p‘ contain orthonormal bases of X and X ’ , respectively, and so generate everywhere-dense subspaces X ( G X ) and X’ (cX ’ ) . I f x j , y j ~ X( j = 1 , . . . , m + n), we have
(14x1
, x m + n ) , p ( Y l ~ . * * ,Ym+n))
7 * *
=(XI,YI)
(Xm,Ym)(Xm+l,Ym+l)
*
a
*
(xm+n,Ym+n)
= ( ~ 1 6 * * * 6 x m , Y 1* 6* * Q Y m ) ( ~ m + l O. . * O X m + n , Y m + l O . . . @ Y m + n ) = (P’(XI
9
*
*
rXm+n),P’(Yl,
-
*
9
ym+n))*
From this,
k= 1
k=l
1=1
wheneverakE@andx)k)EX,(j=1,..., m + n ; k =
1 ,..., q).
137
2.6. CONSTRUCTIONS WITH HILBERT SPACES
The remainder of the argument is of frequently recurring type. The equation
\k=
1
1
k=l
defines a norm-preserving linear mapping Uo from Xonto X’. The definition is unambiguous since, given two expressions Ca,p(xy), . . . and Cb,p(yy),...,y$)+,,) for a vector x in X, it follows (upon replacing Cakp(x‘f),. .. ,x:!+,) by Cakp(xy),.. . ,x:’+,,) - 1blp(yy),. . . in the last chain of equations) that the two corresponding expressions
,XI’+,) ,YE)+,,)
1a,p’(xY),. . . , m +
X(k) n)
and
14 P’(Y:“
1
. . Y:)+ n ) *
9
for Uox are equal. By continuity, Uo extends to an isomorphism U from X onto X ’ , and
By use of the “associativity” established in the preceding proposition, questionsconcerning the n-fold tensor product of Hilbert spaces can usually be reduced to the particular case n = 2. Our next few results are directed toward this case. We consider first the question of linear dependence of simple tensors. 2.6.6. PROPOSITION. Suppose that Xl and X2 are Hilbert spaces, X = S10 X 2 ,and X o is the everywhere-dense subspace of 2‘generated by the simple tensors.
cjn= x j 0 y j
(i) U x , , . . . ,x, E Xl ,y , ,.. . ,ynE X 2 ,then there is an n x n complex matrix [cjk] such that
= 0 ifand only
if
n
1 cjkxj=o
( k = 1, ..., n),
j= 1
(ii) If L is a bilinear mapping from Xl @ X2 into a complex vector space X, there is a (unique) linear mapping T from Xo into X such that L(x,y ) = T(x 0 y ) for each x in Xl and y in X 2 .
138
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
Proof. (i) If there is a matrix [ c j k ] with the stated properties, bilinearity of the mapping (x,y) + x @I y implies that n
n
/
\
n
n
n
Conversely, suppose that Cj”= xi 6y j = 0. I f u l , . . . ,ur is an orthonormal basis of the linear subspace of X2 generated by y l , . . . , y n , we can choose an n x r matrix A = [ a j k ] and an r x n matrix B = [ b j k ] such that r
C
y j =
ajpl
( j = 1, ..., n),
b1kyk
(f
1=1 n 01
C
=
= 1,.
. .,r).
k= 1
With
the n x n matrix AB, we have
[cjk]
Yj =
i
ajl( 1=1
i
=
blkyk)
k=l
( j = 1,.
Cjkrk
. .> n),
k= 1
and n
0=
n
1
x j
C
Oy j =
j= 1
x j
O
j= 1
ajiul (1:1
)
=
,Il
~1
o 01,
where n
( I = 1,.
aj,xj
u1 =
. . ,r).
j= 1
F o r e a c h m = 1, ..., r, r
r
0=
(ui O ui, urn O urn> = 1=1
C (ui,urn)(Vi, urn> = IIurnII’. 1=1
Thus u1 = u2 = . . . = ur = 0, and n
C j= 1
n CjkXj
=
r
r
11U j l b r k X j = 1b l & j=ll=l
=0
(k = 1,. . . ,n).
I= 1
(ii) Suppose that L is a bilinear mapping from XI x 2P2 into X . If . . , x , E X ~, y l , . .. ,yn€ 2 2 , and Cj”= x j O y j = 0, we can choose a matrix
XI,.
2.6. CONSTRUCTIONS WITH HILBERT SPACES [cjk]
139
as in (i). The bilinearity of L then entails
j= 1 n
n
c
=
1
cjkl(xj,yk)
= k= 1
k=l j=l
Suppose next that x l , . . . ,x n ,u l , . . . ,um E X l ,yl ,. . .,y,, u l , . . . ,vmE X 2 , and x j 0 y j = Cj”= uj 0 vj. Then
c
m
n
x j o y j +
j= 1
c ( - u j ) o V j = o ; j= 1
the preceding paragraph shows that m
n
and therefore
j= 1
j= 1
From this, it follows that the equation /
n
\
n
2.6.7. REMARK.The first part of Proposition 2.6.6 asserts, in effect, that the only finite families of simple tensors that have sum zero are those that are “forced” to have zero sum by the bilinearity of the mappingp: ( x ,y) + x 0 y. From this, APo can be identified with the algebraic tensor product of Xl and X 2 ,which was defined, traditionally, as the quotient of the linear space of all formal finite sums of simple tensors by the subspace consisting of those finite sums that must vanish ifp is to be bilinear. The second part of the proposition shows that Xo has the “universal” property that characterizes the algebraic tensor product. We can identify X with the completion of its everywhere-dense subspace Xo.Accordingly, the Hilbert tensor product Xl 0 X 2 can be viewed as the completion of the algebraic tensor product Xo,relative to the unique inner product on Sothat satisfies (x1
@ Y l , X 2 0 Y 2 )
=(X1,X2)(Yl,Y2)
(Xl,X?EJEoll
Y19Y2EX2).
rn
140
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
2.6.8. REMARK.We show that the tensor product of Hilbert spaces A? and X can be viewed as the n-fold direct sum of A? with itself (that is, the direct sum of n copies of A?),where n is the (finite or infinite) dimension of X. For this purpose, let { Y b : b E B} be an orthonormal basis of X, and for each b in B let A?b be X. The mapping w b :x x @ Y b : A?b A? @ is a norm-preserving linear operator from A?b onto a (necessarily closed) subspace A?; of A? Q X. Since c
0 Y a ~ @~Y b2>
f
= (XI,X2)(Ya,Yb)
=0
for all x 1 and x 2 in A?,when a and b are distinct elements of B, it follows that the subspaces {A?::be B} are pairwise orthogonal. With {za} an orthonormal basis of A?,the closed subspace V A?; of A? @ X contains the orthonormal basis {za @ ' y b } , so VA?; = A? Q X. Accordingly, A? @ X is the internal , we have isomorphisms W direct sum of its subspaces A?; ( ~ E B ) and (= I@ w b , from 10A?b onto Z@ A?:) and V (from 10A?: onto A? @ X ) , defined by Thus VW is an isomorphism U , from
Z@A?b onto A? Q X, and
u(c@x b ) = c x b @ Y b * (1 1) From this, each element of A? @ X can be expressed (uniquely, once the orthonormal basis { y b } is specified) in the form 1 x b @ Y b , where x b E A?(bE 5 ) and 1llxbl12 < co.When .fis finite dimensional, the elements of A? @ X are finite sums of simple tensors; the same is true when A? is finite dimensional, since the roles of A? and X are interchangeable. W We show next that the tensor product of Hilbert spaces X and X can be represented as a certain linear space of operators from the conjugate Hilbert space 2 into X. For this, note first that the equation YEX) X ) onto the set of all defines a one-to-one linear mapping T -+ bT from a(%, bounded bilinear functionals on A? x 2 (since these are, precisely, the bounded conjugate-bilinear functionals on A? x X ) .With T i n a(*,X ) , it follows by applying Proposition 2.6.1 to bT that the (finite or infinite) sum b,(X,Y)
= (%Y)
( X E S ,
has the same value, for all orthonormal bases X of A? and Y of X. From Parseval's equation, this sum can be written also in the alternative forms
2.6. CONSTRUCTIONS WITH HILBERT SPACES
141
We describe T as a Hilbert-Schmidt operator if the value of the sums is finite; equivalently, T is a Hilbert-Schmidt operator if and only if bT is a Hilbert-Schmidt functional on X x 2. With X Y 9 the linear space of all Hilbert-Schmidt functionals on S x 9, the Hilbert-Schmidt operators from Y? into X form a linear su bspace XYU = {TEB(X,X):bTEY?Y%} of B(2,X ) .By means of the mapping T + bT, the Hilbert space structure on as described in Proposition 2.6.2, can be transferred to XYO. Accordingly, XYU is a Hilbert space, when the inner product and norm are defined by ( S , T) =
c (SX,Y)(Y, Tx), [ c c I(TX,Y)l’] XEX YEY
1/2
lIT112 =
9
x e x yeY
these being independent of the choice of the orthonormal bases X of X and Y of X , Of course, the mapping T bT is an isomorphism from Z Y O onto X Y R The equality of the four sums appearing in (12) and (13) implies that there are three other, equivalent, expressions for I(T(12 ; and similarly, the inner product ( S , T) can be expressed in the alternative forms
c c (T*Y,XXX,S*Y),
(SX,
Tx),
X€X
YEY X E X
c (T*Y,S*Y). YEY
If X,,, X,, are Hilbert spaces, A E ~ ( X X,,), ,, EEB(Y?,,, X ) , and T is a Hilbert-Schmidt operator from A? into X, then ATE is a Hilbert-Schmidt operatorfromS,, into X,, with IIATEl12 G IlAll llTl12IlEll. For this, let X , bean orthonormal basis of So,and observe that
1 IITW2 =
lIB*T*Yl12, YEY
XEXO
since these sums are the analogues, for TE, of the ones in (13). The stated result now follows from the inequalities
c IIATBxl12 G llA1I2 c IITBxl12 xexo
xexo
= llAl12
IIB*T*Y1I2
YEY
G
11~11211~*112
c IIT*Y1l2 YEY
=
I I A II211 TllzZllBll 2 .
This result can be proved also by means of Proposition 2.6.1.
142
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
The identification of X 6 X with the Hilbert space of all Hilbert-Schmidt operators from 2 into X is described in the proposition that follows. r f X and X are Hilbert spaces, then,for each x in % 2.6.9. PROPOSITION. and y in X, the equation
(~€2)
T x , y=~( u , x ) - ~= ( x , u ) ~
defines a Hilbert-Schmidt operator Tx,yfrom 2 into X. With 29 0 the Hilbert space of all Hilbert-Schmidt operators from 2 into X, there is a unitary transformation U from X Q X onto 2 9 0 ,such that w x €3 Y ) = T x , y
YE.%?-).
(XEX,
Proof. As constructed during the proof of Theorem 2.6.4, X €3 X is the Hilbert space X Y 9 of all Hilbert-Schmidt functionals on R x 9. Moreover, when x E X and y E X, x 6y ( = p(x,y ) ) is the bilinear functional ( P ~defined, , ~ throughout 2 x 9,by (PX,Y(U,
0) =
0,U ) ( Y ,
v).
The discussion preceding Proposition 2.6.9 shows that there is an isomorphism U from X Y 9 onto &‘YO that associates with each Hilbert-Schmidt functional on R x 9 the corresponding Hilbert-Schmidt operator from 2 into X. It is apparent that Tx,y,as defined in the proposition, is the bounded linear operator from 2 into X that corresponds to the bilinear functional qx,y. Since ( P ~E, X ~ Y 9 , it follows that Tx,y E 2 9 0 ,and
u x 6 Y ) = Uqx,, = T X J .
rn
2.6.10. EXAMPLE.With A and B arbitrary sets, we can associate with each x in /,(A) and y in 12(B) a complex-valued function px,y defined throughout AxBby
Px,y(a,b) = x(aly(b).
We shall show that there is a (unique) unitary transformation U from [,(A) Q I,@) onto 12(A x B) such that
w @ v)=
Px,y
(XE12(A),
For this, note first that p , , , ~ l ~ ( xA B) since
c
YEMB)).
2.6. CONSTRUCTIONS WITH HILBERT SPACES
143
Moreover, (P x , y , P”,”)=
c
(o,b)oA x B
(the series manipulations being justified by absolute convergence), when x , u E /,(A) and y , u E 12(B). By expressing norms in terms of inner products, it
now follows that, for any finite linear combination of elements px,y,
We sketch the remainder of the argument, which follows the same pattern as the second paragraph of the proof of Proposition 2.6.5. The linear span Xo of { P ~ ,x ~~ :l ~ ( ~A E) I, ~ ( Bis) everywhere } dense in &(A x B) since it contains the usual orthonormal basis (consisting of functions with value 1 at a single point of A x B and 0 elsewhere); and the linear span Soof the simple tensors is everywhere dense in ],(A) @ 12(B). From (14), there is a norm-preserving linear mapping Uo from Xo onto Z0 such that UOpx,,= x @ y ; and this mapping extends by continuity to an isomorphism U from I,(A x B) onto M A ) @ MB). 2.6.1 1. EXAMPLE.We now consider the tensor product of the L2 spaces m) and (S’, Y ‘ ,m’).We show associated with 0-finite measure spaces (S,9, that this can be identified with the L2 space of the product measure space (S x S ‘ ,Y x Y ‘ ,m x m’),in such a way that x @ y corresponds to the function px,ydefined throughout S x S’ by PX,Y(S,
s? = x(s).Y(s’).
For this, note first that p x , yis a complex-valued measurable function when x E L 2 ( S , Y , m )(= 2)and yEL2(S‘,Y’,m’)(= 2’); moreover, px,yE L2(S x S’,Y x Y ‘ ,m x m‘) (= x ) ,
144
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
since
Also,
=
(
x(s)u(s) dm(s)) S
(1
y(s’)uo)dm’(s’))
S‘
= (x, u > ( y ,v> = (x
6y , u 0 v>,
whenever x, U E A ? and y , V E A?‘.From this, we have n
n
II 1 cjPxj,yjII = II 1 cjxj 6YjII, j= 1
j= 1
for every finite linear combination of elements p x , y . Accordingly (by the argument already used in the preceding example and in the proof of Proposition 2.6.5), there is a norm-preserving linear mapping Uo, from the linear span Xo of { P ~ ,x ~~: 2 , y ~ Xonto ’ ) the linear span Xo of the simple tensors in A? 6 X ‘ ,such that U o p , , = x @ y . Now X0is everywhere dense in X 6X ’ , and Xois everywhere dense in X since it contains the characteristic extends by function of every measurable rectangle of finite measure. Thus 17, continuity to an isomorphism U from X onto A? 6W . H We now introduce tensor products of bounded linear operators. r f %1, . . . ,X n ,Xl,. . . ,Xnare Hilbert spaces and 2.6.12. PROPOSITION. A,,, E~(X,,,, X,) ( m = 1, . . . ,n), there is a uniquq bounded linear operator A from Xl 6 . . * 0 A?n into Xl 6 6Xnsuch /hat
-
A(x1 8 . . . 6x,) = A i x l
6
* *
(XI EX i ,
6Anxn
-
. . . ,X, E 2,).
Proof. The canonical mapping p: X, x x X, -+ Xl @ * * * 6 Xn (= X ) is a weak Hilbert-Schmidt mapping, with llpllz = 1. With u in Y,andp,
145
2.6. CONSTRUCTIONS WITH HILBERT SPACES
defined by (p(z1 . . ,Zn), u>, pu is a Hilbert-Schmidt functional on Xl x . . * x X,, and IIpu112< ((u(1.The equation ( p ( X 1 , . . .,xn) = P ( A l X l t . . . ,Anx,)
P&,
9 .
. ., z,)
=
9 .
defines a bounded multilinear mapping cp: Xl x . . . x X,, + X, and cpU(Xl,.. x,) = (44x1 9.. - 3
=
., xfl),u>
-
(p(A1x1,. . ,Anx,),
4
1x1,. . A,x,). It now follows from Proposition 2.6.1(ii) that cpu is a Hilbert-Schmidt functional on a?, x . . . x H,, with = p,(A
a ,
-
llcpullz G 11Alll . * IlAnll IIPUIIZ G IlAlll * * IlAnll IlullAccordingly, tp: Xl x . . x X, + X is a weak Hilbert-Schmidt mapping, with (Icp((2 6 llAlll . . * llA,,ll. By the universal property of the tensor product (see Theorem 2.6.4(i)), there is a unique bounded linear operator A , from X1Q . . . Q &',, into X, such that cp = Ap', wherep' is the canonical mapping Moreover, from X', x . . . x Z,into H I Q * . . Q a?,,.
IlAll = I I d l z 6 IlAlll . . *
11~4.11.
Also, A(x1 Q ' . . Q x,) = Ap'(x1,.. . ,x,)
. . .,x,) =p(Alxl, ...,A,x,)= A l X , Q ... @A,x,, when x1€a?,,...,X,E a?,,. = cp(x,,
The operator A described in Proposition 2.6.12 is called the tensor product of A l , . . . , A , and denoted by A l Q . . . Q A,. It is apparent that A , Q . . . Q A , depends linearly on each A,,, and that ( A , Q . . . Q A,)(B, 0 - .. Q B,) = A l l ? , Q . . Q A,B,. Since
((A1 0 * . . 0 M x 1 0 . * * 0 X,),Yl 6 . . . QY,> = (AlXl
0 . . . 0 A,x,,y, 0 . . * 8 Y,>
= (AlX1,Yl)
. . * (A,xn,yfl)
* . . (Xfl,An*Yn> = (xl Q . . . Q x,,A:y, Q . . . Q A,*y,)
= (X1,ATYl)
= (X1Q
. . . Qx,,(A:O ' . . QA,*)(y1Q
* * .
Byfl)>,
146
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
it follows by linearity and continuity that ( ( A , 0 . * . @A,)u,u) = ( u , ( A : @
6A,*)u)
. * *
for all vectors u and u in the appropriate tensor product spaces. Thus
(15)
(A,@
* * .
@ A , ) * = A T @ * . *@A,*.
We assert also that llA1 @ . . . @ All = IlAlll * * * IlAnll. (16) Indeed, given by any unit vectors x1 in Sl, . . . ,x, in X,,, we have llA1 6 . * * @ An11 = 11-41 @
04 111x1 @ . . . @ XnII
* *
3 ll(A1 @
0 An)(x1 @
* * *
= llAlx1 6
*
+
*
+
@ XJll
@ Anxnll = IlAlxlll . . * IIAnXnll.
*
Upon taking the supremum of the right-hand side, as the unit vectors . . ,xnvary, we obtain
xl,.
llA1 0 . . . @An11 2 IlAlll .
9
*
IlAnll;
the reverse inequality was noted during the proof of Proposition 2.6.12. XI,. . . ,Xm+,, are Hilbert spaces and Suppose next that X , , .. .,Sm+,,, that Aje99(Xj,X j )( j = 1,. ..,m + n). We can construct isomorphisms u:X1@
@ ~ m + n ~ ( ~ , @ . . . @ ~ m ) 6 ( ~ m + *1. .@@ X m + n ) ,
v:
6X m + n
@
* * *
+
(XI@ . . 0 X m ) @ ( X m + l @ . . . 0 X m + n ) ,
as in Proposition 2.6.5, and it is at once verified that
v(A,O * . . OAm+n)U-’
=(A1
O
*
*
a
OAm)O(Am+I O
1
.
.
OAm+n)*
This proves the “associativity” of the tensor product of bounded linear operators on Hilbert spaces. With X and X Hilbert spaces, the linear mapping A
+A
6I:a(%)-+ 99(S@ X )
preserves operator products, adjoints, and norms; from this last, it is norm continuous. We consider next its continuity properties relative to the strongoperator topology. With v a simple tensor x @ y in % @ X,
ll(A 0 0 0 - (A0 0 Oull = ll(A - Ao)x OYll = ll(A - Ao)xlI Ilvll, for each A and A . in a(&).From this, it follows that, if ol,. . . ,v,,, are simple tensors in 2 @ X and c > 0, then the set { A E B ( X )ll(A : @ Z ) u j - ( A , @I)ujll
-=
E
( j = 1 , . ..,m)}
is a strong-operator neighborhood of A . in g(S).Since the simple tensors
2.6. CONSTRUCTIONS WITH HILBERT SPACES
147
have closed linear span X‘ Q Y, they suffice to determine the strong-operator topology on bounded subsets of X‘ 0 X ; so the preceding sentence implies that the mapping A + A Q I is strong-operator continuous on bounded subsets of a(*).When one or other of X‘ and X is finite dimensional, the simple tensors have algebraic linear span X ’ Q X (Remark 2.6.8), and therefore suffice to determine the strong-operator topology on (the whole of) Q X ) ; so, in this case, the mapping A + A @ I is strong-operator continuous on a(%). Suppose, finally, that { y b : b E B} is an orthonormal basis of X. As noted in Remark 2.6.8, the equation defines an isomorphism U from CbBQ3 X‘b onto X‘ With A in a(X),
6X when each x b is X.
( A 8 I)u(c@ x b ) = ( A 8 I > ( cx b 8 y b ) = x ( A @ I)(xb =Z A X b
8y b )
@ Y b = u(c@ AXb),
so U-’(AQI)LI= C @ A
(17)
(AEW(X‘)).
b€B
Matrix representations. We conclude this section with an account of the matrix representations of operators acting on a direct sum CbB@ %b, where each X‘b is the same Hilbert space 3. Before embarking on this program, we consider the numerical matrices of operators relative to orthonormal bases. Suppose that { Y b : b E B} is an orthonormal basis of a Hilbert space X. With S in a ( X ) ,each vector s y b has an expansion
(18)
syb
=
c
( b E B),
SabYu
aEB
in which the coefficients are given by sub = < S Y b , Y a ) * (19) In this way, we associate with each S in 9 ( X ) a complex matrix [Sabla&B, relative to the orthonormal basis { y b } .When the index set B is finite, every complex matrix [ S a b l u , & ~corresponds, as in (18), to some element S of B ( X ) . When 5 is infinite, however, boundedness of S imposes certain restrictions on its matrix. For example, Parseval’s equation gives babl’ a&
=
1 asB
I(SYb,ya>l’
=
llSyb11’
< lls112,
148
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
so the “columns” and “rows” of the matrix [ s o b ] form (bounded sets of) vectors in &(B). The algebraic relations between operators and matrices follow the pattern familiar in the finite-dimensional case. From (19) and (18), the matrix elements S,b depend linearly on S, and [ s o b ] is the zero matrix only when S = 0. Since (s*Yb,Yo)
= ( Y b , s Y a > = ( s Y o , Y b ) = sba,
the matrix of S* has &,a in the (a,b) position. If two elements S and T of g ( X ) have matrices [ s u b ] and [ f o b ] , respectively, and R = ST, then (RYb,Ya)
=(sTYb,Ya) =(TYb,s*Ya),
and Parseval’s equation gives (RYb,Ya)
=
(TYb,Yc)(yc,S*ya) CEB
1( = 1 =
S Y c 9 Y a > ( TYb 9 Y c )
CEB
Sactcb.
ceB
Accordingly, the matrix
[rgb]
of R (= S T ) is given by rob
=
1
Soctcb.
CEB
The results of the preceding paragraph can be summarized in the assertion that the matrices, corresponding (through a fixed orthonormal basis) to bounded operators on X, form an algebra relative to the usual concepts of sum, product, and scalar multiple of matrices. Moreover, the mapping from bounded operators to the corresponding matrices is an isomorphism. 0 x b , with each x b We now consider operators acting on a direct sum CbeB the same Hilbert space X. For this purpose, we introduce a closed subspace 3Pb of &, and bounded linear operators
c@
ua :
Z@x b ,
v g
:10 x
b
,?%c
for each a in B,as follows. When X E Y and u = { x b } EX@ x b , V,u = x, and U,x is the family { z b } in which z, = x and all other z b are 0;A?; is the range of U,,and so consists of all elements { z b } of I@ x b in which z b = 0 when b f a. Observe that V,U, is the identity operator on 2 and U,V, is the projection E, from x b onto 2;. Since the subspaces 2;(a E B) are pairwise orthogonal, and V X ; = 10 %b, it follows that the sum LEBE,is strong-operator convergent to I . Note also that U, = V,*, since
z@
( uax, { x b } ) = ( x , xa> = ( x , Va{xb})
whenever x E
~
and
{xb}E
c0 X b .
149
2.6. CONSTRUCTIONS WITH HILBERT SPACES
With each bounded h e a r operator T acting on I@ s matrix [TablaqbB, with entries Tabin g(#) defined by (20) If u
=
Tab
= {xb}
EX@
s b ,
Ya =
vaTU
VaT
c ) EbU
we associate a
vaTub.
then Tu is an element { y b } of =
b ,
=
I@ #b,
and
1v a T u b v b U =
( b
b
T&b. b
Thus (21)
T(c@ xb)
=
10y b
where
=
ya
c
Tabxb
(a E 8).
k B
The usual rules of matrix algebra have natural analogues in this situation. From (20), the matrix elements Tabdepend linearly on T. Since vaT*ub
= UtT*
vt = ( V b T U a ) *
= (Tba)*,
the matrix of T* has ( Tba)*in the (a, b) position. If S and Tare bounded linear operators acting on s b , and R = ST, then
c@
Rub
=
vaRub
=
VaSTUb
=
VaSEcTUb C€B
=
c
VasucVcTub
CE EJ
=
c
SocTcb,
CEB
the sum converging in the strong-operator topology if the index set B is infinite. In this way, we establish a one-to-one correspondence between elements of with entries Tabin B(&‘).When @ #b) and certain matrices the index set B is finite, each such matrix corresponds to some bounded operator Tacting on I@ s b ;indeed, Tis defined by (21), and its boundedness follows at once from the relations II{yb}112
=
1lly011~= 1 1 1c a
G
a
b
TabXb112
G
cc a ( b
IITabll l b b l l
>’
~a ( b~ ~ ~ T a b ~ ~ z )=(( ~~a ~b ~~ ~x Tb a ~ b ~~ 2~ )2 ) ~ ~ { x b } ~ ~ 2 .
When the index set B is infinite, it is apparent that some matrices with entries in
a(#)do not arise in the above manner from bounded operators. In formal matrix calculations, it is necessary to ensure that no such “unbounded” matrices appear at any stage. While there is no simple general procedure for determining whether or not a given matrix corresponds to a bounded operator, a criterion that is sometimes useful is set out in Proposition 2.6.1 3 below. In the meantime, we describe certain special types of “bounded” matrices that arise frequently in applications.
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2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
In the first place, a matrix [Tab]gives rise to a bounded operator if it has only a finite number of non-zero entries Tab.Indeed, the proof used above, for the case in which B is finite, applies also in the circumstances just described. More generally, the same argument shows that the matrix corresponds to a bounded operator whenever the sum c ~ c ~ J J isT finite, ~ ~ J since J ~ it is easily verified in this case that the series for y,, in (21), is absolutely convergent. However, there are bounded operators on I@ X b (for example, I ) whose matrices do not satisfy these conditions. Second, suppose that n is a permutation of B, and { Tb:b E El} is a bounded family of elements of g ( H ) .Since
defines a bounded linear operator Ton the equation T(C@ xb) = I@ Tbxn(b) C @ X b . The matrix [Tab]of T is given by Tab= da(a),bTa,where d a b is the Kronecker symbol ( d a b is 1 when a = b, 0 when a # b). If each Tbis unitary, the same is true of T . When n is the identity mapping on B, T is the direct sum 10 Tb, and corresponds to the diagonal matrix [dabTa]. Finally, we consider the matrix representation of certain tensor products of operators. Suppose that {yb : b E EB} is an orthonormal basis in a Hilbert space X, and Uis the isomorphism from C@ x b onto H @ X (where each Z b is H ) , definedby U(C@Xb) = CXb@yb. W h e n A € g ( X ) , U - ' ( A @ I)Uisthedirect sum CkB@ A , and has matrix [ & , A ] This . characterizes the operators To of the form A @ I acting on 2 @ X as those for which U - ' ToU has a diagonal matrix with the same element of @ ( X in ) each diagonal position. We assert also that an element To of a(&'@ X ) can be expressed as I @ S, with S in .@(X), if and only if the matrix of U - ToU has the form [ & I ] , with each Sob a scalar. For this, suppose first that S E ~ ( X so ) , that S has a complex matrix satisfying (18). With x in H and b in B, UUbx = x @ yb, so
'
va(
u- ' ( I @ s ) u ) u b x = vau- ' ( I @ s ) ( x @ yb) =
v a u - ' ( x @ s y b ) = VaU-'
=
@va: (
LB
)
cscbx@yc
scbx) = Sabx.
Hence v,(U-'(Z@ s)u)&= &blr and U - ' ( I @ S)U has matrix [&(,I]. Conversely, suppose that ToE a(# @ X ) and the matrix of U - ToU (= 7') has the form [ & I ] . With x a unit vector in A? and f an element of lZ(5), u = C@f(b)x is a vector in C@ %b and Tu = xb, where
'
c@
xa =
c sabf(b)x*
as
2.6. CONSTRUCTIONS WITH HILBERT SPACES
151
Hence the sum CkBsabf(b)converges, and
so
Accordingly, the equation
defines a bounded linear operator S on X. Since S has matrix [sob], U - ' ( I 0 S ) U has the same matrix [SabI] as does U - ToU, so To = I 0 S. We now establish a criterion for determining whether or not a matrix [Tab] &B, with entries Tabin i?d(X), corresponds to a bounded linear operator acting on CkBQ #b, where each X b is X. As noted above, each such matrix gives rise to a bounded operator when the index set B is finite.
'
2.6.13. PROPOSITION. Suppose that X is a Hilbert space and [ Tabla,bs~ is a matrix, with entries Tabin a(*). For eachfinite subset ff of B,let T(F)be the bounded linear operator, corresponding to the matrix [Tab] a,kF, that acts on the Hilbert space ChF0 X b (where each s b is 2 ) . 0) IlW1)ll 6 IlT(mll i f E 1 F2. a,bB corresponds to a bounded linear operator T acting (ii) The matrix [Tab] on CkBQ ixg ifand only ifthe set { 11 T(IF)II : IF afinite subset of B} of realnumbers is bounded above. When this is so,
IlTll = sup{l(T(IF)II:ff afinite subset of B}, Proof. Let 9denote the class of all finite subsets of B, and when ff E let F E and ~ u is an element
*(IF) be the Hilbert space ChEF O X b .Note that if I CkF0 xb of ,)FI(% then
llu112
=
c
t€F
llXb112,
llT(F)u112 =
11
c TabXb112.
hb
(i) Suppose that IF1, IF, €9 and ff E IF2. With u an element ChF,Q X b of 2(ff'), let w be the element CkFl0 xb of #(IF2) obtained by taking X b to be 0
152
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
when bEIFz\IF1. Then
= I I ~ ( ~ z ) w 1G I 2llm2)l1211w11z = l
l ~ ~ ~ 2 ~ l l z l l ~ l l z ~
and therefore IIT(!F1)ll< IIT([Fz)ll. (ii) Suppose first that the set { llqlF)ll: IF E F}has finite supremum k. In view of the discussion preceding the proof of (i), it follows that
whenever IFE 9and x b E %b ( bE IF). Let u be an element CkB0 x b of Eke @ %b. Our proof that [Tab]is the matrix of a bounded linear operator is now divided into two stages. First, we show by the Cauchy criterion that, for each c in B, the sum CkB T c b X b converges to an element yc of X. For this, suppose E > 0, and choose IF, in 9 so that
1
lIxb(12
< 2/k2
whenever IF €9 and
F nF, = @.
kF
By enlarging IF, if necessary, we may suppose that C E [F,. When IF E 9 and ff n IF, = 125, let IFo = IF u {c}, and define xi = X b ( bE IF), xi = 0. From (22),
11
TcbXbIIZ
= 11
bsf
1
TcbXi112
kF0
6
c 11 c
aeFo
Tabxb1Iz
&F,
< kZ 1 IlxJl’ = kZ C llxbllZ < E kFrJ
~ .
k F
Thus IICkFTcbXbll < E whenever F E and~ IF n [F, = 0,the Cauchy criterion is satisfied, and CbEB T c b X b converges to an element yc of X. Second, we prove that
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2.6. CONSTRUCTIONS WITH HILBERT SPACES
For this, suppose that ff ff, E% let ff = [F1 u [F2, and define xb = x b ( b IF,) ~ xb = 0 ( bE ff\F2). From (22),
When IF2 increases to B, llCbFlT,,bXbl( from the preceding inequalities that
-,IlypII;and since IF1
C IIYaII’ < k2
is finite, it results
IIxbII’. b B
ad,
Since the preceding relation has been proved for each
1 llYa1I2 < k2 1 ll€B
[F1
in
g
IIXb1l2.
b B
From the two assertions just proved, there is a bounded linear operator T, acting on CbB0 Z b , with
T(C@x b ) = 10Y b
where
Ya =
C TabXb
(a E B),
bsB
Since this is a restatement of (21), T has matrix [ T a b ] . Conversely, suppose that [ T a b ] is the matrix of a bounded linear operator T. With IF in 9and u an element CbEF 0 x b of Z(Q, let u be the vector Eke 0 x b in CkB0 x b , obtained by defining x b = 0 when b E B\F. Then
Ilr(F)ul12
=
111 1 a€F
b€F
TabXbl12
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2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
Thus IIT([F)II < IlTll, and the set {Ilr([F)II:[ F supremum at most IlTll. This, with (23), gives
E ~ } is
bounded above, with
IlTll = suP{llT(~)Il:F E 5 ; ) . In the circumstances described in Proposition 2.6.13, we regard the operator T(F)as the “finite diagonal block” of the matrix [Tab]corresponding to the finite subset [F of B. The main result of the proposition is the assertion that [Tab]is the matrix of a bounded linear operator if and only if the set of norms of all the finite diagonal blocks is bounded above.
2.7. Unbounded linear operators
In Section 1.5, we discussed linear transformations from one normed space into another. We noted in Theorem 1.5.5 that the continuity of such a transformation is equivalent to its boundedness (on the unit ball), so that we speak, interchangeably, of “continuous” and “bounded” linear transformations. In Section 2.4, we specialized the discussion of bounded linear transformations to Hilbert space. In this section, we take up the study of discontinuous (and, necessarily, unbounded) linear transformations between Hilbert spaces. We have only to think of the process of differentiation to be convinced that unbounded linear operators arise in the most natural way and that they are important. Without proceeding carefully, let 9 be the linear manifold of allfin L,(R) (relative to Lebesgue measure) almost everywhere differentiable with Then D is a linear transformation and derivativef’ in L2(R);and let D ( f ) kf’. D is not bounded. (If fk(t) = exp(- kltl), with k a positive integer, then IlDfklllllfkll = k.) Although D is defined on a dense submanifold of L,(R) (as follows from classical approximation results), it is certainly not defined on all of L,(R). We must expect, then, in dealing with unbounded linear operators, to specify a domain of definition 9(T)for our operator T (and, thereafter, to exercise care not to apply T, in a formal way, to each element that suits our convenience). Not only is the subject of unbounded linear operators natural and important, but the literature devoted to it is vast (almost as a consequence). Not to divert ourselves from the purpose at hand, we restrict the examples and results described in this section to a bare minimum. Unbounded operators will appear again in Section 5.6, when we extend to unbounded self-adjoint operators the spectral theory developed there for bounded self-adjoint operators. A “polar decomposition” for (closed) unbounded operators appears in Section 6.1, and a formulation of Theorem 7.2.1 in terms of unbounded operators appears in Section 7.2; but the essential use of
2.7. UNBOUNDED LINEAR OPERATORS
155
unbounded operator theory occurs, for us, in the presentation in Section 9.2 of modular theory. For the most part, the naturally arising unbounded operators retain some vestiges of orderly “limit properties” - notably, the possibility of extending them to operators with closed graphs. While this assumption may seem like a negligible replacement for continuity, it can be turned to remarkable advantage, as we shall see. In Section 1.8 we associated a graph B(T) with a linear transformation T, where B(T ) = { (x, Tx) : x E 9(T)}.The closed graph theorem (1 3 . 6 ) tells us that if Tis defined on all of A? (mapping into the Hilbert space X ) and B(T )is closed, then T is bounded. (Conversely, if T is bounded and everywheredefined, B(T )is closed.) This provides us with the possibility of an assumption intermediate between continuity and the totally unrestricted linear operator. Let T be a linear mapping, with domain 9 ( T ) a linear submanifold (not necessarily closed), of the Hilbert space A? into the Hilbert space X. We say that T is closed when B(T) is closed. The unbounded operators T we consider will usually be densely defined, that is, 9(T)is dense in S. Whatever T we consider, it has a graph ’3(T),and the closure B(T)- of B(T ) will be a linear subspace of 2 0 X. It may be the case that Y(T)- is the graph of a linear transformation T, but it need not be. If it is, T “extends” T and is closed. We say that To extends (or is an extension of) T, and write T G To, when 9 ( T ) E 9(To) and Tox = Tx for each x in 9(T).If B(T)- is the graph of a linear transformation T, clearly T is the “smallest” (“minimal”) closed extension of T. In this case, we say that T ispreclosed (the term closable is also used) and refer to Tas the closure of T. If ’3(T)- contains elements ( x , y ) and (x,y’) such that y # y’ (equivalently, -since B(T)- is a linear space, if (O,Z)EB(T)-with z not 0), then B(T)- is not the graph of a (single-valued) mapping and Tis not preclosed. This is, of course, the only way in which Tcan fail to have a closure (for, otherwise, the mapping that sends the first to the second coordinate of Y(T)- defines T). Interpreting B(T)- as the closure of B(T) in limit terms, we see that T is preclosed if and only if convergence of the sequence {x,} in 9 ( T ) to 0 and {Tx,} to z implies that z = 0. From the point of view of calculations with an unbounded operator T, it is often much easier to study its restriction Tlgo to a dense linear manifold goin itsdomain 9(T)than to study Titself. If Tisclosed andB(TIQ0)- c= B(T),the information obtained in this way is much more applicable to T. In this case, we say that gois a core for T. Each dense linear manifold in B(7‘)corresponds to a core for T. 2.7.1. EXAMPLE. With the notation of Example 2.4.10, remove the restriction that g be bounded and let Tbe defined as in that example for those x in 3Ep such that CyEY Ig(y)(x,y)12 is finite (so that 9 ( T ) consists of such vectors x). Of course 9(T) contains the submanifold go of all finite linear
156
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
combinations of the basis elements in Y, from which 9 ( T ) is dense in X. At the same time, with u and u in 9(T),from the Cauchy-Schwarz inequality,
so that B(T) is closed. The submanifold 9,,is easily seen to be a core for T.
2.7.2. EXAMPLE.With the notation of Example 2.4.11, once again remove the restriction that g be bounded (requiring only that g be measurable and finite almost everywhere) and let MBbe defined as in that example for those x in %' such that jsI(Mex)(s)12dm(s) is finite (so that 9 ( T ) consists of such x). The present example extends the preceding example to the case of non-discrete (a-finite) measure spaces (so that the important case in which Y is denumerable is included). Again 9 ( T ) is dense since it contains the submanifold g o of measurable functions on S with support in a set of finite measure on which g is essentially bounded. The Cauchy-Schwarz inequality assures us once again that 9(7')is a linear manifold. A more general measure-theoretic argument of the character of that appearing in the preceding example establishes that M Bis closed and g ois a core for it. (See the comments following Theorem 5.2.4.) W 2.7.3. EXAMPLE.IS%' is a separable Hilbert space with orthonormal basis {y,,}n=1,2,... and TyP= ny,, then T extends linearly to the (dense) linear
manifold g oof finite linear combinations of basis vectors y n .If we denote this extension by T again (so that 9(T)= go),then T is densely defined, unbounded, and not preclosed. To see this, it suffices to note that n-'y, -+ 0 while T(n-'y,,) = y1 + y l . H
2.7. UNBOUNDED LINEAR OPERATORS
157
We define the operations of addition and multiplication for unbounded operators so that the domains of the resulting operators consist precisely of those vectors on which the indicated operations can be performed. Thus 9 ( A + B) = 9 ( A ) n 9 ( B ) and ( A B)x = Ax Bx for x in 9 ( A + B). Assuming that 9 ( B ) E X and 9 ( A ) c X , where B has its range in X, A B is defined as the linear transformation, with { x :x ~ 9 ( Band ) B x E ~ ( A )as} its domain, assigning A(Bx) to x. Of course 9 ( a A ) = 9 ( A ) and (aA)x = a(Ax). More care is needed in defining the adjoint of an unbounded operator.
+
+
2.7.4. DEFINITION. If Tis a linear transformation with 9(T)dense in the Hilbert space X and range contained in the Hilbert space X, we define a mapping T*, the adjoint of T, as follows. Its domain consists of those vectors y in X such that, for some vector z in Z, ( x , z ) = ( T x ,y ) for all x in 9(T).For such y , T*y is z. If T = T*, we say that T is self-adjoint. In connection with this definition, we must note that there is at most one z (for a given y ) since x can assume values in the dense set 9(T);so T* is well defined. Note, too, that the existence of z is equivalent to the boundedness of the linear functional x + ( Tx,y ) on 9(T )(for, given that it is bounded, it has a unique bounded extension from 9 ( T ) to X, and Riesz's representation theorem (2.3.1) provides us with z). The formal relation ( T x , y ) = ( x , T * y ) , familiar from the case of bounded operators, remains valid in the present context only when x E 9(T ) and y e 9(T*).
2.7.5. REMARK.If To is densely defined and Tis an extension of To,then TO*is an extension of T*. To see this, suppose that y ~ 9 ( T *and ) uc9(T0). Then
so that y ~ 9 ( T g *and ) Tgy = T*y.
2.7.6. REMARK.If Tis densely defined, T* is aclosed linear operator; for, with u and u in 9 ( T * ) and x in 9 ( T ) ,
+ T*u) = (ax, T*u) + ( x , T*u) = ( T x , au) + ( T x , u ) = ( T x , au + u ) , so that au + U Eg ( T * )and T*(uu + u) = aT*u + T*u. Thus 9 ( T * )is a linear (x,aT*u
manifold and T* is a linear operator. If {u,} is a sequence in 9 ( T * )converging to u such that {T*un}converges to u', then, with u in 9(T),(u, T*u,) = (Tu, u,); and { (u, T*u,)} converges to (Tu, u ) . Thus (Tu, u ) = ( u , u ' ) for each u in 9(T);so u ~ 9 ( T * )and , T*u = u'. It follows that T* is closed.
158
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
2.7.7. REMARK.There are several ways in which we can use the hypothesis that T (that is, % ( T ) )is closed. The mapping P taking (u, u) to u is a bounded linear transformation of the Hilbert space %(T)into S. Thus P has a bounded adjoint P* mapping 2 into %(T).Since ( O , z ) € % ( T ) only when z = 0, P has null space (0). From Proposition 2.5.13, the range of P* is dense in Y(T). Thus, with u in 2,if P*(u) = (w, w') (in %(T)),then Tw = w' and, for each u in 9(T), (u, U )
= (P*(u),(u, Tu)) =
(w, U )
+ (w',Tu).
Hence ( Tu, w') = (u, u - w ) and w' E 9(T*). Moreover, T*w' (= T*Tw) = u
- W,
so that (T*T + I)w = u. While it is not clear, apriori, that B ( T * T )consists of more than the vector 0, our brief computation, relying on the information that %(T) is closed, allows us to conclude that B ( T * T )contains (and, therefore, is) a core for T (namely, the first coordinates of the range of P*). We learn, at the same time, that T*T + Z has range 2 (for D was an arbitrary element of 2).
Making use of the preceding remarks, there is no difficulty in proving the main theorem of this section.
2.7.8. THEOREM.If T is a densely defined transformationfrom the Hilbert space S to the Hilbert space X, then
(n*
(i) if T is preclosed, = T* ; (ii) T is preclosed if and only 9(T*) is dense in A!-; (iii) if T is preclosed, T** = T ; (iv) if T is closed, T* T + I is one-to-one with range 2 andpositiue inverse of bound not exceeding 1 ; (v) T* T is self-aa'joint when T is closed.
(n*
Proof. (i) Since T E T ; from Remark 2.7.5, s T*. Suppose y ~ g ( T * )For . each x in 9(T),there is a sequence {x,,} of vectors in 9(T) converging to x such that {Tx,,} converges to Tx. Thus ( T x , ~=) lim( Tx,,y) = lim(x,,, T * y ) = ( x , T*y),
so that y ~ g ( ( n * and ) (T)*y = T*y. Hence (T)* = T*. (ii) If T is preclosed, from Remark 2.7.7, %(T)contains a dense linear manifold (the range of P*) consisting of pairs (x,Tx) with Tx in 9 ( T * ) ( = 9(T*)).If y is orthogonal to the range of T, then 0 = ( T x , y ) = ( Tx, y) for each xin 9(T);and y is in 9(T*)(y is annihilated by T*).Thus 9(T*)contains a dense subset of the range of T as well as the orthogonal complement of this range. Since 9 ( T * )is a linear manifold, it is dense in X.
159
2.7. UNBOUNDED LINEAR OPERATORS
Suppose, now, that 9(T*) is dense in X and {u,} is a sequence in 9 ( T ) converging to 0 such that {Tu,} converges to u. With y in 9(T*), (Tu,,,~)= (u,,, T*y); so (u,,, T*y) converges both to 0 and to ( u , y ) . Since 9(T*) is dense in X, u = 0 and T is preclosed. (iii) If Tis preclosed, 9(T*) is dense, from (ii), and T* has an adjoint T**. If y ~ 9 ( T * and ) x ~ 9 ( T ) then , (T*y,x) = (y, Tx), so that x ~ 9 ( T * * and ) T**x = Tx. Thus T** is a closed (from Remark 2.7.6) extension of T, and T E T**. From Remark 2.7.5, T*** c T* = T*. Since T* is closed, we have, as well, T* c (T*)**. Thus T* = T***. As noted T c T** (equivalently, %(T)E %(T**)).If ( x , T**x), in Q(T**), then (x, u ) + (T**x, Tu) = 0 for each u in 9( This is orthogonal to %( holds, in particular, when T U E ~ ( T *(= ) g(T***)); and, for such u,
n,
n.
0 = (x,(T*T+ I ) u ) .
But, from Remark 2.7.7, (T*T + I)u takes on all values in 2.Thus x = 0, %(T**)= and T** = T. (iv) We noted in Remark 2.7.7 that the domain of T*T (and, hence, of T*T + I)is a core for T when T is closed and densely defined. We noted, too, that T*T + I has 2 as its range. If X E ~ ( T *+TI>, then
%(n,
llxll’
< (x,x) + (Tx, Tx)
= ((T*T+ I)x,x)
< II(T*T + 0x11 IIxII.
+
Thus T* T I has (0) as null space, is one-to-one, and has a bounded inverse H of bound not exceeding 1. From this same computation, and since each z in 2 has the form (T* T + I)x, it follows that ( z , H z ) is (( T*T + I ) x , x ) , which is real and non-negative. Thus H is positive. (v) As noted in (iv), 9(T*T) is a core for T; hence it is dense in 2. Since
+(XJ) when x E 9(T* T), we see that (T* T)* and (T*T + I)*have the same domain and that (T*T)* + I = (T*T + I)*. With y in 9(T*T), ((T*T+ I ) x , y ) = (T*Tx,y)
(T+Tx,y) = (x, T*TY), so that T*T G (T*T)* and T*T + I c (T*T + I)*. Itfollowsthat(T*T+ I)* has 2 as its range. If (T*T + Z)*y = 0, then, for each x in 9(T*T),
0 = ((T*T+ I ) * ~ , x )= (y,(T*T+ I ) x ) . Since T*T + I has range 2, y = 0. Thus (T*T + I)*is one-to-one, extends T*T + I, and has the same range as T*T + I. It follows that T*T
+ I = (T*T + I)*= (T*T)* + I,
so that T*T = (T*T)*. H
160
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
The statement that Tis self-adjoint (T = T*) contains information about the domain of T as well as the formal information that (Tx, y ) = (x, Ty) for all x and y in 9(T).When 9(T ) is dense in 2 and (Tx, y ) = (x, Ty) for all x and y in 9(T), we say that T is symmetric. Equivalently, T is symmetric when T c T*. Since T* is closed and 9(7') c 9(T*), in this case, Tis preclosed if it is symmetric. If Tis self-adjoint, Tis both symmetric and closed. The operation of differentiation on an appropriate domain provides an example of a closed symmetric operator that is not self-adjoint. In Proposition 2.7.10 we describe conditions that guarantee that a given closed symmetric operator is selfadjoint. If A c T with A self-adjoint and T symmetric, then A c T c T*, so that (T** c ) T * E A* = A G T G T* and A = T. It follows that A has no proper symmetric extension. That is, a selfadjoint operator is maximal symmetric. 2.7.9. LEMMA. Zf Tis closedandsymmetric, T f ilhave closedranges. Zf T is closed and 0 < (Tz, z ) for z in 9(T), then T I has a closed range.
+
Proof. Suppose {x,} is a sequence in 9 ( T ) such that {( T f il)x,} tends to y . Note that, with z in 9(T), (Tz, z ) is real, so that
< ( ( T z , ~ )+~( z , ~ ) ' ) ' ' ~= I((Tf il)z,z)l < ll(Tf il)zlJ11z11. Thus Ilx, - xmll< ll(T f il)(x, - xm)lland {x,} isconvergent. Suppose x, llzll'
-+
x.
Since {Tx,} converges to T ix + y and T is closed, x ~ 9 ( T and ) Tx = f ix + y . Thus y = ( T f il)x, and T f i l have closed ranges. Suppose, now, that T is closed and 0 < (Tz, z ) for each z in 9(T). Then
llzl12 < (Z,Z> + (Tz,z) for z in 9 ( T ) , and, as above, T
< ll(T+ Ozll 11z11,
+ I has closed range.
W
2.7.10. PROPOSITION. If T is a closed symmetric operator on the Hilbert space 2, the following assertions are equivalent; (i) (ii) (iii) (iv)
T is self-adjoint; T* f i l have (0) as null space; T f il have 2 as range; T f iI have ranges dense in 2.
Pro05 (i) -+ (ii). If T = T*, for each x in 9(T),(Tx, x) (Tx,x) is real. Thus
= (x,Tx) ; and
f i l ) x , x ) = ((T f i l ) x , x ) = (Tx,x) f illxl12 = 0 only if x = 0. Hence T* f i l have (0) as null space. ((T*
2.8. EXERCISES
161
(ii) + (iii). From Lemma 2.7.9, T k il have closed ranges. Thus, it suffices to show that these ranges are dense in X . If ( ( T f il)x,y ) = 0 for all x in 9(T),then ( T x , ~=) f i ( x , y ) , so that y ~ 9 ( T *and ) T * y = f iy. Since T* t- il have (0) as null space, y = 0. Hence T f il have dense ranges. (iii) -(iv). This follows from the preceding discussion. (iii) + (i). Since T is closed and symmetric, T G T* and Y ( T )is a closed subspace of the closed space Q(T*).If ( y , T*y)in Q( T*)is orthogonal to Y( T ) , then (y,X>
+ ( T*Y,T x ) = 0
for each x in 9(T).Since T f i l have range X, there is an x in 9(T)such that ( T x e 9 ( T ) ,and) y = ( T + il)(T - il)x ( = ( T 2 + Z)x). For this x, (Y,Y> = ( y , ( T 2 + O x ) = ( Y , x > + ( T * y , T x ) = 0.
Thus ( y , T*y) = (O,O), Y ( T ) = Y( T*),T = T * , and T is self-adjoint. 2.7.11. REMARK.If T is self-adjoint, it follows from (iii) of Proposition 2.7.10 and the inequality at the beginning of the proof of Lemma 2.7.9 that T f il have everywhere-defined, bounded inverses with bound not exceeding 1 . rn 2.8. Exercises
2.8.1. Show that a finite set { x l , .. . ,xn}of n vectors in a Hilbert space X is linearly independent if and only if the n x n matrix that has ( x j ,x k ) in the ( j ,k ) position is non-singular. 2.8.2. Show that a Hilbert space is uniformly convex (in the sense defined in Exercise 1.9.13). 2.8.3. Show that, if X is a real Hilbert space, then X x X becomes a (complex) Hilbert space Xc when its linear structure, inner product, and norm, are defined by ( x ,y ) + (u, u ) = ( x + 4 y (a
+ 01,
+ ib)(x,y ) = (ax - by, bx + uy), + ( y , u > + i - 0,u>, l l ( X ~ Y ) I l 2= llX1l2 + llY1I2,
( ( x , Y ) (u, , ~ 1= ) ( x , u>
for all x , y , u, u in X and a, b in R.
162
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
Prove also that the set {(x, 0) :x E X } is a closed real-linear subspace XRof Zc,that Xc = { h + i k : h , k ~ X ~and } , that the mapping x + (x,O) is an isometric isomorphism from X onto (the real Hilbert space) XR. 2.8.4. Suppose that {xl, x2,x3,.. .} is an orthonormal basis in a Hilbert space X and that
Prove that Y is a bounded closed convex set that has no element with greatest norm. 2.8.5. Suppose that Y is a closed convex set in a Hilbert space X, 4!y is the set of all unitary operators U acting on X for which U ( Y ) = Y, and Yo = {YE Y : Uy = y for each U in
aY}.
(i) Prove that Yo is not empty. [Hint. Use Proposition 2.2.1.1 (ii) Show that, if Yo consists of a single non-zero vector yo, then Y is a subset of the hyperplane
EX: Re(x - y o , y o ) = 0). 2.8.6. Prove that a bounded sequence of vectors in a Hilbert space has a weakly convergent subsequence. 2.8.7. Suppose that A is an uncountable set and, for each a in A, ma is Lebesgue measure on the a-algebra of Bore1 subsets of the interval [0,1] (= S,). Show that, if (S,g m ) is the corresponding infinite-product measure space (see [H: p. 158]), then L 2 ( S , g m )is non-separable. 2.8.8. Suppose that X is a Hilbert space in which the inner product is denoted by ( , ) and that K e g ( % ) + . Show that the equation (X,Y)l
= (KX9.Y)
(X,YEX)
defines an inner product ( , on X. By means of the Cauchy-Schwarz inequality for ( , ) 1 , prove that
IIKIJ= min{a:aER, K G a l } . 2.8.9. Let &' be a Hilbert space in which the inner product and norm are denoted by ( , ) and 11 1 , respectively. Suppose that ( , )1 is another definite inner product on Z and the corresponding norm 11 Ill satisfies llxlll < llxll for each x in X. Prove that there is a positive self-adjoint operator K, acting on X,
163
2.8. EXERCISES
2.8.10. Suppose that X is a Hilbert space in which the inner product and norm are denoted by ( , ) and 11 I), respectively. Let K be a positive element of a(%'), and define an inner product ( , )1 on &' by (X,Y)l = (KX,Y) Let IIxIll
=
(XlYEX).
[(x,x)~I~'~.
(i) Prove that 11 [I1 is a norm on X if and only if K has null space (0). (ii) Show that, if K has null space {0},then the norms 11 11 and 11 [I1 give rise to the same topology on 2 if and only if K has an inverse in a(&'). (iii) Suppose that K has an inverse in a(%). If A* denotes the adjoint of an element A of a(X)relative to the inner product ( ,),find a formula for the adjoint of A relative to the inner product ( , 2.8.1 1. Suppose that T is a bounded self-adjoint operator acting on a Hilbert space X and k is a positive real number such that - kZ < T < kZ. By using the identity 4 Re(Tx9.Y) = (T(x
+ Y ) , x + r>- (T(x - Y ) ,x - Y>,
show that IRe(Tx9Y)l d ;kk(llxl12 + llY1l21 for all x and y in X. Deduce that llTll < k and that
IlTll =min{a:aER, - a Z < T < a Z } = sup{l(Tx,x)l : X € X : llxll =
l}.
2.8.12. A bounded linear operator A , acting on a Hilbert space X, is said to attain its boundif llAxll = llAll for some unit vector x in 2.Give examples of (a) a bounded self-adjoint operator with an orthonormal basis of eigenvectors, (b) a bounded self-adjoint operator with no eigenvector, neither of which attains its bound. 2.8.13. Let X be a Hilbert space. (i) Prove that each unit vector x in X is an extreme point of the unit ball (X)l. (ii) Prove that each isometric linear operator V from X into X is an extreme point of the unit ball (a(#)), .
164
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
2.8.14. Show that the projection E from a Hilbert space &' onto a closed of all positive operators in subspace X is an extreme point of the set the unit ball of a(&'). 2.8.15. Determine a necessary and suficient condition for the operator MB,defined in Example 2.4.1 1, to have a bounded inverse. ) each a in A, 2.8.16. Suppose that T = CacA@ T,, where T , E ~ ( & ' ,for and sup{11 Tall:a E A} < co. Show that T has a bounded inverse if and only if the following two conditions are satisfied :
(i) each T, has a bounded inverse, (ii) sup{llT,-'Il:a~A} < 00. 2.8.17. Let 9 denote the set of all projections from a Hilbert space &' onto its closed subspaces, and suppose that F E ~0 # , F # I. Prove that the mappings E+EAF,
E-EvF
are not continuous, from 9 with the norm topology into 9 with the strongoperator topology. 2.8.18. Let 9'denote the set of all bounded self-adjoint operators acting on a Hilbert space X. If A, B, CE% we say that Cis a lower bound of { A , B} if C < A, C < B. We say that Cis the greatest lower boundof { A , B } if it is a lower bound of {A, B}, and D < C whenever D is a lower bound of {A, B}. (i) Show that, if A , B E % then {A, B} has a lower bound in 9 (ii) Suppose that A, Bare non-zero elements of a(&')'. Show that there is a vector xo such that ( A x o ,xo) > 0 and ( E x , , xo) > 0. Prove that, if Po is the projection onto the one-dimensional subspace containing xo,a and b are suitable positive real numbers, and T = aPo - b(I - Po),then T < A, T < B, T 6 0. Deduce that 0 is not the greatest lower bound of {A, B}. (iii) Suppose that A, B E % and {A, B} has a greatest lower bound Cin 9 By applying the result of (ii) to {A - C, B - C}, show that either A < B or B < A. 2.8.19. Suppose that A and B are mappings from a Hilbert space X into itself, and ( A x , y ) = ( x , B y ) for all x and y in 2.Prove that A and B are bounded linear operators; and A = B*. 2.8.20. Suppose that &' is a Hilbert space and A €a(&'). Prove that the following five conditions are equivalent.
165
2.8. EXERCISES
(i) A is continuous as a mapping from the unit ball (%), (with the weak topology) into % (with the norm topology). (ii) If x, x l , x 2 , . . . € 2and {x,) is weakly convergent to x, then { A x , } is norm convergent to Ax. ( 5 ) Every bounded sequence {x,} in A? has a subsequence {x,(k)} such that {A+..} is norm convergent. (iv) The set { A x : x E ( # ) , } is relatively compact in the norm topology of x. (v) The set { A x : x ~ ( # ) ~is} compact in the norm topology of S. [An element of a(%)that has any (and, hence, all) of the above properties is described as a compacf linear operator.] 2.8.21. Prove that the identity operator, acting on an infinite-dimensional Hilbert space, is not compact (in the sense of Exercise 2.8.20). 2.8.22. Suppose that % is a Hilbert space and
9 = { A €&?(A?): A has finite-dimensional range}. (i) Prove that, if A €9and {yl,. . . ,y,} is an orthonormal basis of the range of A, there exist vectors x l r . .. ,x, in % such that n
Ax =
C ( x , x,)yn
(xEH).
j= 1
(ii) Prove that 9 is a two-sided ideal in a(%)and that every non-zero two-sided ideal in g(%) contains A (iii) Prove that an element A of a(&)lies in 9 if and only if A is continuous as a mapping from % (with the weak topology) into 2' (with the norm topology). (iv) Prove that the elements of 9 are compact linear operators (in the sense of Exercise 2.8.20). 2.8.23. Suppose that {A,} is a sequence of compact linear operators acting on a Hilbert space 2, A E&?(%), and ( [ A ,- ,411 -,0. Prove that A is compact. [Hint. Use condition (i) of Exercise 2.8.20 as the defining property of a compact linear operator.] 2.8.24. Suppose that A is a compact linear operator acting on a Hilbert space A? (see Exercise 2.8.20).
(i) Prove that the (closed) range space [A(%)] is separable. (ii) Suppose that [A(%)] is infinite-dimensional, and let {yl , y z ,y,, ..-} be an orthonormal basis of [A(%)]. For each positive integer n, let P, be the
166
2. BASICS OF HILBERT SPACE A N D LINEAR OPERATORS
projection from 2 onto the subspace spanned by yl , . . . , y n . Prove that $4 - P,,A(I + 0 as n 3 00. (iii) Deduce that A lies in the norm closure of the ideal f (see Exercise 2.8.22), and A* is compact. 2.8.25. Let X denote the set of all compact linear operators acting on a Hilbert space X. By using the results of the three preceding exercises, show that:
(i) Z is the norm closure of the ideal 9 in a(%); (ii) X is a norm closed two-sided ideal in g ( 2 ) ; (iii) each non-zero norm closed two-sided ideal in @ ( X contains ) X. 2.8.26. Suppose that {yl,y2,y3, .. .} is an orthonormal system in a Hilbert space S, and { A1,A 2 , A,, . . .} is a sequence of real numbers such that 11112 IAzI 2 2 . * Suppose also that the sequences {y,,} and {A,,} are either both finite and of the same length or both infinite, with {A,,} converging to 0. Show that the equation a .
Ax
=
C An(X, yn)yn
(X E 2 )
n
defines acompact self-adjoint operator A on 2,with IlAll = /All.[The result of Exercise 2.8.29 below shows that every compact self-adjoint operator has the form just described.] 2.8.27. Suppose that A is the compact self-adjoint operator constructed in Exercise 2.8.26 from an orthonormal system { y , ,y 2 ,y,, . ..} in a Hilbert space X and a real sequence {A,, A 2 , A,, . . .} that satisfy the conditions set out in that exercise. Extend the orthonormal system to an orthonormal basis {YI r ~ ~ , * ~ 3u ,{ Z a } ,
(i) Prove that, if 3, is a non-zero scalar that does not appear in the sequence {An}, the operator A - AI has an inverse in a(%'), and
[Hint. Consider the matrix of A.] (ii) Show that, if A is a non-zero scalar that appears in the sequence {A,,} and x E X, the equation ( A - AI)z = x
167
2.8. EXERCISES
has a solution z in X if and only if ( x , y k )= 0 for each integer k satisfying = 1. What is the most general solution z , when this condition is satisfied?
,Ik
2.8.28. Let A be a bounded self-adjoint operator acting on a Hilbert space X. (i) Show that each eigenvalue of A is real. (ii) Show that eigenvectors corresponding to distinct eigenvalues of A are orthogonal. 2.8.29. Let A be a compact self-adjoint operator acting on a Hilbert space X. (i) By using the result of Exercise 2.8.11, show that there exist unit vectors xl, x2,x3, . . . in 4 such that the real sequence { ( A x , , ,x,,)} converges, Show that IIAx,, - px,,ll-, 0 as n + 00. with limit p equal to IlAll or - JIAJJ. (ii) Prove that A x = px for some unit vector x in 2 (so that a non-zero compact self-adjoint operator has a non-zero eigenvalue). (iii) Show that, if A is a non-zero eigenvalue of A , then the null space of A - LZ has finite dimension. (We call this finite dimension the multiplicity of Iz as an eigenvalue of A . ) (iv) Prove that, if E is a positive real number, there are only a finite number of different eigenvalues p of A such that 1p( > E. Deduce that the distinct non-zero eigenvalues of A either form a finite set or form a sequence converging to 0. (v) Let {pl ,p 2 , p 3 , . . .} be the (finite or infinite) sequence of all distinct and non-zero eigenvalues of A , arranged so that lpl(2 Ip21 2 Ip31 2 . suppose that p,, has multiplicity m(n). Let {Al, 12,,I3,. ..} be the real sequence consisting of p1 (m(1) times), followed by p2 (m(2) times), followed by p3 (m(3) times), and so on. Let {yl ,y 2 ,y,, . . .} be a sequence of unit vectors consisting of an orthonormal basis of the null space of A - p u l l , followed by an orthonormal basis of the null space of A - p21, followed by an orthonormal basis of the null space of A - p3Z, and so on. Show that {y,,}is an orthonormal system, and Ay,, = Any, for each n. Prove that, if A . is the compact self-adjoint operator defined by
-
Aox = CAn(x,yn)yn
a ,
(XEW
n
(see Exercise 2.8.26), then A - A . has no non-zero eigenvalue. Deduce that A = Ao. (vi) Show that A 0 if and only if A,, 2 0 for all n. Deduce that, in this case, A has a (compact) “positive square root” A l (that is, A: = A and A l 2 0) such that IJA11(2= ( [ A [ ( .
168
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
2.8.30. Let A be a self-adjoint operator acting on a Hilbert space X, and = JJAJJ. let x be a unit vector in X such that JJAxJI (i) Show that x is an eigenvector for A' corresponding to the eigenvalue
llA1I2. (ii) Show that either Ax = llAllx or Az = - IJAllzfor some unit vector z.
[Hint. Consider the vector JlAlJx- Ax.] (iii) Under the added assumption that A 3 0, show that x is an eigenvector for A corresponding to the eigenvalue llAJ1. 2.8.31. Let E and F be projections acting on a Hilbert space X.
(*I
(i) Show that, if E and F commute, EvF<E+F.
(ii) By means of a two-dimensional example, show that (*) need not hold when E and F do not commute. 2.8.32. Suppose that &' is an infinite-dimensional Hilbert space. Let { y l , yz ,y , ,. . .} be an orthonormal sequence in X. By consideringthe sequence {V,} in @X),where Vflx = (X,Yn)Yl
(XEX),
prove that the adjoint operation is not strong-operator continuous on (the unit ball of) W(X).Deduce that the strong-operator topology on g ( X )is strictly coarser than the norm topology. 2.8.33. Let X be an infinite-dimensional Hilbert space. Given any finitedimensional subspace F ( # (0)) of $', let AF = n(Z - PF),where n is the dimension of F and PF is the projection from X onto F. Choose any orthonormal system of 2n vectors, {xl,. . . ,x , , y l , . ..,y,}, the first n of which form an orthonormal basis of F, and define V,, in .@(%),by
In this way, we obtain nets { A F ) ,{ V F ) ,{ A F V F )where , the finite-dimensional subspaces Fare directed by the inclusion relation. Show that: (i) { A F }is strong-operator convergent to 0; (ii) { V,} is bounded, norm convergent to 0, and hence strong-operator convergent to 0; (iii) { A F V F }is not strong-operator convergent to 0. Conclude that multiplication is not (jointly) strong-operator continuous from W(X) x into a(*).
169
2.8. EXERCISES
2.8.34. Show that, if X is a separable Hilbert space, there is a countable strong-operator dense subset of 9?(X). 2.8.35. Show that, if 2 is a separable Hilbert space and { y , ,y,, y,, . . .} is an orthonormal basis of 2,the equation m
4s T ) =
c 2-"llsY"
-
mll
n= 1
defines a translation-invariant metric d on g ( X )(that is,
d ( S + R, T + R ) = d(S, T ) for each R in 9?(2)), and the associated metric topology coincides on bounded subsets of B ( X )with the strong-operator topology.
2.8.36. Suppose that XI,. . . , X nX, are Hilbert spaces and L: XI x . . . x Xn+ X is a bounded multilinear mapping. Suppose also that for each u in X, the bounded multilinear functional L,, defined by L,(Xl
9 .
. .,x n )
= ( W ,9 ' *
f
9
xn), u>,
is a Hilbert-Schmidt functional on XI x . x X,,. Prove that the mapping u + L, from the conjugate Hilbert space 3 into the Hilbert space X Y 9 of Proposition 2.6.2 is linear and has closed graph. Deduce that there is a positive real number d such that
IILullz 6 dllull where 11
[I2
(UEX),
denotes the usual norm on XY'R
2.8.37. Suppose that X is a Hilbert space, and let 2 9 0 denote the Hilbert space of all Hilbert-Schmidt operators from X into X (see the discussion preceding Proposition 2.6.9). (i) Prove that IIT((< llTllz for all Tin 2 9 0 ,where 11 11 and II (1, denote the usual norms on g ( X )and .%YO, respectively. (ii) By identifying %YO with 9 @ X, prove that the ideal {A E B ( X ):A has finite-dimensional range}
in B ( X ) is a 11 1I2-densesubset of XYO. (iii) Prove that the elements of 29'0 are compact linear operators.
2.8.38. Suppose that X is the L2 space associated with a a-finite measure , qy,z in L2(S x S , Y x x m x m) by space ( S , g m ) . When y , Z E X define q y . h ,t ) = z(sly(t). ~
170
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
(i) Show that, if Y and Z are orthonormal bases of 2, then the set { qy,z:y E Y, z E Z} is an orthonormal basis of L2(S x S, Y x 9, m x m). (ii) Show that, if k E L2(S x S , Y x g m x m),the equation (TkX)(S)
= Jsk(s, t)x(t)dm(t)
(XEX)
(where the integral exists for almost all s in S) defines an element T, of a(%). Prove also that Tk is a Hilbert-Schmidt operator acting on X, and that llTkl12 =
llkll*
(iii) Prove that every Hilbert-Schmidt operator on X arises, as in (ii), from an element of L2(S x S , Y x g m x m). 2.8.39. Suppose that ( S , Z m ) is a a-finite measure space, k E L2(S x S, Y x z m x m), and Tk is the Hilbert-Schmidt operator defined in Exercise 2.8.38(ii). Show that T: = T,, where h(s, t) = k(t,s). ~
2.8.40. Suppose that X and X are Hilbert spaces, A E B ( X ) ,and B Eg ( X ) .Prove that A @ Zhas an inverse in B(X 0 X ) if and only if A has an inverse in g ( X ) ,and that A @ B has an inverse in B(X 0 X ) if and only if A has an inverse in W ( X ) and B has an inverse in B(X). 2.8.41. Let {yl,y2,y3,. ..} be an orthonormal basis of a Hilbert space 3; and let [ a j , k ] , [ b j , k ] be the matrices, with respect to this basis, of bounded linear operators A, B acting on X. Prove that [ a j , k b j , k ] is the matrix of a bounded linear operator. [Hint. Let P be the projection from X 0 X onto the subspace X spanned by the orthonormal system {yl 0 yl , y 2 B y 2 , .. .}, and consider the operator T obtained by restricting P(A 0 B)P to X.] 2.8.42. Suppose that A is a closed linear operator with domain dense in a Hilbert space X and with range in X ; and let B be in g(X).Prove that AB is closed. Show also that, if ABis densely defined and bounded, then A B E ~ ( X ) . 2.8.43. Let {yl, y 2 , y 3 , .. .} be an orthonormal basis for a Hilbert space X, and let m
m
Define Bin g ( X )by Bx = (x, z ) z ; and define mappings Sand Twith domain 9 by m
SX =
C n2(x,y,,)yn, n= 2
TX = SX
+ (Sx,z)yl
(~€9).
2.8. EXERCISES
171
Show that Sand Tare closed densely defined operators, but that neither T - S nor BS is preclosed.
2.8.44. Suppose that A and B are linear operators with their domains dense in a Hilbert space X and their ranges in X. Prove that A* + B* G (A + B)* if A + B is densely defined, and that B*A* c (AB)* if AB is densely defined. 2.8.45. Suppose that Tis a closed linear operator with domain dense in a Hilbert space X and with range in A?. Show that the null space { x E .9(T ): Tx = 0} of T is a closed subspace of 2. Let N( T) and R(T) denote the projections whose ranges are, respectively, the null space of T and the closure of the range of T. Prove that R(T) = I - N( T*), R( T* T) = R( T*),
N ( T ) = I - R(T*), N( T* T ) = N( T).
[For the case in which T E W ( X ) ,these relations have been established in Proposition 2.5.13.1
2.8.46. Let T be a closed operator with domain dense in a separable Hilbert space X and with range in X. Show that the ranges of R(T) and R( T*) have the same dimension. 2.8.47. Let X be a Hilbert space and E be a projection with finitedimensional range. Let F be a projection such that the dimension of E ( X ) is less than the dimension of F ( X ) . Show that (I - E) A F # 0. 2.8.48. Show that, if Tis a linear operator with domain dense in a Hilbert space 2 and with range in X, and if (Tz, z) is real for each z in 9(T),then Tis symmetric. 2.8.49. Let X be the Hilbert space L a , corresponding to Lebesgue measure on the unit interval [0,1], and let gobe the subspace consisting of all complex-valued functionsfthat have a continuous derivative S’ on [0,1] and satisfyf(0) = f(1) = 0. Let Do be the operator with domain goand with range in 2 defined by D o f = f . Show that iDo is a densely defined symmetric operator and that (iDo)M - M(iDo)= iZ190,
where M is the bounded linear operator defined by
(Mf)(s) = sf(s)
(f€L2 ; 0 6 s 6 1).
172
2. BASICS OF HILBERT SPACE AND LINEAR OPERATORS
2.8.50. Let 2,go,and Do be defined as in Exercise 2.8.49, and let Xl = {f1€3EP: ( f i , u ) = 0}, where u is the unit vector in X defined by u(s) = 1 (0 < s < 1). WhenfEX, define Kfin X by
(Kf)(s)=
[
f(t) dt
(0 < s < 1).
0
(i) Prove that KE?#(%), that K has null space {0}, and that K(X1) 1 . 9 0 . (ii) Show that the equation DlKfl
=f1
(fl
defines a closed linear operator D 1with domain a1= K(Xl).Prove also that D1 is the closure of D o . (iii) Show that the equation D2(Kf
+ au) =f
(YEX,
a E C)
defines a closed linear operator D 2 ,with domain g 2= {Kf+ au:fe X, a E C}, that extends Dl . (iv) Prove that ( K f l ,f) + (fl,Kf+ au) = 0 for all fl in Xl,f in H, and a in C. (v) Show that DO* = D : = - D 2 . (vi) Let g3= {Kfl au:fle X 1 , a € @ } ,and let D3 be the restriction D2193. Show that D3 is a closed densely defined linear operator, that D1 E D3 = - DT E D 2 , and that iD3 is self-adjoint.
+
CHAPTER 3 BANACH ALGEBRAS
In this chapter, we study algebras that have a Banach-space structure relative to which the multiplication is continuous. The operator algebras, which form the principal object of study for us, are a special subclass. Our purpose is to locate those constructs (for example, spectrum and spectral radius), develop the techniques, and prove the results that are natural to this general setting.
3.1. Basics
Let B be a Banach space (complex or real) and, at the same time, an algebra with identity I, in which multiplication is separately continuous (that is, (A, B) + AB is continuous in A for each fixed B and in B for each fixed A). Denote by L A and R E the operators on % such that LA@) = AB = R,(A). From the continuity assumption, L A and RB are in a(%).Thus llLA(B)ll G IlRBll . llAll,and{IILA(B)II:llAll < 1)isaboundedsetofnumbersfor each B in %. From the uniform-boundedness principle (see Theorem 1.8.9), {llLAll:llAll 6 l} is bounded. Similarly {IIRBll:llBll < l} is bounded. It follows that the mappings 2:A + L A and 9 :B + RE, which are linear isomorphisms (note that L,(Z) = A and R,(Z) = B) of B into B(%), are continuous (bounded). The mapping 9 is an algebraic isomorphism, while W is an algebraic anti-isomorphism. Now,
< I l l L A l l l Ilrll, where, for the moment, we denote the norm on a(%) by 111 111. IlAll = I I L A ( 0 l l
~
~
~ G~ lllLAlll ~ = - ll19(A)lll 1 ~
*
Thus
1G 1l911~ ~ llAll*~ '
Hence 9(B) is a norm-closed subalgebra of a('%) (see Corollary 1.5.10(ii))and 9is an algebraic isomorphism and homeomorphism of % onto 2(%) As .far as the (combined) algebraical and topological properties of % and 9(B) are concerned, they are indistinguishable when identified by the isomorphism 9. The norm on a(%) enjoys some special properties: 111 ~ 1 1 1= 1, where I (= L,) is the identity mapping on %; and IIIq+III < lllqlll * 111+111. (Note that these 173
174
3. BANACH ALGEBRAS
properties of the norm on a(%) are valid for each normed space 2l, from the by that discussion preceding Theorem 1.5.6.) Thus the norm induced on 9(%) has these special properties. on a(%) If we assume that the norm on % satisfies
IIABII
(1) then, since llAB - "'1
= llLA(B)ll
< IlAll
*
llAll
JIB- B'll
*
llB1ly
+ IIA - A'll . IIB'II
G l l 4 * IP - B'll + IIA - A'll(llBll + when IIB - B'1I < 1, multiplication isjointly continuous on %. Thus multipliin any event, and since 9is an algebraic cation is jointly continuous on Y(%), isomorphism and homeomorphism, multiplication is jointly continuous on % - independent of the norm assumption. (The uniform-boundedness principle did the work in getting us joint from separate continuity of multiplication.) From the joint continuity, it follows at once that the closure of a subalgebra (ideal) is, again, a subalgebra (ideal). If we assume, now, that the norm on % satisfies 11111 = 1 as well as IlABll < l l 4 . 11B11, then from (I), IILA(r)ll
=
l l A l l < lllLAlll < IlAll,
so that Y is an isometry as well as an algebraic isomorphism. While the natural structural assumption on % is that of continuity of multiplication (either joint or separate), the preceding discussion assures us that nothing is lost if we make the convenient normalization assumptions on the norm. 3.1.1. DEFINITION.An algebra 2l (over R or C) with unit I is said to be a normedalgebra when %is a normed space such that llABll < llAll llBll for all A and Bin %, and llZll = 1. If % is a Banach space relative to this norm, % is said to be a Banach algebra. H 3.1.2. REMARK.From the discussion preceding Definition 3.1.1, we see that a normed algebra % is isometrically, algebraically isomorphic to a and that a(%) is a normed algebra. From Theorem 1.5.6, subalgebra of a(%), if 2l is a Banach space, then &?(a) is a Banach space, hence a Banach algebra. If 2l is a normed algebra, completing it to a Banach space @ (see Theorem 1.5.1) allows us to view each L A as a bounded linear transformation from % to '$I and extend it (uniquely) in a norm-preserving fashion (see Theorem 1.5.7) to an operator LAon @. The resulting mapping, 8 :A + LA,is then an isometric Moreover, 8 algebraic isomorphism of % into the Banach algebra a(@). extends to an isometric linear isomorphism of '$I onto the norm closure of p(%)in 99('$I). Thus '$I becomes a Banach algebra. We may say, briefly, that the completion of a normed algebra is a Banach algebra. H
175
3.1. BASICS
3.1.3. REMARK.The assumption that ‘u has an identity is not an essential restriction. If ‘u does not have an identity, we can employ the standard process for adjoining an identity to an algebra. We embed 2I in the algebra ‘ul of pairs (al,A), where a is a scalar and A E 2I. The multiplication (al,A) . (bl,B) = (abl,aB bA A B), addition (al,A ) (bl, B) = ((a b)l, A B), and multiplication by scalars c(al,A) = (cal, cA) impose the structure of an (associative) algebra on ‘ul,and (I, 0) is a unit for it. The algebra 2I appears as (that is, “is isomorphic to”) the subalgebra ((0, A): A in a}.Defining Il(al,A)ll to be la1 + llAll, we map ‘u isometrically onto a subalgebra of ‘ul by means of this identification. It is easily checked that 211 is a normed algebra; and if 2I is a Banach space, then 211 is a Banach algebra (with identity). Despite the possibility of adjoining an identity to a Banach algebra, it is sometimes artificial and inconvenient to do so (as in Subsection 3.2, The Banach algebra L,(R) and Fourier analysis, concerned with certain Banachalgebra generalizationsto topological groups of the concept of group algebra). In these cases, one develops the appropriate techniques for dealing with the algebras without identity. For our purposes, this assumption will cause us no difficulty and is a considerable convenience.
+ +
+
+
+
3.1.4. EXAMPLE.An important class of Banach algebras (Section 3.4 is devoted to a study of their properties) is made up of the algebras of (complexor real-valued) continuous functions on compact Hausdorff spaces. The algebraic operations are the usual pointwise addition and multiplication of functions. If X is a compact Hausdorff space, we shall denote this algebra of continuous functions (for the most part over C) by C(X).In Example 1.7.2, we studied C ( X ) as a Banach space with its so-called “supremum norm”
llfll = suP{If(x)l:xEX). Of course the identity of C ( X )is the constant function 1, and 11111 = 1. We note, too, that
I l f . 911 = SUP{lf(X)l. lg(x)l:xEXl G suP{lf(x)l:xExl suP{lg(x)l:xEXl *
=
llfll Ilsll. *
From this and the fact that C(X),in the given norm, is a Banach space, we see that C ( X )is a Banach algebra. While we could have used the fact that eachfin C ( X ) attains its norm at some point of X(that is, If(xo)l = llfll for some x,, in X) in our norm considerations, by avoiding its use, the discussion applies without change with the assumption that X is compact omitted and C ( X ) denoting the bounded continuous functions on X. H 3.1.5. LEMMA.I f A is an element of the Banach algebra 2I and IlAll < 1, then Ak (where A’ = I ) has a limit B, as n tends to co, in ‘u and B(Z - A ) = (1 - A)B = I.
176
3. BANACH ALGEBRAS
Proof. Noting that, if n < m,
II
i
k=O
m
Ak -
1 Akll = II k=O
i
m
Akll <
k=n+ 1
c
IIAllk,
k=n+l
we conclude, from the convergence of the geometric series, Ckm,oIIAIJk(with IlAll < l),and the fact that 8 is complete, that C;= Ak tends to a limit Bin 8 as n -+ co. Since
and IIAnf1ll < IIAll"" -+ 0, wesee that B(I - A ) = ( I - A ) B = I , by continuity of multiplication in 8. H With the notation of Lemma 3.1.5, we write B = C,"= A" and say that B is an inverse (two-sided) to I - A (denoted by ( I - A ) - I ) . Numerous consequences result from the small observation of Lemma 3.1.5. 3.1.6. PROPOSITION. If 8 is a Banach algebra, the set .Af of invertible elements is an open subset of 8, and the operation inv of inversion on Jf is continuous.
Proof. If A E Xthen LA, left multiplication by A on 8,is a continuous mapping of 8 onto 8 with the continuous inverse LA - I. Since B - ' A - is an inverse to AB, if A and B are in M,LA maps .Af onto Jf and I onto A . From Lemma 3.1.5, the open ball of radius 1 with center Zis a subset of N Thus, the homeomorphism LA maps this ball onto an open set in 2I containing A and contained in N Hence .Af is open. If 111 - BII < 1, writing A for I - B, A k = ( I - A)-' = B - ' ; and
'
k= 1
In particular, if 111 - BII ( = llAll) < E < $, then JIB-' - Ill < 211AJI< 2 ~Thus . inv is continuous at I on N For each A in & inV = RA-IOinVOLA-L, and LA-' maps A onto I , inv maps I onto I, RA-I maps I onto A - ' , each mapping continuous at the specified element. Thus inv is continuous at A and hence on JK 3.1.7. REMARK. The elements that are not invertible in 8 are said to be singular. From Proposition 3.1.6, the singular elements form a closed subset of 8 .We refer to the invertible elements as regular and non-singular elements as well.
177
3.1. BASICS
After showing that inv is continuous at I with the series estimate (2), the proof that inv is continuous on JV is given in a "structural" manner. This (in terms structural argument points the way to an estimate for 11A-l of I1A - Aoll),which establishes, again, the continuity of inv. For this, note that lJL",I(A - A0)ll = 1 1 4' A - Ill 6 IlAi lll . IIA - A0119
so that, from (2), when llA;lll . [ [ A- AolJ c 1, lIA-'Ao - 41 < IlAo'II . IIA - AoII(1 - lA;ll
. IIA - Aoll)-';
and
(3)
11.4-1
- Ail11
- I)ll 6 IIA-'Ao - Ill . IlA;111 < llAi 111211A - AOIIU - llA;lll . IIA - Aoll)-'. = IIR,&4-'Ao
In particular, if [ [ A- Aoll (4)
-= (211A;111)-1,
I(A- - Ail11
then
< 2114 '((21(A- '4011.
Making use of the fact that elements near the identity in a Banach algebra are invertible, the possibility of norm-dense, proper ideals and its associated difficulties can be eliminated.
3.1.8. PROPOSITION. If9 is aproper (left or right) ideal in a Banach algebra 'u, then the norm closure 3 of 9 is aproper (left or right) ideal in %. If9 is a maximal (left, right, or two-sided) ideal in %, then 9 is norm closed. If9 is a proper closed two-sided ideal in 'u, then the quotient algebra 'uJ9,prouidedwith
the quotient Banach space structure, is a Banach algebra. Proof. If I E ~then , there is an A in 9 such that 111 - ,411 c 1. From Lemma 3.1.5, A ( = I - (I - A)) is invertible. Hence A - 'A (= A A - ') is in 9. But then B . I (or I * B) is in 9 for all B in 'u, contradicting the assumption that 9 is proper. Thus Z $ 3 , and 3 is a proper ideal. If 9 is maximal, then 9 = 3 ; so that 9 is closed. It follows from Theorem 1.5.3 that 'u/9is a Banach space in its quotient norm when Y is a closed ideal in 'u. If 9 is a (proper) closed two-sided ideal in %, then %/9is a Banach space and (with its natural structure as a quotient algebra) an algebra with identity I Y. We have noted that inf{llZ- A ~ ( : A E= ~1 } (and, thus, 111 + 9 1 1 = l), since 9 is proper. Since IIA + $11 * llB + 9 1 1 = inf{llA - Clll: C1€ 9 ) * inf{llB - C211:C 2 € 9 }
+
= inf{llA - C1((* ( ( B - Cz((:C1,C2EY)
>inf{llAB-(C1B+AC2 - C1Cz)(l:C1,C2E9}
2 inf{llAB - CII: C E ~=}ll(A %/Sis a Banach algebra in its quotient norm.
+ 9 ) ( B + $)I[,
178
3. BANACH ALGEBRAS
3.2. The spectrum Our Banach algebras are intended to provide the general framework for the study of algebras of (linear) operators on a Hilbert space. In the case of a finitedimensional space (and when they are present in the infinite-dimensional case), the eigenvectors and their associated eigenvalues play an important role in the analysis of the individual operator. The concept of spectrum, which we study in this section, is devised as the replacement, in the general setting of Banach algebras, for the set of eigenvalues in the finite-dimensional case. Henceforth our Banach algebras are assumed to be complex. 3.2.1. DEFINITION. If A is an element of the Banach algebra 2l,we say that a complex number A is a spectral value for A (relative to a) when A - AZdoes not have a two-sided inverse in N. The set of spectral values of A is called the spectrum of A and is denoted by spW(A). When there is no danger of confusion, we write sp(A) in place of sp,(A). Before beginning the general study of the spectrum, let us note that it serves the purpose for which it is designed. If 3’ is a finite-dimensional (Banach) space and A is a linear transformation of 3’ into itself, then A - AZ will fail to have an inverse (in a(#),the family of all linear transformations of # into itself) if and only if it annihilates some (unit) vector xo in 2 -that is, if and only if A has some unit eigenvector xocorresponding to the eigenvalue A. At the same time, let us note that while an eigenvalue is always in the spectrum, the reverse need not, in general, be the case. 3.2.2. EXAMPLE.Let 2 be the Hilbert space of complex-valued squareintegrable functions on [0,1] relative to Lebesgue measure, and let A be multiplication by the identity transform on [0,1] (so that (Af)(x) = xf(x)). Then A has no eigenvalues; for if Af= AJfmust be 0 at all points of [0,1] other than A. Hencefis 0 almost everywhere, andfis the element 0 in X. Nonetheless, sp,,,,(A) = [0, 11. To see this, let y, be the characteristic function of [A - 1/2n,A+ 1/2n], and let x, be nl”y,. The obvious modification is made when A is 0 or 1 ;and, for 1 in (0, l), the sequence {x,) has as first element xno, where no is so large that [A - 1/2n, A + 1/2n] E [O, 11, when no 6 n. Each x, is a unit vector, and ll(A - AZ)x,ll = >/6n (so that {x,} is a sequence of “approximate” eigenvectors for A corresponding to the eigenvalue A). If B is a left inverse to A - AZ, then
179
3.2. THE SPECTRUM
A - IZdoes not have a two-sided inverse in A?(%). (In essence this is part of the argument of Corollary 1.5.10, which could be applied here.) It follows that 11 SPB,,,(A)If J.$[O, 11, then A defined as (x - I)-' for x in [0, I], is continuous in [0, I]. Multiplication by f o n &' is a bounded operator, which is the two-sided inverse (in A?(%)) to A - IZ. Thus I $ s ~ ~ ( ~ , (and A )sp,(,,(A) , = [O, 11. r o 9
We shall see presently (see Lemma 3.2.13) that for normal operators (see Section 2.4) the situation of Example 3.2.2 holds generally: spectral values correspond to (sequences of) approximate unit eigenvectors. 3.2.3. THEOREM. I f A is an element ofthe Banach algebra 2l then sp,(A) is a non-empty closed subset of the closed disk in C with center 0 and radius llAll.
Proof. If I $ s ~ p l ( A )then, , by Proposition 3.1.6, A - I'Zis invertible for all 1'in a small open disk with center 1.Let p be a continuous linear functional on 8. Since p ( ( A - I ' Z ) - l ) - p ( ( A - I Z ) - 1 ) - p((I' - A)(A - I'Z)-l(A - I z ) - ' )
A' - I
I' - I
= p ( ( A - A'Z)-'(A -+
p((A - u
- IZ)-')
- 2 )
as I' -+ I , by continuity of inversion (see Proposition 3.1.6) on the set of invertible elements of 8, and the continuity of p , the function I -+ p ( ( A - IZ)- l ) is holomorphic on C \ sp,(A). Note, too, that p((A - u as l,ll-+
00; for
- 1 )
= I-lp((I-1A - Z ) - l ) + O
A-'A - I is invertible when (IA(I< 111,and
(I-lA - I ) - '
-+
-I
as 121 -+ 00.We see, at the same time, that I ( I - ' A - Z) (= A - AZ) is invertible when llAll < 111;so that sp,(A) is a subset of the closed disk in C with center 0 and radius IlAll. If sp,(A) were empty, the function I -+ p((A - AZ)-') would be an entire function that vanishes at a.By Liouville's theorem, this function would vanish everywhere on C. In particular, we would have p ( A - l ) = 0, for each continuous linear functional p on a. From the Hahn-Banach theorem (see Corollary 1.2.1l), it would follow that A - = 0, a contradiction. Thus sp,(A) is not empty. We observed, during this argument, that C \ sp,(A) is open, so that sp,(A) is a non-empty closed subset of the disk C with center 0 and radius llAll. H Despite the fact that the spectrum of A is not empty, it may consist ofjust 0. If { e l ,e 2 }is a basis for two-dimensional Hilbert space and A is the operator on
180
3. BANACH ALGEBRAS
this space that maps e l to e z and e2 to 0, then sp,,#,(A) consists ofjust 0. Note, too, from this example, that A may be non-zero and have just 0 in its spectrum. If each element of 2l other than 0 has an inverse in 2I (so that ‘21 is a division algebra), then, if A E spm(A),A - IIl must be 0 (being a singular element of ‘91). Thus, in this case, 2I consists of just scalar multiples of I. Since M is, then, isomorphic to @, we say, loosely, that 2l is @. A (complex) Banach division algebra (or field) is @. 3.2.4. COROLLARY.
We noted, in Proposition 3.1.8,that a maximal ideal .M in a Banach algebra 2I is closed. Since .A is maximal, if 2I is commutative, %/.Ais a field -and a Banach algebra. From the preceding corollary, ‘%/.Ais C,and the quotient mapping is a continuous multiplicative linear functional on ‘i!l(that is, a homomorphism of 2I onto C). Conversely, if p is a homomorphism of 2I onto @, its kernel .A is a maximal two-sided ideal in 2I (since %/A! is the field C). Hence A! isclosed and, from Corollary 1.2.5,p iscontinuous. In general, if ’21is not commutative and .Ais a maximal two-sided ideal in 2I, we cannot conclude that ‘%/A is a field; so that no multiplicative linear functional need be associated with .I.
3.2.5. COROLLARY. If ’21 is a commutative (complex) Banach algebra and .Iis a maximal two-sided ideal in %, then %/Ais C and the quotient mapping from ‘91 to 2lIYXis a (continuous) multiplicative linear functional on a.If2l is an arbitrary (complex) Banach algebra and p is a multiplicative linearfunctional on PI, then p is continuous with kernel A! a maximal two-sided ideal in CU such that Vlldt is @. We saw that sp,(A) is contained in the disk in C with center 0 and radius (IA((.The radius of the “smallest” disk containing the spectrum will appear in our considerations.
3.2.6. DEFINITION. The spectral radius r,(A) of an element A of a Banach algebra 2I is sup{(k(:AESPyl(A)}.
w
3.2.7. REMARK.When no confusion can arise, we write r(A) in place of r4((A).As noted in Theorem 3.2.3, r(A) < l1All. It is apparent from the definition that r(A) is the radius of the smallest disk in C with center 0 containing sp(A). W 3.2.8. PROPOSITION. I f A and B are elements of a Banach algebra PI, then sp(AB) u ( 0 ) = sp(BA) u {0),and r(AB) = r(BA).
181
3.2. THE SPECTRUM
Proof If2 # O a n d l ~ s p ( A B )then , A B - Aland,hence,(A-'A)B - l a r e not invertible. On the other hand, if l#sp(BA), then BA - A/ and, hence, B(i.- ' A ) - I are invertible. Our task, then, is to show that I - A B is invertible in V l if and only if I - BA is invertible in 2l, for arbitrary elements A and B of
PI. Arguing formally, for the moment, (I
-
A B )- ' =
c (AB)" x
=
I
+ A B + ABAB +
n=O
and
B(I- AB)-'A
=
BA
+ BABA + BABABA + . . * = ( I -
BA)-' - I .
Thus if I - A B has an inverse, we may hope that B(I - A B ) - ' A inverse to I - BA. Multiplying, we have ( I - BA)[B(I - A B ) - ' A =
B(I - A B ) - ' A
+ I is an
+I]
+ I - BAB(I - A B ) - ' A
-
BA
=B[(I-AB)-' -AB(I-AB)-']A + I - B A = I ,
and similarly for right multiplication by I - BA. W
3.2.9. R E M A R K . It is apparent that sp(A + I ) = { l + a : a E s p ( A ) ) .We shall prove the more general result concerning the relation between sp( p ( A ) ) and sp(A), for an arbitrary polynomial p , in the proposition that follows. (We prove the full spectral mapping theorem (Theorem 3.3.6) in Section 3.3.) Combining the simple initial observation with the preceding proposition yields the fact that the unit element I of a Banach algebra 2l is not the commutator A B - BA of two elements A and B of 2l. (If I = A B - BA, then sp(AB) = 1 + sp(BA), which is not consistent with sp(AB) u (0)= sp(BA) u { O } . ) This fact is familiar in quantum theory where it takes the form the commutation relations are not representable in terms of bounded operators. However, there are unbounded operators whose commutator is I restricted to a dense linear manifold. (See Exercise 2.8.49.) 3.2.10. PROPOSITION. I f A is an element of the Banach algebra 91 andp is a polynomial in a single cariable, then
SP(P(A)) = M A ) : 1 E SP(A)J ( = P(SP(A))).
182
3. BANACH ALGEBRAS
If A is int:ertible, then sp(A-') = {1-':1Esp(A)) ( = (sp(A))-'). r f A and B are elements of the commutative Banach algebra 'u, then
sp(AB) G sp(A)sp(B), r(AB) < r(A)r(B),
sp(A r(A
+ B) G sp(A) + sp(B),
+ B) d r(A) + r(B).
Prooj: If 1~ sp(A), then A - 11 does not have a two-sided inverse in PI. Thus one of ( A - 11)BI or ' u ( A - 11) is a proper ideal .a in PI. If p ( x ) = a,x" + . . . + ao, then p ( A ) - p ( i L ) l= an(A"- 1")
+ . . . + a,(A - 11).
Noting that
+ lAk-' + ' . + Jk-'f) = (Ak-1 + A A k - 2 + . . . + Ak- ' I ) ( A - I I ) ,
Ak - IkI = ( A - l l ) ( A k - '
we conclude that p ( A ) - p ( 1 ) 1 ~ 9so, that p ( A ) - p ( 1 ) l does not have a twosided inverse in %, and p(1) E sp(p(A)). If y~ sp(p(A)) and I1,. . . ,A, are the n roots of p ( 1 ) - y, then p(A) - y l = ( A - 1'1) *
. . ( A - 1J),
so that at least one of A - 1'1,.. . , A - &I is not invertible. If A - AjI is not invertible, then l jE sp(A) and y = p(Aj)~p(sp(A)).Thus sp(p(A)) = p(sp(A)). Suppose A is invertible in 'u (equivalently, 0$spw(A)). If ?, f 0, then A - ' - 1 - ' 1 = 0.1- ,4)(2A)-', so that L-'Esp(A-')ifand onlyifAEsp(A). Thus sp(A-') = sp(A)-'. Suppose, now, that A and B are elements of the commutative Banach algebra 'u. If 1 E sp(AB), then AE - 21 lies in a proper ideal (necessarily, twosided) 9 of %. Since 2l has an identity, Zorn's lemma, applied to the set of proper ideals in 'u containing 9,shows that 9 is contained in a maximal ideal .X of 'u. From Corollary 3.2.5, .I is the kernel of a multiplicative linear functional p on %. Thus 1 = p(AB) = p(A)p(B). Since A - p ( A ) I and B - p(B)I are in the kernel JZ of p, p(A)~sp(A)and p(B)~sp(B).Thus 1 E sp(A)sp(B) and sp(AB) E sp(A)sp(B). Again, if 1 E sp(A + B), there is a multiplicative linear functional p on our commutative 'u such that p(A + E ) = 1. As p(A) E sp(A), p(B) E sp(B), and 1 = p ( A ) p(B); l.~sp(A) sp(B), and sp(A + B) 5 sp(A) + sp(B). The inequalities for the spectral radius are immediate consequences of the corresponding relations for the spectra. m
+
+
3.2.11. R E M A R K . We make special note of the fact established at the end of the proof of Proposition 3.2.10. If A is an element of a commutative Banach
3.2. THE SPECTRUM
I83
algebra CLI and E. E sp,(A), then there is a multiplicative linear functional p on such that p ( A ) = E.. Conversely, if p is a (non-zero) multiplicative linear functional on ‘21 (not necessarily commutative), then ~ ( A )sp%(A) E for each A in ‘$1. For this last assertion, note that A - p ( A ) I is in the kernel of p, a proper two-sided ideal in 91. The examples that follow illustrate the concepts of spectrum and spectral radius in the Banach algebra g ( X ) of bounded operators on the Hilbert space .X. 3.2.12. EXAMPLE.Let { e n } be an orthonormal basis for a separable Hilbert space X . Recalling Example 2.4.10, we have a bounded operator A on d such that Ae, = %,,en,where {A,,} is an arbitrary bounded (denumerable) subset of @.We saw that llAll = sup{lA,,l}, that A is normal, in general, and selfadjoint exactly when all A,, are real, unitary when all A,, have modulus 1, and positive when all A,, are real and non-negative. Since each A,,is an eigenvalue (with eigenvector en),{All} E sp,,,,(A). From Theorem 3.2.3, sp(A) is closed, so that {A,,} -,the closure of {A,,}, is contained in sp(A). If 1 is not in this closure, then inf{li - in[} > 0, and {(A,, - A)-’} is a bounded subset of @. Thus there is a bounded operator B on A? such that Be,, = (A,,- 1)-‘en.Since ( A - Al)e, = (All - ;,)en,we have B(A - AI)e,, = e,,and ( A - 1I)Be,, = e,,for all n. Thus Bis a two-sided inverse in g ( H )to A and A#sp,,,,(A). Hence {A,,}- = sp,,,,(A). If {EL,,) is an enumeration of the rationals in [0,1], then sp(A) = [0,1]. In Example 3.2.2 we considered an operator with spectrum [0,1] but no eigenvectors. Although the present example and Example 3.2.2 exhibit selfadjoint operators with the same spectrum, and, in a sense still to be made precise, both of these operators have spectra without “multiplicity”; these operators are quite different structurally. One has an orthonormal basis of eigenvectors, while the other has not a single eigenvector. In the finitedimensional case, self-adjoint operators having the same spectrum, each without multiplicity, have identical structure (are “unitarily equivalent”).
With the aid of an extension of the “approximate eigenvector” technique encountered in Example 3.2.2, we shall be able to extend to the spectra of selfadjoint, positive, and unitary operators, the information we have about the eigenvalues for the corresponding operators with an orthonormal basis of eigenvect or s. 3.2.13. LEMMA.’I .x‘ is a Hilbert space and A is a normal operator in g ( X ) ,then I E sp(A) ifand only ifthere is a sequence {x,,} of unit veclors in *x‘ such that “ ( A - ,IJ)x,,ll + 0 as n -+ 00.
Proof. Since A is normal, A - 1I is a normal operator in LA?(#). From Lemma 2.4.8, A - AI fails to have a bounded two-sided inverse (that is,
184
3. BANACH ALGEBRAS
i ~ s p ( A )if) and only if inf{ll(A - Al)xll: llxll = 1 , X E X=}0. Thus I E sp(A) if and only if there is a sequence {x,} of unit vectors in X such that l((A - Il)x,ll+ 0 as n + co. 3.2.14. THEOREM.If2 is a Hilbert space and T E ~ ( . @then ),
(i) sp( T ) consists of real numbers if T is self-adjoint ; (ii) sp( T ) consists of non-negative real numbers if T is a positive operator; (iii) sp(T) E { O , I } if T is a projection; (iv) sp(T) consists of complex numbers of modulus 1 if T is a unitary operator ; (v) sp(T*) consists of the complex conjugates of numbers in sp(T). Proof. Suppose Tis self-adjoint and 1E sp( T). From Lemma 3.2.13, there is a sequence of unit vectors x, such that ll(T - AZ)x,ll + 0 as n + 00. Then (( T - Al)x,,,x,) + 0 as n + 00. Since (Ax,, x,) = A, ( Tx,, x,) tends to 1.But (Tx,, x,) is real. Thus A is real, and (i) follows. In the same way, if T 2 0, then ( T x , , ~ , 2 ) 0 and I b 0. Thus (ii) is established. If T is a projection, then T 2 = T, so that
T(T - Al)x, = (1 - A)Tx, + 0. Thus (1 - A)Ax, + 0. But l(x,,ll = 1 . Hence ( 1 - l)A = 0, and A is either 0 or I , so that (iii) is established. If T is unitary, then 1 = (x.,x,) = ( T x , , Tx,). Since (Axn,1xn) = Ill2, and ( T x , , Tx,) - (Ax,, Ax,) -+ 0 as n + 00, 1 = 1AI2 and (iv) follows. From the properties of the adjoint operation on W ( 2 )(see Theorem 2.4.2), B* is a bounded inverse to T* - 11if and only if B is a bounded inverse to T - AZ, and (v) follows. With the added assumption that T is normal, the converses to (ik(iv) of Theorem 3.2.14 are valid. It is more convenient to establish these after the spectral theory of normal operators has been developed (see Theorem 4.4.5). 3.2.15. PROPOSITION. If 2 is a Hilbert space and A is a self-adjoint operator in @(#), then at least one of llAll or - IlAll is in sp(A). Proof. By working with IIAII-lA in place of A , we may assume that (IA((= 1. In this case, there is a sequence {x,} of unit vectors such that llAx,ll + 1 as n -+ co. Thus
- A2)x,1I2 =
+ llA2~,112- 2 Re(A2x,,x,) < 2 - 211A~,()~ -+0
as n + 00. From Lemma 3.2.13, 1 esp(A2); and by Proposition 3.2.10, 1 ~ ( s p ( A ) )Thus ~ . 1 or - 1 is in sp(A).
185
3.2. T H E SPECTRUM
It follows from the preceding proposition that r(A) = llAll when A is selfadjoint. More generally, r( T ) = IlTll when Tis normal. These facts will follow directly from a general formula for the spectral radius that will be developed in Section 3.3 (see Theorem 3.3.3 and Proposition 4.1 .l(i)). 3.2.16. EXAMPLE.There is no difficulty extending the construction used in Example 3.2.2 to identify the spectrum of more general “multiplication operators” (see Example 2.4.1 I). If ( X , p) is a o-finite measure space and fis an essentially bounded measurable function on X , then MJ(g) = f g defines a bounded operator M , on L 2 ( X ,p ) . The essential range s p ( n offis the set of complex numbers i. such that p ( f - ’ ( O ) ) > 0 for each open subset 0 of C containing L. Suppose L ~ s p ( f ) .For each positive integer n, let yn be the characteristic function of a measurable subset of f - ’ ( O , ) of finite positive p-measure a,, where 8, is the open disk in C with center 1 and radius n-l. Then {x,} is a sequence of unit vectors, where x, = ~ , - ” ~ y , ,and
Thus ll(MJ - A I ) x , , ~+~ 0 as n + SO, and IESP(M/). Conversely, if L $ sp(f) there is a disk 0, of radius n - with center i, such that p ( f - ’ ( O , ) ) = 0. Then l/(f- A) is a measurable function g with an essential bound n, and M, is a two-sided bounded inverse to MJ - At. Thus i$sp(M,). It follows that sp(M,) = s p ( n (and that r(M,) = IlfllX, = IlM~ll). H 3.2.17. EXAMPLE.Let ,P be a separable Hilbert space and {e,:n = 0, 1, & 2 , . . .} an orthonormal basis for 2.The transformation U on 2 such that Ue, = en+ is a unitary operator. From Theorem 3.2.14, sp(U) is a subset of C1, the complex numbers of modulus 1. If 1 E C1, then, in a formal sense,
If= - ].-,en is an “eigenvector” for U corresponding to the “eigenvalue” 1. Although this sum is not a “genuine” element of ,X; its partial sums, multiplied by suitable normalizing factors, provide us with a sequence of approximating eigenvectors. Specifically, let x, be (2n + x i = -nA-kek.Then Ilx,ll = 1 and ‘1
I~(U i l ) x , J ~= (2n +
n
1)-~”11
C
n
A - k e k + l-
k= - n
= (2n
+ I)-’i2IIA-”e,,+
=21’2(2n as n
+
+ 1)-”2+0
x.Thus i ~ s p ( ( i ) and , sp(U) = C l .
k-“-l)ek1I k= -n
- An+ le-,ll
186
3. BANACH ALGEBRAS
Let Y?’ be L2(Cl) relative to Lebesgue measure on C1 normalized so that the total measure on C,is 1. Then {z”:n = 0 , f 1 , . . .) (where z denotes the identity transform on C1) is an orthonormal basis for 8’ [the Weierstrass approximation theorem (see Remark 3.4.15) is used to show that this system generates X ‘ ] . There is a unitary transformation V of ,X’ onto .X such that V(z”)= en. The multiplication operator M , on H ’has spectrum C1, from Example 3.2.16. Note that VM,V-’ = U . From this “unitary equivalence” of the “two-sided shift” operator with “multiplication by z,” we can deduce the spectral properties of one from those of the other. W 3.2.18. EXAMPLE.With .K a separable Hilbert space and { e , : n = 0,1, 2 , . . .) an orthonormal basis for 2,let W be the bounded operator on X such that We, = en, In this case, W , the “one-sided shift” operator, is not a unitary operator on .X, since eo is not in its range (although W is a unitary transformation of .X onto the range of W ) . Note that if 111 = 1, then U , C ; = ~l - k e k( = x,) is a unit vector in ,X, where a, = (n + 1)-1’2,and n
Il(W - II)x,ll
= anll
C A-kekt
k=O
n
-
C j.-(k-llekll k=O
as n + cx;, since a, + 0. Thus ).ESP( W ) . If 111 < I , then W * x = A x , where x = X;=oIkek, for ( W * x , e k )= (x, We,) = ,Ik+’. Thus A€sp(W*) and ~ T Esp( W ) (see Theorem 3.2.14(v)). Since 11 WII = 1 , sp( W ) is contained in the closed disk of radius 1 with center 0 in C (see Theorem 3.2.3).Thus sp( W )is this closed disk. W 3.2.19. EXAMPLE.Returning to H and U of Example 3.2.17, we let CLI be the Banach subalgebra of B ( H )consisting of the norm closure of the algebra of polynomials of a single variable in U (and I ) . If U has an inverse in $11,then that inverse must be U * . Each polynomial in U , and hence each element of CLI, however, maps the closed subspace generated by {en:n = 1 , 2 , . . .) into itself, whereas U* does not map this space into itself ( U*el = eo).Thus U* 4 CU and U has no inverse in ’11. Stated in terms of spectrum, we have O E S P ~ ( Ubut ) 0 4 SP,B,,I(U). Of course, as we pass from a Banach algebra to a Banach subalgebra, an element of the subalgebra may “lose its inverse.” Thus, in theory, the spectrum may “grow” on passage to a subalgebra. The present example indicates that this increase of spectrum can occur in practice. We shall note (Proposition 4.1 .5) that no change occurs in the spectrum when passing from a C*-algebra to a C*-subalgebra. This fact plays a crucial role in the application of spectral theory to C*-algebras. W
3.2. T H E SPECTRUM
187
3.2.20. PROPOSITION. The non-zero multiplicative linear functionals on a Bunach algebra '$S form a weak* compact subset of the unit ball of a'. Proof: From Remark 3.2.1 1, if p is a non-zero multiplicative linear functional on VI, then p(A)~sp,(A), for each A in '$1. Thus, from Theorem 3.2.3, Ip(A)I ,< llAll, and p lies in the unit ball of a'. such that p ( A B ) - p(A)p(B) = 0 is weak* The set of elements p in closed. The intersection of these sets (as A and Brange through 2l)is the weak* closed set of multiplicative linear functionals in a'. The further condition, p ( l ) = I , singles out the weak* closed subset consisting of non-zero multiplicative linear functionals on VI -a subset of the unit ball of a'. From Theorem 1.6.5(i), the unit ball of rU' is weak* compact, as is this closed subset. H The Banach alyebra L , ( R ) and Fourier analysis. In this subsection, we study the maximal ideals of the special Banach algebra Ll(R) provided with convolution multiplication. By letting L , ( R ) act on L,(R) as a convolutionmultiplication algebra, we define an algebra , d , ( R ) of operators acting on the Hilbert space L,(R). We adjoin I to d , ( R ) and take the (operator) norm closure to obtain another algebra VS0(R) of operators on L,(R). The algebra !&(R) is an example of a class of operator algebras, abelian C*-algebras, whose general properties will be studied intensively in Chapter 4. For the present, we identify the maximal ideals of PSo(R) and use this information to develop some of the basic theory of Fourier transforms. The Banach algebra L , ( R ) and its ideal structure is the general framework for this theory. We shall have occasion to use Fourier transforms in Sections 9.2, 13.2, and 13.3. The results we obtain here on the ideal theory of L , ( R ) will play an important role in the analysis of the (continuous) homomorphisms of the additive group R into the group of unitary operators on a Hilbert space (see Stone's theorem (5.6.36)). The various algebras we define may be viewed as generalizations of the complex group algebra of a finite group to the case of the group R. The methods we describe apply to more general (locally compact) topological groups and can be extended without great difficulty to such abelian groups.
3.2.21. DEFINITION. With f and y measurable functions on R, the conrdution of fand y is the function f * y whose domain consists of those real numbers s for which the integral f ' ( t ) g ( s- t ) dt converges and whose value at s is this integral. H
Sw
Since Lebesgue measure on R is invariant under the transformations -+ t + s, for each real s, we have, for each h in Ll(R),
t -, - t and t
h(s + t ) d t =
h(s - t ) d t = J R
188
3. BANACH ALGEBRAS
3.2.22. PROPOSITION.
f*Y
=
(i) I f f and g are measurable functions on R, then
s*f.
(ii) I f f € L1(R) and g E LJR) (where 1 < p), then f * g E L J R ) und
Ilf*sll p G llflll . Ilsllp.
(1)
(iii) I f f € L , ( R ) , g,hELp(R), and a E C , then both f * ( a . g a . j * g + f * h are in L,(R) and f * ( a .g
+ h) = a . f * g
+ h ) and
+f*h.
(iv) I ~ J ~ E L ~andhELp(R), ( R ) then both (f*g)*h a n d f * ( g * h ) are in L p ( R ) and
( f * g ) * h=f*(g*h). (v) Provided with the mappings ( J g ) + f * g and f - Ilflll, Ll(R) is a commutative Banach algebra.
Proof. (i)
(f*g)(s) = J =
flt)g(s - t ) dr = R
lR
s ( t ) f ( s - 4 dt
JR
= (9
f(s
+ r)s(- t ) dt
*ncs>.
(ii) If h,(t) = g(s - t ) and p(S) = Js If(t)l dt for each measurable subset S of R, then h S ~ L p ( Rand ) p is a finite measure on R. Applying the Holder inequality [R: p. 62, Theorem 3.51 to h, and the constant function 1 relative to p, we have
and
Now (s,t ) + Ig(s - t)lPf(t) is in L , ( R x R) since
3.2. T H E SPECTRUM
189
Thus, using Fubini’s theorem,
from which (1) follows. (iii) From (ii), f * (a . g
and u
+ h) and a .f*g +f* hare in L,(R). In particular
[flI)co . g(s - t )
+ h(s - t ) ] dt
s
s
f l t ) g ( s - t ) dt
+
f(t)h(s - t ) dt
converge for almost all s, and (iii) follows. (iv) From (ii), (f*g) * h and f*(g * h) are in L,(R). Now
and
Since If1 and IgI are in Ll(R), and Ihl E L,(R); whenf, g, and h are replaced by their respective absolute values, the last two integrals converge for almost all s. Fubini’s theorem applies, and, for almost every s,
Since
s
g ( r - r)h(s - r)dr =
( f * 9 ) * h = f*(Y * h).
s
g(r)h(s - t - r)dr,
(v) From (i)-(iv), the Ll-norm and convolution multiplication provide L , ( R ) with the structure of a commutative Banach algebra (without unit).
I90
3. BANACH ALGEBRAS
If we define LJy) to kf*y, wherefg Ll(R) and g E L,(R), (ii) and (iii) of Proposition 3.2.22 tell us that L, is a bounded linear operator on Lp(R) and llLs 11 < IIJ'II1. From (iv) of that proposition (and right-distributivity of convolution multiplication - proved as in (iii)], we have that the mapping f --,L , is a homomorphism of the algebra Ll(R) into B(L,,(R)).In particular, if p = 2, the image d l ( R ) of Ll(Ew) under this homomorphism is an algebra of operators on the Hilbert space L2(R).We denote by %,(R) the norm closure of v d l ( W
3.2.23. PROPOSITION. For each f in t l ( R ) , $ G llLf - 111,so that 14 91 The linear space %o(Ew) generated by I and 911(R)is a norm-closedcommuiatitie algebra of operators on L2(Ew)and %,(R) is a (proper)maximal ideal in &,(R).
Proof. Let tlnft)be (n/2)1i2 for tin [ - n - I , n - J and 0 for other values o f t . If f i n L,(R) is such that llLr - ZII < i, then
=
11"
r
lj*in f(s - t)dr - 1 ds, 2 - I/. - lin for all positive integers n. Since feL,(R), we can choose n so large that ft';/,If(t)\dt < $. With s in [ - n - ' , n - $ 3 ,
so that
+ < If!':,,f(s
Is":,.
I
f(s - i)dr g
- r)di
If(t)l dr
1
<2
I
- 112 and
It follows, from this contradiction, that d ilts- Zit for each f i n L,(Ew), so that Z$9Il(R). From Corollary i 5 4 , ~~~~) is norm closed. It is a commutative algebra and %,(R) is a (proper) maximal ideal in it, We denote by y, the multiplicative linear functional on 9l0(R) that assigns 1 to Zand 0 to each element of %,(R). There are other non-zero multiplicative linear functionals on (equivalently, by Corollary 3.2.5, proper maximal ideals in) %& OurI) present . goal is to describe each of these and establish a homeomorphism between the set of these functionals with the weak* topology (see the discussion preceding Proposition 1.3.5) and R.
191
3.2. THE SPECTRUM
Although L , ( R ) has no unit, the sequence { u , } , appearing in the lemma that follows, functions as a unit for L , ( R ) in many circumstances. The sequence {u,} is referred to as an approximate identity for L,(R). We shall encounter “approximate identities” again when we study the ideal theory of C*-algebras (notably Lemma 4.2.1 1 and Proposition 4.2.12). 3.2.24. LEMMA.I f { u n } is a sequence ofpositive functions u, in L,(R) such that llunlll = 1 andu,(t) = 0 when t $ [ - n - I , K ’ ] , then Ilf*u, -fll,+Ofor each f in L,(R).
Prooj: Assume, first, that f i s continuous on R and vanishes outside the finite interval [a, 61. Choose E positive. There is a positive integer m such that If’(s) -f(t)l < F. when ( t - SI < m-I. If m < n ,
s
G If(t) -f(s)lu,(s - 1)dt G E .
In additionf*u, - f vanishes outside of [ a - n - ’ , b + n - ’ I . Hence
[If*
111’. u, -fII,
u,
-flip 6 E(b - a
+
l)I’P,
as n + 00. For an arbitrary f in L,(R), we can choose { f m } so that I l f - f m l l p -+ 0, l l f m l l p 6 I l f l l p , where each f m is continuous and vanishes outside a finite interval. Then
and
IIf*
un
+0
Ilf*un - f m * unllp + l l f m * un - f m l l p + Ilf, -flip G IlUnlll . I l f - f m l l p + l l f m * un - f m l l p + I l f m -flip G 2llf-fmllp + I l f m *un - f m l l p .
-flip G
Choosing m large enough, for all n, we have
I l f * un -flip 6 243 + l l f m * un - f m l l p . From what we have proved, with n large enough, llfm * u, -fmll, Ilf* u, -l’llp 6 E .
6 4 3 ; and
Iff€ L , ( R ) and { u,} is as described in Lemma 3.2.24, then IIL,un -fill + 0. Thus L p , = 0 for all n if and only i f f = 0. Since u, can be chosen in L J R ) for all n andp, the homomorphismf+ L,: L , ( R ) + B(L,(R)) is an isomorphism. From ( I ) , this isomorphism does not increase norm. We denote byf* the function whose value at s isf( - s). If g and h are in L,(R), then (2)
L,. = L;
192
3. BANACH ALGEBRAS
since
Thus .dl(R), 211(R), and 'u,(R) contain A* when they contain A . The algebra %,(R) is an abelian C*-algebra (with unit). Such algebras will be studied in Section 4.4. We shall prove in Theorem 3.2.26 that the non-zero multiplicative linear functionals on L1(R) are in (natural) one-to-one correspondence with R-more precisely, with k, the dual group of R. The elements of k are the continuous homomorphisms (characters) of the (additive) group R into the (circle) group U, of complex numbers of modulus 1 (and the product in If4 is pointwise multiplication of characters). We describe the characters of R in the lemma that follows. 3.2.25. LEMMA.For each real number r, the equation <,(t) = ei"
<,
defines a character of R. The mapping r -+ is an isomorphism of (the group) R onto k. Proof: As defined, 5, is clearly a character of R. It remains to show that eachcharacter of R has the form <,for some real number r. Note, for this, that the continuity of at 0 implies that maps some interval [ - a , a ] , with a positive, into the neighborhood
<
<
<
v=
{e'S:
- +7l
< s < +7l}
of 1 in U The equation arg(exp is) = s defines a homeomorphism arg from V onto the interval ( - $n,+n). The continuous mapping q = arg 0 5 : [ - a, a ] + ( - +n,$T)
is additive, in the sense that q(s + t ) = q(s) [ - a, a ] . From this it follows easily that
+ q(t) when s,t, and s + t all lie in
v(ma/n) = ( m / n M a ) = (ma/n)r,
(where ro = a - 'q(a)),when rn and n are integers and Iml c n. By continuity of q, we have that q(t) = tr, (and therefore 5 ( t ) = expitr,) when - a Q t Q a. Accordingly, the two homomorphisms 5 and t + exp itr, coincide on the interval [ - a, a ] (which generates R as an additive group), and so coincide throughout R. H
193
3.2. THE SPECTRUM
The function 6, that takes the value 1 at r and 0 at other points of R corresponds, of course, to the element 0 in L , ( R ) . If we treat R as a discrete space (for the purpose of heuristics) and replace integration by discrete summation, then S, *fbecomesf,, wheref,(t) = f ( t - r). (That is, convolution by 6, is translation by - r.) Of coursef, E L,(R) iff€ L,(R) and Ilfill, = Ilfll, so that f ' - f i is an isometry of L,(R) onto itself. If p is a multiplicative linear functional on L , ( R ) , then p ( f , ) / p ( f ) = p(6,) ( = [(r)), provided p ( f ) # 0. Again, in a purely formal sense, 6, * 6, = 6, +s, so that [ is a homomorphism on R. While 6, is not available to us in a rigorous presentatioqf, is. To reconstruct p ( f ) from (, we replace f by f ( r )6, or, more appropriately, by Sf(r)6, dr. Then p ( f ) is
x,
where ( ( r ) = expiry (as in Lemma 3.2.25). The accurate version of the preceding discussion appears in the theorem that follows.
3.2.26. THEOREM.The non-zero linear functional p on L , ( R ) is rnulfiplicative if and only if there is a unique real number r such that
J
p(f) = f(t)e'l'dt = f ( r ) .
(3)
ProoJ We show, first, that p , as defined in (3), is multiplicative. (It is nonzero, for if p(f) = 0 for eachfin L , ( R ) ,then the function f + exp itr is 0 almost everywhere, which is absurd.) I f x Y E L , ( R ) ,
j( (
j f ( s ) g (t - s) ds
= j f ( s ) { g (t - s)e'" dr) ds = { f l s )
(
j g ( l ) e i i i + d l ) ds
Assume, now, that p is multiplicative. If we adjoin a unit to L,(R), as described in Remark 3.1.3, and extend p linearly to the Banach algebra so obtained, after assigning 1 to the unit element, the extended functional is multiplicative. Proposition 3.2.20 applies, and the extended functional lies in the unit ball. In particular, we conclude that p is a bounded linear functional (with norm not exceeding 1) on L,(R).
I94
3. BANACH ALGEBRAS
Since p # 0, there is a functionfin L , ( R ) such that p(f’) # 0. Define ((r) to be p(f;)/p(f), and note that, with h and g in L,(R), (5)
sSh(n,r(s
(h, * g)(s) = h(t - r)g(s =
-
1
t ) d t = h(t)g(s - t - r)d/
*
- t ) dt = ( h gr)(s).
It follows that
(6)
p ( h r ) p ( g ) = Ah)p(gr),
so that 5 does not depend on the choice off(provided p ( f ’ ) # 0). If p(f;) for some r, then
=0
0 = p(f,)p(g) = d f ) p ( g r ) ; and p(gr) = 0 for each g in L,(R). In particular, 0 =~C(f-r)rl
=
~(f),
contradicting the choice off. Thus p(f,) # 0 for all r , and ( ( r + s)
= P(f,+,)P(f)-
’ = Pc(fi)slP(fi>- ‘ p ( f , ) p ( f ’ ) -
= ((4 . Hr).
Since llf;lll = llfll I and p is bounded, { 1((r)l: r E R} is a bounded set of positive numbers (((r) # 0 as p ( f i ) # 0). But ((nr) = ((r)” for each integer n, so that 1((r)l = 1. Hence ( is a homomorphism of R into U1. The continuity of ( on R will follow once we establish the continuity of ( at 0; for I((r) - ((s)! = [((r - s) - 1). The set of continuous functions vanishing outside some finite interval in R is dense in L,(R), so that there is such anf’with p ( f ) not zero. Supposefvanishes outside of [ - n + 1, n - 11. If Irl < 1,
llfi -fll1 =
[
If(t - r ) -f’(t)ldr;
-n
and this integral is small for small r by (uniform) continuity off’on [ - n,n]. Now,
1m - 11 = Mf, -f)l . lP(fll-l
d llfi -f’Il1 . Ip(f)l- l ? which proves the continuity of ( at 0, hence on Iw. By Lemma 3.2.25, there is an r in R such that ((1) =.expitr for each t in R. To prove (3), note that, from Theorem 1.7.8, there is a function g in L , (R) such that p ( f ) = Jf(t)g(t)dr for each f in L,(Iw). Hence dt
=
s
s
( ( s ) p ( f ) = p ( j J = f ( t - s)g(r)di = f(t)g(r
+ s)dt
3.2. THE SPECTRUM
195
for all f in L , ( R ) , and g - s = (expisr) . 9 almost everywhere. If h(t) = [exp( - itr)]g(t),it follows that h is a bounded measurable function that is (essentially) invariant under all translations. Thus there is aconstant c to which h is equal almost everywhere. Hence g( t ) = c exp itr for almost all t , and (7)
c - 1p(f) = c - 1 J f ( t ) c exp itr dt = 3 ( r ) .
Using (4), (7), and the assumption that p is a multiplicative, we have c - ~ p ( f )=~ ( c - ' p ) ( f * f )
=f5&r) =3(r)' = c - 2 p ( n 2 .
Since p ( j ) # 0, c = I and (3) follows from (7). If p(f') = R r ) = 3 ( r ' ) for eachfin L,(R), then expitr = expitr' for almost all t . Choosing t small enough, we conclude that r = r'.
In the theorem that follows, we use the relation between L,((w)and PI,(R) to identify the space .X,(R) of non-zero multiplicative linear functionals on QIo((w) different from p a . We show that .Xo(R) with its weak* topology is homeomorphic to R. 3.2.27. THEOREM.There is a homeomorphism A of A 0 ( R ) onto R such that, if' po E,K O (R) und r = A(po), (8) for ruch f in L , ( R ) .
Po(L,)
=3w
Proof: The restriction of po to d 1 ( R ) gives rise to a non-zero multiplicative linear functional p on L , ( R ) though the isomorphismf- L , of L l ( R ) with ..ll(R) described in the comments following Lemma 3.2.24. From Theorem 3.2.26, there is a unique real number r such that (9) Po(L,) = p ( f ) =3m for eachfin L , ( R ) . We define A ( p o ) to be r . Since distinct functionals in .Mo(R) give rise to distinct functionals on L , ( R ) by the process just described, (3) of Theorem 3.2.26 implies that A is a one-to-one mapping into R. Given r in (w. (9) defines a non-zero multiplicative linear functional p 1 on d 1 ( R ) . Although Ipl(L,)I < llflll, it is not evident that (10) lPl(L/)I < IlLJll. We prove (lo), from which it will follow that p I has a (unique) bounded extension to i!l,(R) and, thence, to a functional po in .Mo(R) (by taking the extension to be 1 at I ) . This will show that A maps .Mo(R) onto R. To prove (lo), we note, first, that .Xo(R) is not empty. Withf'a non-zero element of L , ( R ) ,from (2) and the comments following Proposition 3.2.22, L,.,,
=
L,.L,
=
L,*L,;
196
3. BANACH ALGEBRAS
and L,*Lj is a non-zero positive operator A in dl(R). From Proposition 3.2.15, llAll is in the spectrum of A relative to 99(L2(R)), and, hence, relative to the smaller algebra 210(R). From Remark 3.2.11, there is a multiplicative linear functional t on %,(R) assigning llAll to A. Since 0 # t # par,,T E M ~ ( [ W From ). the foregoing, t corresponds to some real number to and
J
t(Lj) = f(s) exp ist, ds for eachfin L,(R). If ( ( t ) = expitr' and M,(J) = 4 *f,then M , is an isometry with inverse M , when viewed as a mapping on either Ll(R) or L,(R). Since
(M
- &sr' Jf(t -
+ s)(Mtg)(- t)dt
+ s)ei*r'g(- t )dt
= Sei(l+s)rf(t
+ s)g( - t ) dt = ( L M C / ( g ) ) ( S ) ;
we have, (1 1)
LMsj =
M,LjMt
for eachfin L,(R). Thus llL,+,cjll= 11Lj11. If IJLjJJ < 1, then
llLMcjll
< 1 and
If we replace r' by r - to in (12), we have l p i ( L j ) [ 6 I , and (10) follows. Thus A is a one-to-one mapping of .Xo(R) onto R. We prove next that A is continuous, where .do(R) is provided with the weak* topology. Suppose p and t correspond to rand t , respectively. LetJ(s) be exp( - isr) for s in [0,1] and 0 for other s. Then
where u = t - r . If
7c
6 IuI, then
Since ReCp(L.1) - r ( L J ) ]= 1 - u - l sinu, which has an alternating series expansion, u2/3! - u4/5 ! + . . ., whose terms are monotone decreasing to 0
3.2. THE SPECTRUM
197
for such 14. Thus It - r( is small when Ip(L,) - r(L,)l is small. The space .U(R) of non-zero multiplicative linear functionals on %,(R) is weak* compact, from Proposition 3.2.20, so that it is &,(R) “compactified” by adjoining the point pa. Let A’ be the one-to-one mapping of A@)onto the one-point compactification {R,a}of R that restricts to A on .Mo(R)(and assigns x, to p T ) . If we prove that A’ is continuous at p a , then, since A is continuous on . l o ( R ) , A’ is a continuous one-to-one mapping between the compact spaces .X(R) and {R, a}and, hence, A’ is a homeomorphism. It will follow that A is a homeomorphism of ,,4!o(R) onto R. To prove the continuity of A‘ at p I, choose an integer n greater than 1 and let f be the characteristic function of [O,n-’]. I f r ~ . 1 , ( R ) ,t = A ( 7 ) , and Is(L,) - p,(L,)I < n-3, then t(L,) =
I:.’
eIs’ds = t-lsinn-lf - ir-’[cosn-’t - 11.
From the earlier alternating series computation (with n- ‘t for u), 11 - n t - ’ sinn-’t( < t2/6n2, when It1 ,< nn, so that (l/n)(l - t2/6n2) Q t - l s i n n - l t Q IT(L,)I Q n-’. Thus 4n2 6 6(n2 - 1) Q f 2 , and 2n < It[. We have shown that IA(r)l is large if Ir(L,)l is small, for 7 in .Mo(R).Hence A‘ is continuous at pm on A ( R ) . The function3corresponding to f in L,(R) (defined by (3)) is called the Fourier transform off: It will be convenient to use this transform with other normalizations - that is, multiplied by constants (such as ( 2 ~ ) l”), - appropriate to the circumstances in which it is used. For the present, wecall attention to some of the special information we have obtained about Fourier transforms. 3.2.28. COROLLARY. The Fourier transformj+fdefined on L,(R) has the following properties : (i) I t is linear. (ii) (Uniqueness.) It is one-to-one. (iii) (Riemann-Lebesgue lemma.) I t maps L , ( R ) into the space of conrinuousfunctions on R ranishiny at 00. (iv) (ConLlolutionjiurmulu.) F & r ) = f(r)i(r). Proof. (i) This assertion amounts to the linearity of p in ( 3 ) . (ii) From (i), it sufices to prove that f is the element 0 in L , ( R ) iff(r) = 0 for all real r. SupposefZ 0. In the third paragraph of the proof of Theorem
198
3. BANACH ALGEBRAS
3.2.27, we show that there is a T in .M,(R) such that z(L,-) # 0. But T(L,-)= ] ( t ) for some real t . (iii) Again, from Theorem 3.2.27, A is a homeomorphism, so that
is continuous, where continuity of the second mapping is a consequence of the definition of the weak* topology on .X,(R). Since A ’ + ’ is continuous on {R, 301, A ‘ - ’ ( c n ) = p x , and p , ( L J ) = 0;fvanishes at XI. (iv) This assertion is a restatement of (4). One important aspect of the Fourier transform that has not appeared in our discussion up to this point concerns “inversion,” the process by which a functionfmay be “reconstructed” fromJ Some of the known results in this area are delicate. We shall need only an easily available sampling (for application in Section 13.2),which we prove in Theorem 3.2.30. In preparation for the proof of that theorem, we recall that the operation f + f * of “translating” functions is an isometry of L p ( R )onto itself. Another fact about this operation is proved in the lemma that follows. 3.2.29.
LEMMA.
If
fE Lp(R), then
(13)
111; - A 0 l l p
-+
0
(t
+
to).
Proof. Since
111; -1;CJp (13) will follow from
=
1u;
=
-f;o)-rollp
111; -flip -,0
IIA-ro
-flip*
0). If we have proved (14) for each function g in a dense subset B of L,(R), then, given a positive E and choosing g in B such that 119 -flip < 43, we have (14)
(t
+
IIA -flip d 111; - srllp + llYl - Yllp + 119 -flip c: + IIYI - Yllp. By assumption (on F), there is a positive 6 such that llgr - yllp < 4 3 when 111; -flip < E . For F, we choose the set of continuous functions on R,each of which vanishes outside some finite interval. With f i n 9, (14) follows from uniform continuity offand the fact thatf; -fvanishes outside a fixed finite interval for all small t . It1 < 6, and, in this case,
It will be useful for the proof that follows to recall that (from an integration by parts) sin t dt = -dt,
{
o
t
199
3.2. THE SPECTRUM
and that
We are now in a position to prove our inversion theorem. 3.2.30. THEOREM. If'fE L , ( R ) and
f(p ) = (274
-
then (15)
f(s) = (271)-'12 lim U-
cc
(:,(
1
-
$)r-isv(p)dp
for euch s at which f is continuous i f fe L , (R). all s, f(s) = (2n)-'12 (16)
L,(R), then, for almost
s.
e-isP'(p)dp.
Prooj: Let h,(p) be ( 2 7 ~ - ~ / ' (-1 Ipl/a) when IpI 6 a and 0 when a Then, since h,( p ) = A,,( - p ) ,
-= IpI.
200
3. BANACH ALGEBRAS
Now
and, iffEL,(R) and is continuous at s, then
for almost all t. Moreover, for all t ,
Thus, from the dominated convergence theorem and (1 7),
Noting that J.I;.(t)
dt = 2
Jr
&(t) dt =
1
1 - cosat
at2
dt = 1,
If a is large, llga -fill < E, so that, as a + co, {ga}converges in L l ( R ) to$ For
20 1
3.2. THE SPECTRUM
some sequence of integers { n ( j ) } monotone , increasing to 00, {g,(j,} converges to f almost everywhere. If we assume, now, that 3 € L I ( R ) , then p + h,, j,(p)e-ispfip)is a sequence of functions with absolute values dominated by If1 and tending to (27~)-112e-isp3(p) for eachp and each s. Hence {g,( j)(s)} tends to (27~)-l'~ J R e - i S q ( p ) dfor p each s, and (16) is valid for almost every s. We turn now to the L2 theory of Fourier transforms. The theorem that follows, Plancherel's theorem, will be used in Section 13.2. There is a unitary operator T o n L2(R) such that 3.2.31. THEOREM. (Tf)( p ) = (271) - 1/2
s,
eispf(s)ds
( p E R) ~
when f is continuous with support in a finite interval. If Rg = T(g)f o r all g in L2([w),then R = T*. Proof. Iff is a continuous complex-valued function on R with support in a finite interval, then so is the function g defined by g(s) =
1.
f(t)f(s
+ t)dt.
Thus g E Ll(R), and, by Fubini's theorem,
j
e-igPf(t)dt = (2n)'/213(p)12.
R
Since g is continuous at 0, it follows from the inversion theorem (3.2.30(15)) that
jR
lf(t)12
dt
= g(0) =
lim (271)- 1'2 a+ m
c
202
3. BANACH ALGEBRAS
where
Since {kn} is an increasing sequence of L1 functions with pointwise limit 13(p)12,it follows from the monotone convergence theorem that?€ L,(R), and
1,
If(0l2dt =
1
R
I?(P)12
4.
The set N(W)of continuous functions with support in a finite interval forms a dense linear subspace of L2(R). It follows from what we have proved thus far that there is an isometric linear operator T acting on L2(R),such that Tf =f when fE X(R). The mappingf+ris a conjugate-linear isometry of L,(R) onto L,(R), so that R, as defined in the statement of this theorem, is an isometric linear mapping on L,(R). To verify that R = T*, it will suffice to prove that (Tf,g ) = (f, Rg) forf, g in X(R) (since T and R are bounded and N(W)is dense in L2(W)).For suchfand g , we have, with the aid of Fubini’s theorem,
=
a)
f(s)((2,)-1’z
[;mg(r)ei~%!r)ds=
(f,rcs>)
= (f,Rg).
Now the null space of R is the orthogonal complement of the range of T, from Proposition 2.5.13. Thus, since R is an isometry, T maps L2(R) onto L2(R). Hence T and R are unitary operators. H
3.3. The holomorphic function calculus We develop, in very brief form, the theory of holomorphic functions of a complex variable with values in a Banach space, together with the associated holomorphic function calculus of a Banach algebra element. One of our main aims is to derive the spectral radiusformulu (Theorem 3.3.3). Although no other serious use of the Banach-algebra-valued holomorphic function calculus will be made, it is a powerful technique in the subject; and its results follow easily from classical complex function theory and the basic principles of functional analysis. Holomorphic functions. We need some concept of line integral of a Banach-space-valued function for this program. It will suffice for our
3.3. THE HOLOMORPHIC FUNCTION CALCULUS
203
purposes to use a “Riemann sum” limit, Jc f ( z ) dz, of a continuous Banachspace-valued functionfon a “smooth” curve C ( = t + z(t),a < t < b, where z is a continuously differentiable complex-valued function on [a, b]). The integral Jc f ( z )dz (= J,bf(z(t))z’(t)dt) is the norm limit of Riemann sums of the form n
C f(z(ti))[z(tj)- z(tj-
(1)
j= 1
where. as customary, a = to < t l < * . < t, = b, t j - < ti < t j , the limit taken as max{ltj - t j - : j = 1,. . . ,n} tends to 0. This theory is a straightforward extension of the classical theory (as presented, for example, in [R, Chapter lo]). By considering the Riemann sums (l), it follows that
where A is a bounded linear transformation from the range space offto some other Banach space. Our applications of (2) occur most often in the case where A is a continuous linear functional. From (1) we also have
(3)
II S c f ( z ) dzll
<
s’
Ilf(z(2))ll . Iz‘(0l dt
=
J
Ilf(z)ll . I d 4
C
With (u a Banach space and 0 an open subset of C, a mappingffrom 0 into (u is said to be holomorphic (on 0 )when it is differentiable at each point zo of 0, in the sense that the limit of the usual difference quotient exists in the norm topology on ‘ill. In this case, we denote the limit by f’(zo). I f f is holomorphic on 0 with values in ‘9I and p is a bounded linear functional on (u, then p ofis a (classical) complex-valued holomorphic function on 0. This fact, coupled with the Hahn-Banach theorem and the other basic principles of linear functional analysis, provides one of the main methods for proving results in the Banach-space-valued holomorphic function theory. (An illustration of this technique occurs in the proof of Theorem 3.2.3, where we proved in the course of the argument that z -+(zZ - T ) - is holomorphic on the set on which it is defined.) Applying this method to a function f holomorphic in a region 0 containing a curve C for which the classical Cauchy theorem and formula are valid, we have, as a consequence of the classical results,
and (for each zo “interior” to C )
204
3. BANACH ALGEBRAS
for every bounded linear functional p on %. Thus (4)
and (5)
for each zo "interior" to C.For the purposes of application in this subject, the curves and regions needed present none of the possible topological intricacies appearing in the more general versions of the classical results: piecewise "smooth" curves and regions with "uncomplicated" boundaries suffice. Without the detailed discussion of such matters (appropriate to classical complex function theory), we speak of smooth closed curves. The method of applying bounded functionals combined with an application of the Hahn-Banach theorem yields the fact that if a power series I,"=T,(z - z0)n,with coefficients T,, in a,converges to 0 in norm throughout some open disk containing zo, then each T, is 0. Repetition of the classical argument establishes the following theorems. 3.3.1. THEOREM.Each function f holomorphic on an open set 8 in C and taking values in a Banach space % can be represented as a power series T,,(z - z0)n, with coefficients in a, throughout the largest open disk with center zo contained in 8,for each zo in 0. 3.3.2. THEOREM. If Iz - zoI < ( ~ ~ ~ 7 ' , , ~ ~then 1 ~ n ) - llTnll 1 , * Iz - zOln converges. I f ( ~ ~ ~ T n ~ <~ Izl ~-"zoI, ) -then l I,"=o T,,(z - z0)"does not converge in norm.
By analogy with the classical situation, we refer to ( ~ ~ ~ T , , ~ as ~ lthe ~n)-l radius of convergence of the power series C=;o T,(z - z0),,.We use these considerations to establish the spectral radius formula in the theorem that follows. 3.3.3. THEOREM(Spectral radius formula). An element A of a Banach algebra 2l has spectral radius r(A) given by the formula
r(A) = lim (lAfllll/".
(6)
n- m
In particular, { llAflll' I n } has a limit. Proof. The existence of the limit in (6) is established by proving that -
(7)
1imllA"II""
< r(A) < l&llA"ll""
205
3.3. THE HOLOMORPHIC FUNCTION CALCULUS
(so that ~&IIIA"II"" = ~ ~ l A " l ] ' ' which " ) , proves (6) as well. If a E s p ( A ) , then a"~sp(A")(see Proposition 3.2.10), so that (from Theorem 3.2.3) la"[ < llA"ll. Hence la1 < &(JA"I("" and r(A) < li~~~llA"ll'/". The argument used in the proof of Theorem 3.2.3 shows that z + ( I - z A ) - ' ( = f ( z ) ) is holomorphic where defined (with derivative A(l - Z A > - ~and ) that the set of such z is open and contains all (small) z such that IIzAIl < I . Thus, for small z,fis defined and is represented by the power A'z". From Theorem 3.3.1 (and the comment preceding it on the series uniqueness of series representation), this series represents f on the largest open disk with center 0 on whichfis defined. On the other hand, Theorem 3.3.2 informs us that this series failsto converge for z of modulus exceeding ~ ' i nis, an a such that a' < la1 for (lim IIA"Illi")-'.Thus, if 0 < a' < lim ~ ~ A n ~there which I - a - ' A and, hence, A - a l fail to have inverses in 2l. Therefore a E sp(A) and a' < r(A). Since a' is an arbitrary non-negative number less than lim IIA"llli",the inequality (7) follows. H In case lim llAnlllin= 0, the inverse of limIIA"II"" is interpreted, as is customary, as co.When this occurs, r(A) = 0 and sp(A) consists of 0 alone. We discussed an instance of this in the comment following Theorem 3.2.3. An element A in 2I for which r(A) = 0 is said to be a generalized nilpotent in 2I. If A" = 0 for some positive integer n, we say that A is nilpotent (so that a nilpotent element in N is, in particular, a generalized nilpotent). One notes from Theorem 3.3.3 that r,(A) = r & 4 ) when A lies in the Banach subalgebra 98 of %. If A and Bare commuting elements of 2l, applying Proposition 3.2.10 to the commutative Banach subalgebra of N that A and B generate, together with this observation, we have the following result. 3.3.4. COROLLARY. If A and B are commuting elements in the Banach algebra 2I, then r(AB) 6 r(A)r(B)and r(A B) < r(A) r(B). The holomorphic function calculus. Turning now to the case where 2l is a Banach algebra, we note that
+
+
'S
'
A" = - z"(zZ - A ) - dz, (8) 271i where n is a positive integer, A EN,and C is a smooth closed curve whose interior contains sp(A). To see this, observe that z + ( z l - A ) - is holomorphic on C\sp(A) (as proved in Theorem 3.2.3). Employing the Cauchy theorem (see (4)) in the case of the 2l-valued function z + z"(zl - A ) - we may replace C by the (circular) perimeter of a disk with center 0 and large radius. Assuming C is this circle and z is on C,
'
',
(9)
z"-l(l-
~ ~ - 1 ) -= 1 zn-l
m
a3
k=O
k=O
1 Akz-k = C Akf-k-1
9
206
3. BANACH ALGEBRAS
where convergence is in the norm topology (and uniform on C). It follows from (3) and this convergence that term-by-term integration of (9) is justified. Now
s,
AkZn-k-l
dz=( ~czn-k-ldz)Ak=O~Ak=O
unless k = n, in which case the integral is 27tiA“.This proves (8). It follows that
‘S
f(A) = 7
2x1 c
f(z)(zZ - A ) - dz
for each polynomialf, when C is as in (8). With the foregoing in mind, we take (10) as the definition of f ( A ) for holomorphic functionsf. More precisely, whenfis holomorphic in an open set containing sp(A), we can choose a smaller open set 8 containing sp(A) whose boundary consists of a finite number of closed piecewise linear curves C1,.. . ,C,,. If C denotes the collection of these curves oriented in the customary way in complex function theory, then (10) definesf(A). To find the smaller open set with boundary as described, an argument involving a square grid in the plane (with squares of diameter less than the distance from sp(A) to the boundary of the initial open set) will suffice. Since the integral in (10) converges in norm, from our discussion of line integrals, it represents an element f(A) in 2l. From (4) (Cauchy’s theorem) f ( A ) is independent of the curve C (consisting of a finite number of smooth closed curves constituting the boundary of an open set in which f is holomorphic). Let %(A) be the set of functions holomorphic in some open set containing sp(A) (the open set may vary with the function). The following two results constitute a “calculus” of such functions - the holomorphicfunction calculus.
3.3.5. THEOREM. The mapping f 3 f ( A ) is a homomorphism from # ( A ) into 2lfor each A in the Banach algebra 2l. I f f is represented by the power series I,,“=anznthroughout an open set containing sp(A), then
,,
m
n=O
Proof. Since
the mappingf+f(A) is linear. The proof thatf(A) . g(A) = (f. y)(A) requires more effort. Let 0 be an open set, containing sp(A), on which bothfand g are holomorphic. We can choose open sets O1 and O2 such that sp(A) E 01,
01 v C1 G 02,
02 v C2 E 8 ,
207
3.3. THE HOLOMORPHIC FUNCTION CALCULUS
where C1and C2, the boundaries of O1 and 0,, consist of a finite number of smooth closed curves. Then
f(z)g(w)(zl- A)-'(wl- A ) - ' dzdw
[ ( z l - A)-' - ( w l - A ) - ' ] w-z
dz dw
- ( ~2ni ) ' S , , g ( w ) ( w l - A ) - ' ( J c1W-z E d z ) d w 1 2ni
==(
f(z)g(z)(zl- A ) - ' dz-
g(w)(wl-A)- ' ( 0 )dw
c1
f . g)(A)*
To prove the last assertion of the theorem, we may assume that f is defined on the disk of convergence of the series. Let C be a circle with center at 0 containing sp(A) in its interior and contained in an open set on which f is holomorphic and represented by I,"=a,z". Then this series converges uniformly on C, so that, from (8),
c a, (& c a,A". m
=
n=O
Ic
z"(zl - A )- dz
m
=
n=O
In our next result, we identify the spectrum off@). The special case wheref is a polynomial has been treated in Proposition 3.2.10.
3.3.6. THEOREM(Spectral mapping theorem). r f A is an element of a Banach algebra 2l and f is holomorphic on an open neighborhood of sp(A), then (12) SP(f(A)) = { f ( a ) :a E SP(41 ( =f(sp(AN). Proof. Suppose aEsp(A), so that either %(A - al) or ( A - al)% is a proper (left or right) ideal in 2l. Say, %(A - a l ) is a proper (left) ideal in 2l.
208
3. BANACH ALGEBRAS
Then 2 n i [ f ( A ) -f(a)Z] = =
f(z)[(zZ
- A)-'
- (z - a)-'Z]dz
(lC
)
[ ( z l - A)(z - a)]- ' f ( z )dz ( A - aZ) E %(A - a l )
and f ( a )E sp(f(A)). Thus (13) f(SP(A)) c S P ( f ( 4 ) . If b$f(sp(A)), then (f- b ) - l (= g ) is holomorphic on an open neighborhood of sp(A). From Theorem 3.3.5, g(A) is a two-sided inverse to f ( A ) - b l in % (since g . (f- b) is 1 on an open neighborhood of sp(A)). Hence b $ sp(f(A)), and
(14) SP(f(A)) s f(SP(4). Combining (13) and (14), we have (12).
An interesting and simple corollary of the holomorphic function calculus and the spectral mapping theorem asserts that if the Banach algebra 2l has an element A whose spectrum is not connected, then 9l has an idempotent E different from 0 and I. To see this, suppose that sp(A) = S1u S2 ,where S1and S, are disjoint closed sets. Since sp(A) is compact (see Theorem 3.2.3), both S1 and S, are compact. It follows that there are disjoint open sets O 1 and 0,such that S1 E O1 and S, E 0,.The functionftaking the values 1 on O1 and 0 on 0, is holomorphic on O1 u O,, an open neighborhood of sp(A). Thenf(A) is an idempotent E since E = f ( A ) = ( f Z ) ( A = ) f ( A ) f ( A ) = E Z , and sp(E) = (0, l}. Thus E is neither 0 nor I. +
3.3.7. COROLLARY. r f A is in the Banach algebra 2l and sp(A) is not connected, then 2I contains an idempotent different from 0 and 1. The composite-function result that follows is an important addition to the function calculus.
3.3.8. THEOREM. IfAisanelementof the Banachalgebra%,gEX(A), and f E W S ( A ) ) ,then f 0 9 E %(A) and ( f o g ) W =f ( g ( 4 ) . Proof. By assumption, f is holomorphic on an open set O1 containing sp(g(A)) and g is holomorphic on an open set 0, containing sp(A). From Theorem 3.3.6, g(sp(4) = sp(g(A)) s
017
so that sp(A) E g - l ( O 1 ) . By continuity of g , g-'(O,) n 0, is an open set O (containing sp(A)) on which f a g is holomorphic. Thus f o g E X(A).
3.3. THE HOLOMORPHIC FUNCTION CALCULUS
209
Choose open sets @ and 6&l with boundaries C and C1consisting of a finite number of smooth closed curves such that sp(A) c 42 E 9 u C G 8 and sp(g(A)) c g(% u C ) c 9
1
e 421 u c1 E 8,.
By continuity of g on 8, there is an open set 9' such that 9u C c 9'E 0 and g ( 4 ' ) 5 +21.Then, for each w on C1, h, is holomorphic on W ,where h,(z) = [ w - g ( z ) ] - ' . From Theorem 3.3.5, h,(A) = [ w l - g ( A ) ] - ' for each w on C1. It follows now that .
P
We conclude this section with a result that allows us to treat convergence in the holomorphic function calculus. 3.3.9. PROPOSITION. ZfS is an open set containing sp(A), where A is an element of the Banach algebra a, and { f n }is a sequence of functions holomorphic on 0 and converging uniformly tof on compact subsets of 0, then f E X( A)and Ilf"(4 -f(A)II 0 as n a* +
+
Proof. Choose an open set 9 with boundary C consisting of a finite number of smooth closed curves such that
sp(A) 5 %! E 42u C c 8.
Since {fn} converges uniformly to f on compact subsets of 0,f is holomorphic on 0.Thus fe.%?(A) and {f,}converges to f uniformly on C. It follows that
Ic
2nllf,(A) - f ( M = II
Cf,(Z)
- f ( z ) l ( z l - A)-'dzll
G klC1 . llfn - f
IIC
+
0 9
210
3. BANACH ALGEBRAS
3.4. The Banach algebra C(X)
In Example 3.1.4 we introduced the algebra C ( X ) of continuous complexvalued functions on the compact Hausdorff space X together with its (supremum) norm and established that it is a Banach algebra. From the point of view of C*-algebras, C ( X ) is, by far, the most important example of a commutative Banach algebra. We shall see in Section 4.4 that C ( X ) is the example of a commutative C*-algebra. In its function algebra form it provides the basis for the spectral theory and “function calculus” of a self-adjoint (or normal) operator. Our purpose in this section is to study C ( X )both with respect to its Banachalgebra structure and with respect to its order structure. We begin by identifying the closed ideals (and, hence, the maximal ideals in C(X)).
rfS is a closed ideal in C(X),there is a closed subset S of 3.4.1. THEOREM. Xsuch that $ is the set of allfunctions vanishing on S. U S is a closed subset of X , the set of allfunctions vanishing on S is a closed ideal in C ( X ) .The maximal ideals in C(X)are those closed idealsfor which the corresponding closed subset of X (on which all the functions of the ideal vanish) consists of a single point. Proof. Since the set of points at which a continuous function vanishes is closed and the set of points S at which all the functions of 9 vanish is an intersection of such sets, Sis a closed subset of X . We use the assumption that $ is closed to show that a functionf that vanishes on S is in 9.Note that this has the implication that 9 = C ( X ) if S is null. Suppose, then, that f E C ( X )andf vanishes on S. Given any positive E , let F, be the set of all pointsp at which If(p)l 2 E , so that F, is compact, and does not meet S. We shall construct an element g, of 9 such that 0 < g , ( p ) < 1 for all p in X , while g , is 1 throughout F,. Once this is done, we have fg,EY (since g c E 9 ) ; moreover, 1) f - fg,ll < E , because 11 - g,(p)l never exceeds 1, and is zero at all points where If(p)I 2 E . Since 9 is closed, we can conclude thatf E A We now construct g , with the properties set out above. By definition of S, and since F, does not meet S,for each point p of F,, there is an f, in 9 such that f,(p) # 0. Thus3, .fp is (strictly) positive on some open neighborhood ofp. A finite number of such neighborhoods cover the compact set F,. Ifp, ,. . . ,pnare thecorresponding points, then l&J2 + . . . + 1f P J 2 is a function h,, in Y,which is non-negative on X and positive throughout F,. From the compactness of F,, h, has infimum c (> 0) on F,. The equation k,(p) = max(h,(p), c ) defines an element k, of C ( X ) , and
k,(P) > 09
k,(P) 2 h&(P)2 0
3.4. THE BANACH ALGEBRA C ( X )
21 1
for all p in X , while k, coincides with h, on F,. Since k; ' E C ( X ) and heE 9, it now suffices to take h,k; ' for g,. The set of functions vanishing on an arbitrary subset of X will be a closed ideal f in C(X),but the set of points at which all the functions of 9 vanish will be a closed subset containing that set, its closure. To establish this Galois-like correspondence between closed subsets of X and closed ideals in C(X), note that if S is a closed subset of X and f is the closed ideal of functions in C ( X ) vanishing on S, the Tietze extension theorem tells us that, given a pointp not in S, there is a continuous functionfon X that is 0 on Sand 1 atp. ThusfE 9 andp is not in the closed set corresponding to 9.Hence the closed set corresponding to Y is S. Of course, now, the maximal ideals in C ( X )are those whose corresponding closed set consists of a point. 3.4.2. COROLLARY. Each non-zero multiplicative linear functional p on C ( X ) corresponds to a point po in X ; and p ( f ) =f ( p o )for each f in C ( X ) .
Proof. The kernel A of p is a proper ideal (since p # 0) and is a maximal ideal of C ( X ) .From Theorem 3.4.1, A is the set of functions in C ( X )vanishing at some point po of X . Since ~ ( 1 ' ) = ~ ( 1 ) '= p(1) # 0, p(1) = 1 ; and f- p(f)l E Afor each f in C ( X ) .Thus f ( p o )= p ( f ) for each f in C ( X ) .
We say that the functional p in Corollary 3.4.2 is evaluation at p o . The closed subset corresponding to an ideal (or just the common zeros of any set of functions) is referred to as the kernel of that ideal (or that set of functions). The (closed) ideal of functions vanishing on a set of points in Xis referred to as the hull of that set. Corollary 3.4.2 (or Theorem 3.4.1) provides us with a means of recapturing the topological space X from the algebraic structure of C(X). 3.4.3. THEOREM.A mapping cp of C ( X ) onto C( Y), with X and Y compact Hausdorff spaces, is an algebraic isomorphism if and only if there is a homeomorphism r] of Y onto X such that q ( f )= f r] for each f in C(X). 0
Proof. If r] is a homeomorphism of Y onto X and q(f)= f0 r], then for each f in C ( X ) , f or] E C( Y ) , and cp is an algebraic isomorphism of C ( X ) onto C( Y ) . Suppose, now, that cp is an algebraic isomorphism of C(X)onto C( Y ) .If .A is a maximal ideal in C( Y ) ,then q - ' ( A is ) a maximal ideal in C(X).Ifp is the point in Y corresponding to A,denote by r](p)the point in Xcorresponding to q - ' ( A )Since . each maximal ideal A. in C ( X )has the form cp-'(cp(Ao)), with cp(d0) a maximal ideal in C( Y ) , q is a one-to-one mapping of Y onto X . If f~C ( X ) , f - f(r](p))lvanishes at r](p)in Xfor eachp in Y. Thus, by definition of r], q(f - f(r](p ) )1) vanishes at p . Hence cp( f)( p ) = f0 r]( p ) for all p in Y, and cp(f) = f o r]. Since X is a compact Hausdorff space, it is completely regular.
212
3. BANACH ALGEBRAS
Hence sets of the f o m f - l(O), with 0 an open subset of C andf in C ( X ) ,form a subbasefortheopen setsinX. Now q-l(f-l(O)) = cp(f)-’(Co);and cp(f)-’(O) is an open set in Y, since q(f) is a continuous function on Y . As the inverse images of subbasic open sets in X, under q, are open in Y, q is continuous. Symmetrically, q - is continuous; and q is a homeomorphism of Y onto Xsuch that cp(f) = f o q for eachfin C(X).
’
3.4.4. R E M A R K . It follows from Theorem 3.4.3 that cp mapsreal functions in C ( X ) onto real functions and positive functions onto positive functions. It follows, too, that cp is an isometry. Thus the assumption that cp is an algebraic isomorphism entails very strict response from cp in terms of other structure that C ( X ) possesses (in particular, the norm and order structure on C(X)).These consequences of the assumption that cp is an algebraic isomorphism become more apparent when we note that cp “preserves” spectrum, that is sp,-,,,(f) = spc0,(cp(f)), and that the spectrum offrelative to C(XJ is the range of the functionf(see Example 3.2.16, in this connection, where the spectrum of M , is the essential range off). That cp preserves spectrum is a consequence of the fact that spectrum is defined in terms of inverses and cp preserves inverses. Of course, f - I 1 fails to have an inverse in C ( X ) if and only if it vanishes at some point of X , that is, if and only if 1is in the range off: The property of being real or positive forfand the norm offare all determined by the range off: We see, at the same time, that cp preserves the operation of complex conjugation on C ( X ) (that is, cp(f) = cp(f))- for cp maps real functions onto real functions. Note the formal similarity between the operation of complex conjugation of functions on C ( X ) and the adjoint operation on g ( X )(see Theorem 2.4.2). Both are conjugate-linear, involutory, (anti-)automorphisms on their respective algebras. With Theorem 4.4.3, this similarity becomes more than “formal.” When Theorem 4.4.3 has been established, the view of a C*-algebra as a non-commutative generalization of C ( X ) (that is, as a “non-commutative function algebra”) will be quite plausible. Up to this point, we have been studying the Banach-algebra structure of C ( X ) . In application to noncommutative C*-algebras, extending the order properties of C ( X ) ,rather than its Banach-algebra structure, proves to be the more fruitful procedure. The order structure of C ( X ) is a natural partial ordering of its real-linear subspace C(X, R), the continuous real-valued functions on X . It is introduced by means of the “cone” 8 of positive functions, which has the properties (of a cone) :
(i) iff and - f a r e in 8, then f = 0; (ii) if a is a positive scalar and f E 8, then af E 9; (iii) f+ g e 9 iffand g are in 9
3.4. THE BANACH ALGEBRA C ( X )
213
A real vector space V with such a cone is said to be a partially ordered vector space. Defining (as is usual in C(X, R)), f < g when g -f E fl induces a partial ordering on .Y:An element I in V (the constant function 1 in C(X,R)) is said to be an order unit when, given any f in "y; we have -aI < f < aZ for a suitable positive scalar a (depending onf -in the case of C(X,R), we may choose a to be
I l f 11). 3.4.5. DEFINITION.If V is a partially ordered vector space with order unit I , a linear functional p on Y is said to bepositive when p ( f ) 2 0 iff 2 0. If, in addition, p ( l ) = 1, p is said to be a state of % If p is an extreme point of the (convex) family Y ( Y )of states of "y; we say that p is a pure state of %
Note that the set of positive linear functionals on Y is a cone relative to which the dual space of Y becomes a partially ordered vector space (generally without an order unit). A simple modification of the condition for a state to be pure proves useful to us. 3.4.6. LEMMA.If V is apartially ordered vector space with order unit I , a state p of Y is pure ifand only ifeachpositivefunctional z on Y such that z < p is a scalar multiple of p . Proof. If the stated condition holds for p and p = upl + (1 - a)p2,with 0 < a < 1 and p1 and p 2 states of "y; then 0 < upl < p , so that u p , = bp. Since pl(l) = p ( I ) = 1, a = b and p1 = p . Similarly p 2 = p , and p is pure. On the other hand, if p is pure and 0 < < p , then 0 < z ( I ) < p ( l ) = 1. If z(Z) = 0, then, for each f in K 0 = z( - aZ) < z(f) < z(aI) = 0 for some scalar a, and t ( f ) = 0; so 7 = 0 (= 0 * p ) . If T ( I ) = 1 (= p ( l ) ) , a similar argument shows that the positive functional p - z is 0 (and z = 1 . p ) . Finally, if 0 < z ( I ) < 1, we have p = (1 - b)pl bp,, where b = z ( I ) and p1,p2are the states defined by p1 = (1 - b ) - ' ( p - z), p 2 = b-%. Since p is pure, p 2 = p , and T = bp. In each case, z is a multiple of p .
+
By expressing elements of C ( X )in terms of their real and imaginary parts, it is apparent that each (real-)linear functional on C(X,)!FI extends uniquely to a linear functional on C(X).A linear functional on C ( X )is said to be positive (or a state, or a pure state) if its restriction to C(X, R) is positive (or a state, or a pure state). Since the states of C(X,R) form a convex set with the pure states as extreme points, the same is true of C ( X ) . The significance of states and pure states for C*-algebras will appear in Sections 4.3 and 4.5. For the present, we establish that the pure states of C ( X ) are precisely the (non-zero) multiplicative linear functionals on C ( X ) . 3.4.7. THEOREM.A non-zerofunctional p on C ( X )is apure state of C ( X )i f and only if it is multiplicative.
214
3. BANACH ALGEBRAS
Proof. If p is multiplicative, from Corollary 3.4.2, there is a point po in X such that p c f ) =f(po) for all f in C ( X ) . Since each continuous function vanishing at po is a linear combination of positive continuous functions vanishing at po; if 0 < z < p for some linear functional t on C ( X ) , then z ( f ) = 0 whenf(po) = 0. Thus z and p are linear functionals on C ( X )with the same (maximal linear) null space and z is a scalar multiple of p. From Lemma 3.4.6, p is a pure state of C(X). Suppose p is a pure state of C(X).If 0 < f < 1 and z(g) = pug),then z is a linear functional on C ( X ) such that 0 < z < p . Thus z = up. If p(h) = 0, then p(fh) = z(h) = ap(h) = 0. Since each function g in C ( X ) is a linear combination of functions between 0 and 1, p(gh) = Ofor all g in C ( X ) .Thus the null space of p is an ideal -clearly maximal since the null space of p is a maximal linear subspace. As p(Z) = 1, p is multiplicative. W The linear order structure in C ( X )is strong enough to characterize it. We say that a mapping cp between two partially ordered vector spaces (or two C ( X ) spaces) that is a linear isomorphism of one onto the other is a linear order isomorphism when q ( f )> 0 if and only iff 2 0.
3.4.8. COROLLARY. A linear order isomorphism cp of C(X)onto C( Y) such that cp(1) = 1 is an algebraic isomorphism. Proof. If po is a pure state of C( Y) corresponding to the point po of Y, by Theorem 3.4.7, then po 0 cp is a pure state of C ( X ) corresponding to a point q(po) of X. Now, q is a one-to-one mapping of Y onto X , and f(q(po)) = po(cp(f)) = cp(f)(po) for all f i n C ( X ) and po in Y. Thus cp(f) =f 0 q. As in Theorem 3.4.3, q is a homeomorphism of Y onto X; and cp is an algebraic isomorphism. W
3.4.9. REMARK.The partial ordering on C ( X )induces a lattice structure on the set of real-valued functions. Iffand g are two such functions, we define (fv g ) ( p ) to be max{f(p), d p ) } and (fA g)(p) to be min{f(p), g(p)} for each p in X . Then fvs=%-+g)+:lf-gL
fAs=t(f+g)-:lf-gI;
so that f v g and f A g are in C(X).Clearly, f v g is the smallest function greater than bothfand g, andf A g is the largest function less than bothfand g. Moreover,f = f - f-,wheref + = f v 0 and f- = - (fA 0); so thatfis the difference of two positive functions in C(X) (with disjoint supports). The lattice structure assures us that C ( X ) has the Riesz decomposition property: iff < g1 g2, wherefl gl, and g2are positive functions in C(X),then f = f l +f2, where 0
+
215
3.4. THE BANACH ALGEBRA C ( X )
remains to note that fz< gz. If gz(p) - = f 2 ( P ) = f ( p ) -f1(P) = f ( p ) - min{f(p),g1(p)} for some p in X , then, since 0 < g2(p), min{f(p),g,(p)} = gl(p); and gl(p) gz(p)
+ ~
~
IP(1 . S>l2,< P(lSIZ)
,< 1.
3.4.10. PROPOSITION. The hermitian linearfunctionals in the (norm-)dualof C ( X )form a lattice.
Proof. With p1 and p 2 hermitian linear functionals on C ( X )andfpositive in C ( X ) , define
(P1 v
(1)
P2)(f)
= SUP{Pl(.fl)
+ Pz(fZ):f1,fZEC(X),
0 Gfl, 0
+fz).
Note that, with f l ,fz as in (I), IPl(f1)
+ Pz(f2)l < lblll . llflll + llP2ll . llfzll < (llptll + IIP2ll)llfll,
so that (pl v p z ) ( f ) is finite. The decompositionf=f+ 0 = 0 + festablishes that p1 G p1 v p z and p 2 < p1 v p 2 . If p1 < 7 and p z < 7 for some hermitian linear functional 7 on C(X), then ~ l ( f 1+ ) pz(fz) G ~(f1)+ 7 ( f d = ~ ( f ) , so that p 1 v p 2 < 7 . Once we show that p1 v p 2 is linear on the cone of positive elements, and, by routine technique, that its extension to C ( X ) is linear, we shall have that that extension is a (bounded) hermitian linear functional on C ( X ) that is a least upper bound for p1 and p 2 .
216
3. BANACH ALGEBRAS
We assume, until further notice, that all functions appearing in the argument are positive functions in C(X).Choose gl, g2 and h l , hz with g equal to g1 + g2 and h equal to hl + h2 such that pl(g,) + p2(gz) approximates (P1 v P 2 ) ( 9 ) and Pl(h1) + P Z ( h 2 ) approximates (P1 v P 2 ) ( h ) . Then 91
+ h l + 9 2 + h2 = 9 + h,
and Pl(91
+ hl) + PZ(92 + h2) = P l b l ) + Pz(g2) + Pl(h1) + P Z ( h 2 ) G (P1 v P2)(9
+ h).
Thus
(2)
(P1 v P2)(9)
+ ( P 1 v P d h ) G (P1 v P 2 ) ( 9 + A).
Suppose g+h=f, + f 2 and Pl(fl)+PZ(h) approximates (PI V P 2 ) ( 9 + h ) . The Riesz decomposition property permits us to expressfl asfl +flz andf, asfzl f Z 2 withf, f Z l equal t o g andf,, + f Z 2equal to h. The see this, note that fl G g h so that fl = f i l + f l z , where f l l G g and f l z G h. Thus fi = g h -fl = g - f l l h - f 1 2 . Lettingf,, beg - f l l andf,, beh - f 1 2 , we have the desired decomposition. Hence
+ + +
+
+
Pl(f1)
+ Pz(f2)
+ PlU-12) + PZ(f21) + P Z ( f 2 2 ) P2)(9) + (P1 v P 2 ) W
= Pl(fl1)
G (P1 v Thus
(3)
(P1 v P2)(9
+ h) G (P1
v P2)(9)
+ (P1
v Pdh).
Combining (2) and (3), we have the additivity of p1 v p2 on the cone of positive elements. approximates (pl v p z ) ( f l , then If0 < a,f1 + f ~=f,and PI(^,) ufl + af2 = a5 so that
+
4Pl(fl)
+ Pz(f2))
G
(P1
v Pz)(af).
Thus 4Pl v
P2)(f)
G (P1 v Pz)(af).
But then a-'(Pl v
P d W ) 6 (P1 v
p d a - ' a . = (P1 v
P2)(f).
Thus 4 P l v P Z ) ( f l = (P1 v Pz)(af). Suppose, now, thatfis an arbitrary real-valued function in C(X).We can express f asfi - fi,withf, andf, positive functions in C(X) (see Remark 3.4.9). Define (P1 v P2)W to be (P1 v P 2 ) d f i ) - (P1 v P d ( f 2 ) . I f f = 91 - 92
3.4. THE BANACH ALGEBRA C ( X )
with g1 and g2 positive functions in C ( X ) , thenf, (p1 v P2)(Sl
+ g 2 ) = (P1 v PZ)(fl) + (PI = ( P 1 v P2)(91) + ( P I
+ g2 = g1 + f 2 , so that
v p2)(g2) = (P1 v P2)(91
v
217
+f2)
p2)(f2),
and (P1
v
P2Ul)
- (PI v
P2)(f2)
= (PI v P2)(91) - (P1 v p2)(92).
Thus p1 v p2 as extended to the space of real-valued functions in C ( X )is well defined (single valued). The additivity and positive homogeneity of p1 v p2 on the positive cone establish the additivity and homogeneity of p 1 v p2 on the space of real-valued functions in C ( X ) .For arbitrary g in C ( X ) ,let ( p l v p2)(g) be (pl v p 2 ) ( g 1 ) + i ( p l v p2)(g2), where g = g1 + ig2 with g1 and g2 realvalued functions in C(X). There is no difficulty in seeing that - (( - p l ) v (- p2)) is a greatest lower bound for p1 and p2 in the space of hermitian linear functionals on C(X). The decomposition of an arbitrary function in C ( X ) as a linear combination of positive functions combined with the observation (noted above) l(P1
v Pz>(f)l G (IIPIII
+ IIP2ll)llfll
for positive functions shows us that p1 v p 2 , as extended to C ( X ) , is bounded. 3.4.11. PROPOSITION. If p is a (bounded) hermitian linear functional on C(X), p + = p v 0, and p - = - ( p A 0), then P = P+
- p-,
p+
A
p - = 0,
and
IIPII = llP+ll + IIP-Il = P + V ) + P - ( 1 ) * Proof. With f a positive function in C ( X ) , P + ( f ) = suP{P(fl):o
(4)
218
3. BANACH ALGEBRAS
(see (4) above). If T is a positive linear functional on C ( X )and z(1) = 0, then T = 0 (since 1 is an order unit), so that 1 1 ~ 1 1= ~ ( 1 )= 0. If ~ ( 1 #) 0, then T(~)-'T is a state of C(X),and, as noted in Remark 3.4.9, 11T(1)-%11 = 1, so that 1 1 ~ 1 1= ~(1). Thus
IIPII 6 IIP+lI + IIP-ll
= P+(l) + P-U) = sup{p(f):O
6 f 6 1,f€C(X)}
- inf{p(g) : 0
6 g 6 1, g E C(X)}.
For suitablefand g in C ( X ) ,satisfying 0 < f < 1 and 0 < g 9)) approximates p f ( l ) + p-(l), and
< 1, p(f)
- p(g)
(= p(f-
P(f) - p ( g ) < llpll - I l f - 911 6 IlPll. Hence p + ( l ) + p - ( l ) 6 llpll, and this, with the previous inequalities, gives
IIPII = lb+Il+ Ib-ll = P + ( l ) + P-(l)3.4.12. R E M A R K . The decomposition of a bounded hermitian linear functional p on C(X)as p + - p - proved in Proposition 3.4.11 is a special case (the commutative case) of the decomposition of positive linear functionals on a C*-algebra established in Theorem 4.3.6. It is proved there that the decomposition with the norm property llpll = IIp+II + I)p-(l is unique. Of course, the special case of C(X)is covered by this result. Nonetheless, it is useful to see an argument for the uniqueness in this simpler case. With the assurance that a precise proof is given for the general case, we can enjoy the luxury of arguing loosely (and possibly, therefore, more lucidly). Suppose, then, that p = p + - p - and llpll = IIp+I( IIp-11. Choosefo in C ( X ) so that llfoll = 1 and llpll = p ( f o ) = p + ( f o ) - p - ( f o ) . (This can be done only approximately, strictly speaking.) From ths, p + ( f o ) = IIp+II and P - ( ~ O ) = - IIp-I). Also p + ( f O + )= ) ) p + ) and ) p-(fO+) = 0, whereJb+ =fo v 0. I f f is a positive function in C ( X ) and 0 < g 6 J with g in C(X), then P(g) = P+(g)-P-(g) 6 P + W G P + ( f ) . Since P+(l)=llP+ll, p+(l -f,'>=O. As 1 - fof 2 0, it follows from the Cauchy-Schwarz inequality applied to p + (see the end of Remark 3.4.9) that
+
Thus p ' c f ) = p+(ffo+). Similarly p - ( f O + f ) = 0, since p - ( f , f ) = 0. Thus
3.4. THE BANACH ALGEBRA C ( X )
219
p ( f ; f l = p + ( f f , ’ ) = p + ( f ) . Now 0 < f , ’ f d L so that
p+(n = sup{p(g):O d 9 d L f E W ) ) ,
which identifies p + with the construction in Proposition 3.4.11. Thus the decomposition p = p + - p - , where llpll = JIp+II+ Ilp-11, and p + , p - are positive, is unique. If we replace the unwarranted assumption that llpll is attained at some f o by an approximation and argue along precisely the same line, using estimates, a valid proof of the uniqueness results. 3.4.13. REMARK.If T is a positive linear functional on C(X), the set Y of functions f in C ( X ) such that t(lf12) = 0 forms a closed ideal in C ( X ) (use Cauchy-Schwarz for t to prove this). From Theorem 3.4.1, 9 is the set of functions in C ( X )vanishing on a closed subset of X.We refer to this closed subset as the support o f t and denote it by s(t). The measure-theoretic approach views t as “integration” relative to an associated positive measure p on C ( X ) (through the Riesz representation theorem). In that context, s(t) is the support of the measure p. If p is a bounded hermitian linear functional on C ( X ) ,then p corresponds to a “signed” measure p, p + to p + , and p - to p - , where p = p + - p - . The decomposition of p as p+ - p- is the Hahn-Jordan decomposition of p. H With the aid of the information we have accumulated concerning functionals (in particular, states) on C(X), we can prove a crucial approximation theorem. 3.4.14. THEOREM.rfd is a norm closed subalgebra of C(X), 1 E d , T e d i f f E d, and, for each pair of distinct points p and p’ in X,there is a function f i n d such that f ( p ) # f ( p ’ ) , then d = C ( X ) . Proof. Suppose d # C(X). From the Hahn-Banach theorem (see Corollary 1.6.3), there is a non-zero bounded linear functional p on C ( X )that annihilates d.We may assume that p is hermitian, since p* (see Remark 3.4.9) also annihilates d.The set of all hermitian functionals with norm at most 2 and annihilating d is a weak* closed bounded convex subset of the (norm) dual of C ( X ) . Hence this set is weak* compact and is the closed convex hull of its extreme points (from Corollary 1.6.6). Let 1 be such an extreme point. Then 1 = q + - q - , where 1’ and 1- are positive linear functionals on C ( X ) such that 2 = ll1ll = IIq+II + 111-(1(from Proposition 3.4.11).Since 1 is 0 on d,and 1 €d, 0 = ?(I) = 1+U)- 1-(1) = lll+ll - ll1-ll.
Thus 1 = Il1+ll = 1+(1) = 1 1 1 1 1= v - ( l ) , so that q+ and 1- are states of d.We shall prove that q+ and 1- are pure states
220
3. BANACH ALGEBRAS
of C ( X ) (based on the extremal property of q). If this has been established, then, from Theorem 3.4.7 and Corollary 3.4.2, there are pointsp. andp- in X such that q+(f) = f ( p + ) and q - 0 = f ( p - ) for allfin C ( X ) . Since q # 0; q + # q - andp, f p - . But 0 = q(9) = q + ( d - vl-(s) = d P + )- g b - 1
for each g in d,contradicting the assumption that for some g in d, # g(p-). It remains to show that q + and q - are pure states of C ( X ) . Using the Cauchy-Schwarz inequality as in Remark 3.4.9, we see that the set 9 of functionsfin C ( X ) such that q+(Jf12) = 0 is a norm closed ideal in C ( X ) on which q+ vanishes (see Remark 3.4.13, where 9 is used to define the support of .)'q Suppose we have proved that q + and q - are multiplicative on d.Then $ n d is a maximal ideal in d.From Zorn's lemma, 9 has an extension to a maximal ideal JZ in C(X),and JZ n d = 9 n d.Thus there is a multiplicative linear functional extending the restriction of q + to d.If there were two distinct such extensions, we would, again, have two distinct points of Xat which all the functions of d take the same values; so there is one and only one multiplicative extension of q + to C ( X )from its restriction to d.The set of all states of C ( X ) that agree with q t on d is convex, weak* compact, and, hence, the closed convex hull of its extreme points. Let z be one of these extreme points. If z = i(pl p2), with p1 and p2 states of C(X),then i p l and ip2 are majorized by q + on d (since their sum is z, hence, q + on d ) .Thus p1and p2 annihilate the same positive functions in d as q+ does; from which (again by using the Cauchy-Schwarz inequality), p1 and p2 annihilate 9 n d.As p l ( l ) = p2(l) = q + ( l ) and 9 n d is the null space of q + (under the assumption currently in force, that q + is multiplicative on d ) ,it follows that pl, p 2 , and q ' coincide on d.Thus z = p1 = p2 (from the extremal property of z). Hence T is a pure state of C(X) that agrees with q + on d. From Theorem 3.4.7, z is multiplicative on C(X),so that it is the unique multiplicative linear functional on C ( X )that agrees with q + on d.It follows that the set of states of C ( X )that agree with q + on d has a single extreme point and hence, consists of this extreme point q + . Thus q + is multiplicative on C ( X ) . The foregoing is predicated on our showing that q is multiplicative on d, which we proceed to do. With 0
+
+
q+(n
q+(n
+
3.4. THE BANACH ALGEBRA C ( X )
22 1
are 0 on d, we have that q = q l = q 2 . Thus q = q : - q ; and 2 = 11r]11 = IIr]:ll Ilq[ll. From the uniqueness of the decomposition of r] as r]+ - q - (see Remark 3.4.12), q: = r]', and a-'r]+(fg)= yl+(g) for all g in C(X). Thus q + ( f g ) = a q + ( g )= q+(f)q+(g), when f e d , 0 6 f 6 1 , and g E C ( X ) . It follows at once that yl+ and q - are multiplicative on d.
+
3.4.15. R E M A R K . The preceding theorem is known as the S t o n e Weierstrass theorem. It was stated by M. H. Stone [22: Theorem lo] in the form given and proved (as it usually is) by making use of the special case discovered by K. Weierstrass where Xis a closed bounded interval in R and d is the norm closure of the set of polynomials on X . This special case (the Weierstrass approximation theorem) can be reformulated as the assertion that each continuous function on a closed bounded interval is the uniform (norm) limit of polynomials on that interval. The Stone generalization allows us to conclude, for example, that the same is true for continuous functions and polynomials in n variables on a closed bounded cube in W. Another immediate application includes the fact that (complex) linear combinations of the functions z -,z" (n = 0, f 1 , f 2, . , .) on the circle C1with center 0 and radius 1 in C are norm dense in C(Cl) (since they form an algebra separating points, containing the constants and closed under complex conjugation). As uniform approximation implies L2 approximation for finite measure spaces, and the continuous functions are dense in L2 (in the L2 norm), it follows that d is (L2-)densein L2(C,).Withfin L2(C,),let ( U f ) ( s )bef(expis) for s in (- n,n). Then U is a unitary transformation of L 2 ( C 1 )onto L2( - n,n) carrying the function z + z" onto s + exp ins. Thus (complex) linear combinations of the functions {expins: n = 0, f 1, f 2 , . . .} (and, consequently, of 1, coss, sins, cos 2s, sin 2s,. . .) are (L2-)dense in L2( - n,n). An application of the StoneWeierstrass theorem allows us to conclude that C ( X )is norm separable (that is, has a countable (norm) dense subset equivalently, is countably generated as a Banach algebra) when Xis a compact metric space. Note for this last that Xcontains a countable dense subset { pn}of points (obtained by choosing one point in each set of a finite covering of X by balls of radius l/n for each positive integer n). If d is the metric on X and f n ( p )= d ( p , , p ) , then the set { f n } separates points of X(for if { p , . } tends t o p andf,(p) =f.(q) for all n, then { p,.} tends to q, andp = q). Thus { f n } generates a norm-dense (complex) subalgebra of C(X),and the finite complex-rational linear combinations of finite products of functions in {fn} is a (countable) norm-dense subset of this algebra, hence of C ( X ) . Concerning the hypotheses of the StoneWeierstrass theorem, the example of the subalgebra of functions in C ( X ) vanishing at some point p in X underlines the necessity of assuming that the constants are in the subalgebra. This assumption appears, first, at the stage where we decompose r] and conclude that q+ and r ] - are distinct and non-zero.
222
3. BANACH ALGEBRAS
The necessity of assuming something that restricts the class of closed subalgebras containing the constants even further (in our case, the assumption is that d is stable under complex conjugation) is illustrated by taking X to be the closed disk D,of radius I with center 0 in C and d to be the norm closed subalgebra of C(Dl)generated by z, the identity transform on D,,and 1. Then d is the algebra of functions continuous on D 1and holomorphic on the open disk. In particular d does not contain 2 ; although 1 ~d and z alone “separates” points of D1.The assumption that d is stable under complex conjugation appears, at the outset, when we locate a hermitian linear functional annihilating d. H The result that follows is crucial for our study of the spectral theory of selfadjoint (and normal) operators on a Hilbert space (see Theorem 5.2.1). It describes the topological property of X associated with a special (and extraordinary) order property of C(X).When C ( X )is a boundedly complete lattice (see the statement of the theorem following), X will be “extremely disconnected” - the closure of each open set is open (as well as closed). Such an X (with more than one point) is certainly disconnected -even “totally disconnected” (each pair of points can be separated by sets that are both open and closed: clopen sets). What is not apparent, though, is that an extremely disconnected space is “more than” totally disconnected. The canonical example of a totally disconnected (infinite) compact Hausdorff space Xis the countable product of two-point spaces (0 , l } (with the product topology). Points of X can be represented as sequences of zeros and ones. Convergence is coordinatewise. If ( a l ,a z , . . .) is a point of X and
q(al , a z , .. .) = a12-’ + aZ2-’
+ ..
*
,
then q is a continuous mapping of Xonto [ 0 , 1 ] .With r an irrational number in [0, 11, let 0 be q-’((r, 11). If
r = a12-’ + a z 2 - 2 + . . * , then (b,,bz ,. . .)E 0 if and only if a1
=b,,...,ak=bk,
ak+l
for some positive k . Since r is irrational, akis 0 and 1 for an infinite number of k . Thus, with k large and a k + , equal to 0, ( a l , a z ,. . . , a k ,1 , 0 , 0 , .. .) is near ( a l ,a z ,. . .) and lies in 0. Hence ( a , ,a z , . . .) E 0 - , the closure of 0. I f j is large and aj+ = 1, then ( a l ,a z , ... ,a j ,O,O, . . .) is near ( a , , a z , .. .) and is not in 0(because no point of 0 agrees with it on the first j + 1 coordinates). Thus (al, a z , .. .) is not interior to 0 - , and 0- is not open. If no examples of (infinite) extremely disconnected compact Hausdorff spaces are apparent, it is because these spaces, and their associated algebras of continuous functions are primarily a (useful) mathematical artifice rather
3.5. EXERCISES
223
than a naturally occurring geometrical construct. With the aid of Theorem 5.2.1 and Example 5.1.6, we shall have examples of such spaces. For the moment, we content ourselves with showing that Xis extremely disconnected if C ( X ) is a boundedly complete lattice. 3.4.16. THEOREM.Ifeach set of functions in C ( X ) that has an upper bound in C ( X )has a least upper bound in C ( X )(so that C ( X )is a boundedly complete lattice), then each open set in X has an open closure (so that X is extremely disconnecfed). Proof. Let 0 be an open subset of X , 0- its closure, 9 the family of functionsfin C ( X ) such that 0 < f < 1 andf(p’) = 0 ifp’#Lo, andf, the least upper bound of 9 in C ( X ) . Since 1 is an upper bound for 4 fo < 1. If p ~ there 0 is an f in 9 such thatf(p) = 1, so thatf,(p) = 1 for each p in Lo (hence, for eachp in Lo-). Ifp‘#Lo-, there is a g in C ( X ) such that 0 < g < 1 , g(p’) = 0, and g(p) = 1 forp in Lo-. Thus g is an upper bound for 4 andf, < g. It follows thatfo is 1 on 0-and 0 on X U ! - . Asf, is continuous, 0-is open. 3.5. Exercises
3.5.1. In Remark 3.1.3 the algebra 2l1obtained from a normed algebra 2l is defined. Complete the indicated check that 211is a normed algebra, and that 2l1is a Banach algebra when 2l is. 3.5.2. Let 2l be a Banach algebra over [w, and let 21c be the Banach space “complexification” of 2l described in Exercise 1.9.6. Define the product (A,B)(CD , ) to be (AC - ED, A D + BC). Show that:
ml
ml
( 0 ll(A3 B)(C, < 211(A, IKC, D)ll; (ii) 21chas another norm 111 111 relative to which it is a Banach algebra, the identity mapping from (ac,11 11) onto 111 111) is a homeomorphism, and the mapping A + ( A , 0) is an isometric (real) algebraic isomorphism.
(ac,
a € A} be a family of Banach algebras, and let 2l be the set 3.5.3. Let {aa: of functions f on A such that f ( a )E 21a and
suP{llf(a)ll:a~Al(=Ilfll) < a. (i) Show that 2l, provided with the operations of pointwise addition, multiplication, multiplication by scalars, and with the norm described, is a Banach algebra. (It is usually denoted by “CoeA@ ‘$la.’’ Compare it with the I , spaces of Example 1.7.1.) (ii) With N in place of A, show that a,, the set of functionsfin 2l such that l i m n + m ~ ~ f (=n 0) ~is~a (norm-)closed proper two-sided ideal in 2l. (iii) Determine whether or not 210 is a maximal (two-sided) ideal in 2l.
224
3. BANACH ALGEBRAS
3.5.4. With the notation of Exercise 1.9.19: (i) Show that I, and c, provided with pointwise multiplication, are (commutative) Banach algebras. (ii) Show that co is a (norm-)closed proper ideal in 1,. (iii) Describe a multiplicative linear functional on c with co as its null space; and conclude that co is a maximal ideal in c. (iv) Is co a maximal ideal in I,? Explain. Reconsider Exercise 3.5.3(iii). [Hint. Consider “dimension” in combination with Corollary 3.2.5.1 3.5.5. Let P(N) be the space of non-zero multiplicative linear functionals on l,(N, C)(= I,) topologized with the weak* topology. Defineap) to be p ( f ) for p in P( N) and f in I,. (i) Note that P(N) is a compact Hausdorff space; and show that the mapping, f + x is an algebraic isometric isomorphism of I, into C(P(N)). (ii) Show t h a t j = z [Hint. Consider the spectrum of a real-valuedfin I, relative to l,.] (iii) Show that 1 = 1, where 1 denotes the constant function “one” in both I , and C(P(N)), and that 1, is (norm) closed in C(p(N)). Conclude that ,7 = C(P(N)). (The space P(N) is called “the P-compactification of N.”) 3.5.6. With the notation of Exercise 3.5.5: (i) Show that p(N) is extremely disconnected. (ii) Let n’((f) bef(n)for n in N andfin 1, ;denote by N, again, the subset {n’:n~ N} of P(N). Show that N is dense in P(N). (iii) Show that the one-point subset {n#} of P(N) is open in P(N). [Hint. Consider in,where x,(m) is 0 unless n = m, and x,(n) = I.] (iv) Show that = 0 when p€P(N)\N andfeco.
p(n
3.5.7. Let G be a countable (discrete) group and {C,} an ascending sequence of finite subgroups with union G. (Such a group is said to be locally finite.) Let p be an element of (I,(C, @)#), such that p ( ( l , l , . . .}) = 1 ; and define p, to be IC,l-’ CBEGn T:(p),where lCnlis the number of elements in G,, and the bounded linear operator TB, acting on Im(G,C), is given by C~,(f)1(90)=f(s- ‘90). (i) Show that ~ , E ( I , ( G , C ) ~and ) ~ that p,,({l,l, ...}) = 1. (ii) Show that the weak* closure of { p , :n E N} contains an element po such that T:po = po for every g in G, and such that po({ 1,1,. . .}) = 1. (The functional po is called an invariant mean on G.) [Hint. With f in l,(G, C), define a function cp on N by q(n) = p n ( f ) . Fix xo in j(N)\N; and let p o ( f ) be @(Xo).l
3.5. EXERCISES
225
3.5.8. Let A be an element of the Banach algebra 2l; and let 8 be an open subset of C containing sp,A. Show that there is a positive 6 such that sp,B G 0 if B E % and IIA - BII < 6. 3.5.9. Let { A , } be a sequence of invertible elements in a Banach algebra 2l and suppose { A , } has a limit A that commutes with each A , . (i) Show, by example, that A need not be invertible. (ii) Show that A is invertible if {r(A,- l ) } is bounded. [Hint. Use Corollary 3.3.4.1
E,
3.5.10. Given an element A of a Banach algebra 2l and a positive number show that:
(i) if A is singular (that is 0 E sp A ) , there is a positive number a0 such that if B in ‘LI commutes with A and IIA - BII < do, then there is a I in sp B with 1111 E ; (ii) if 1E sp A there is a positive number ha such that if B in 2l commutes with A and IIA - BII < da, then, for some 1‘in sp B, [ I - 1’1 < E ; (iii) there is a positive 6 such that if IIA - BII < 6 and A B = BA, with Bin ‘LI, then spA c S,(B), where S,(B) = { I :11 - 1’1 < E for some I’ in sp B } . 3.5.11. Nilpotent and generalized nilpotent elements are defined following the proof of Theorem 3.3.3 (the spectral radius formula). Use Proposition 3.2.10 to show, without the aid of the spectral radius formula, that: (i) each nilpotent element in a Banach algebra has spectrum consisting of 0 alone; (ii) each generalized nilpotent element in a Banach algebra has spectrum consisting of 0 alone. 3.5.12. Let { y l , y 2 , . .} be an orthonormal basis for the (complex, separable) Hilbert space %. Let A be the linear transformation of S into A? that maps y , onto anynrn), where {a,} is a bounded sequence of complex numbers and n is a one-to-one mapping of { 1,2,. . .} into itself. Determine IlAll. Describe Ak, where k is a positive integer. Determine IIAkll. Determine llAll and llAkll under the additional assumptions that la,( < lam[and x(m) < n(n) when rn < n. (v) Show that A is nilpotent when a, = 0 for all n larger than some k and j < n ( j ) f o r j in { 1,. . . ,k } . (vi) Use Exercises 2.8.22(iv) and 2.8.23 to show that A is compact when {a,} tends to 0. (i) (ii) (iii) (iv)
226
3. BANACH ALGEBRAS
(vii) Show that A is a compact generalized nilpotent operator on 3? with infinite-dimensional range when a, = l/n and n(m) = m + 1. 3.5.13. As in Exercise 3.5.12, let {y,} be an orthonormal basis for the where a, = 2-k and k is the largest Hilbert space X and let Ayn be positive integer such that 2k divides n. (i) Show that n- 1
a l a z . . . azn-l =
n 2-jZn-'-'
(= b n ) ;
j= 1
and deduce that b, < JIA2"-'11. [Hint. Observe that 2' has 2"-j- odd multiples among the numbers 1,2,. ..,2" - 1.) (ii) Show that A is not a generalized nilpotent operator. (iii) Let Aky, be anyn+ except when n is an odd multiple of 2k, in which case let Aky, be 0. Show that each Ak is nilpotent and that ( [ A- Akll+ 0. (iv) Deduce that the norm limit of nilpotent operators need not be generalized nilpotent. What relation does this conclusion have to the "spectral (semi-)continuity" considerations of Exercises 3.5.8 and 3.5.10? 3.5.14. With the notation of Exercise 3.5.12, show that: (i) {a,} tends to 0 if A is compact (compare (vi) of Exercise 3.5.12); (ii) A is a compact generalized nilpotent operator on X whenj < n(j) for a l l j and {a,} tends to 0 (compare (vii) of Exercise 3.5.12). 3.5.15. Does each compact operator have a (non-zero) eigenvector? (Example? Proof? See Exercise 2.8.29(ii).) 3.5.16. Can a non-zero self-adjoint operator be a generalized nilpotent operator? (Example? Proof?) 3.5.17. Let A be a compact operator acting on a Hilbert space 2,I be a non-zero complex number, and {x,} be a sequence of vectors in 3? such that { ( A - II)x,} converges in norm to a vector y in A?. (i) Show that if { Ilx,ll} is unbounded, ( A - IZ)x = 0 for some unit vector x in 3.
(ii) Exercise (iii) is (0). (iv) of A*.
Show that if { IlxJ} is bounded, y is in the range of A - IZ. [Hint. Use 1.9.17(ii).] Show that the range of A - IZ is closed if the null space of A - IZ Show that if 1 E sp A either I is an eigenvalue of A or I i s an eigenvalue
3.5. EXERCISES
Ax
227
3.5.18. Let 2 be a Hilbert space and A be an operator on X such that = x for some unit vector x in X.
(i) Show that A*y = y for some unit vectory in 2 when the range of A is finite dimensional. (ii) Show that if A iscompact, there is a sequence { A , } of operators on X converging to A in norm, each A , having finite-dimensional range, and such that A,x = x for all n. [Hint. Use Exercise 2.8.24(ii).] (iii) Show that if A is compact, A*y = y for some unit vector y in X. (iv) Show that if A is compact and 0 # l ~ s Ap, then A is an eigenvalue of A . (v) Suppose that A x , = lnx, (n = 1,2,. . .), where { I , } is a sequence of distinct non-zero eigenvalues of A and each x, is a unit vector. Show that {x,} is linearly independent and that ( A y , , y,) = A,, where {y,} is the orthonormal sequence obtained from {x,} by the Gram-Schmidt orthogonalization process. (vi) Deduce from (iv) that sp A is either a finite set or can be arranged as a sequence of complex numbers tending to 0 and that each non-zero element in spA is an eigenvalue of A of finite multiplicity when A is compact. 3.5.19. Let A be the operator defined in Exercise 3.5.12. Assume that j < ~ ( jfor ) all j and a, + 0.
(i) Show that A has no non-zero eigenvalues. (ii) Show that A has no eigenvalues if a, # 0 for all n. (iii) Deduce again (see Exercise 3.5.14(ii)),with the aid of Exercise 3.5.18, that A is a (compact) generalized nilpotent operator. 3.5.20. Let 2 be L2(0,1) relative to Lebesgue measure, and let (Kf)(s)be
fi f(t ) dr for each f in X.
(i) Show that K is a Hilbert-Schmidt operator and a compact operator on X. (ii) Show that K has no eigenvalues. (iii) Deduce that K is a compact generalized nilpotent operator. 3.5.21. With the notation of Example 3.2.17, C,“=- m I-“e, is an “eigenvector” for U in a formal sense even when Il # 1. Pursue the argument of that example to see how it fails to establish that each I in C is in sp(U). 3.5.22. Let X be a compact Hausdorff space and d be a subalgebra of C(X). Suppose 11 11’ and 11 1)” are norms on d such that d is complete relative to ( 1 11’ and d has a completion d oin C ( X ) relative to 11 11”. Let llfll be sup{If(x)l:x E X > for f in C(X). (i) Show that
llfll < Ilfll‘ and
llgll
< Ilgll” forfin
a? and g in d o .
228
3. BANACH ALGEBRAS
(ii) Show that d is complete relative to the norm that assigns Ilfll’ + Ilfll” t o f i n d. (iii) Show that 11 1)’ and 11 11” are equivalent norms on d (that is, the identity mapping is a homeomorphism from d relative to 11 11’ to d relative to 11 11”) when d is complete relative to 11 11”. (iv) Without the assumption that d is complete relative to 11 (I”, show that the identity mapping from ( d ,1) 11’) to ( d ,11 11”) is continuous. 3.5.23. Let 93 be a commutative Banach algebra, and let W be the intersection of the maximal ideals of 93.(The ideal W is called the radical of 93, and 9? is said to be semi-simple when 9 = (0).) Let 2l be an arbitrary Banach algebra and cp be a homomorphism of 2l into 93 mapping the identity of 2l onto that of W.Let X be the kernel of cp. (i) Show that cp maps X - , the norm closure of X, into 9.Deduce that X is closed when W = (0). (ii) Show that if W is semi-simple, then 9 is isomorphic with a subalgebra dlof C ( X ) ,where Xis the compact Hausdorff space of non-zero multiplicative linear functionals on @ with the weak* topology. [Hint. Let &) be p(A), when A E Band p ~ X . 1 ‘ (iii) Show that if 93 is semi-simple, then cp is continuous. [Hint.Use the mapping of (ii) and this mapping composed with the quotient mapping ‘u -,2l/X together with Exercise 3.5.22(iv).] 3.5.24. Establish the following inclusions for a pair of commuting elements A and B of an arbitrary Banach algebra 2l: SPQdAB) E sp,(A)sp,(B),
sp,(A
+ B ) c SPdA) + SPdB).
[Hint.Apply Proposition 3.2.10 to a maximal abelian subalgebra of 2l.I 3.5.25. Let A be a normal operator acting on a Hilbert space 2. (i) Show that A + a l i s a normal operator for each scalar a and that A is normal when A is invertible. (ii) Is the sum of two normal operators necessarily normal? (iii) Use Theorem 2.4.2(iv) to show that ra,M,(A)= I(All. 3.5.26. Let A, be a bounded operator on a Hilbert space X, for n in { 1,2,. ..}, and suppose that { llA,ll} is bounded. Let A be C 0 A, and 2 be 1 @ X,. Show that: =J:‘[ 1 SPB(.W,)(A,)I - c ~PB(w(A); 0) (*) (ii) the inclusion (*) becomes equality when each A, is normal.
229
3.5. EXERCISES
3.5.27. Let %,, be an n-dimensional Hilbert space, { e l , .. . , e n } be an orthonormal basis for X,,, N,, be the linear operator on X,, determined by N,el = O a n d N , e j = e j - , i f l < j < n , a n d A , , b e I - N , , . (i) Show that llA,ll < 2 and that spyacx,&4,,)= (1) for each n in {1,2, ...}. (ii) Show that A , is invertible in W ( X , , ) with inverse I + N , + N; + . . . Ni-' ( = B,) and that llB,,ll 2 (iii) Show that 0 E sp,,,,(A), where A = 1 @ A, and X = C 0 X,,, and deduce that the inclusion (*) of Exercise 3.5.26 is strict in the present case.
+
&.
3.5.28. Suppose the singular element A in the Banach algebra 2l is a norm limit of the sequence {A,,} of regular elements in 'u and 'uo is a Banach subalgebra of 'u. Show that: ~ ~ } to 0 0 ; (i) { [ ~ A , , - 'tends (ii) if B, = (An- ' - I)/IlA,,- ' - IJI(when An- # I),then {A&} and {B,,A} tend to 0; (When a sequence such as {B,}, not tending to 0, with {AB,,} and {B,,A} tending to 0, exists, we say that A is a (two-sided) topological divisor of zero.) [Hint. Note that A(A,,-' - I) = ( A - &)(A, ' - I ) - A , I.] (iii) if B is a topological divisor of 0 in 'u, then B has no inverse in 'u; (iv) if B € ' u o and 1 is a boundary point of spplo(B),then 1 ~ s p ~ ( B ) ; (v) if B € ' u o and either one of spmo(B) or spa(@ is real, then SPplO(B)= SPdB).
'
+
3.5.29. Let A be an element of a Banach algebra 'u, and let f be in S ( A ) . Suppose f ( z ) = 0 for each z in sp(A). (i) Show that f ( A ) = 0 if sp(A) is infinite and connected. (ii) Find an example where sp(A) is connected but not infinite and f ( A ) # 0. (iii) Find an example where sp(A) is infinite but not connected and f ( A ) f 0.
3.5.30. Let X be a Hilbert space and 1 be an isolated point of sp A with sp A \ {A} non-empty and A in a(#).Let D be an open disk with center 1 and 0 be an open set disjoint from D containing sp A\{1}. Does the idempotent g ( A ) , obtained from the function g that takes the value 1 on D and 0 on 0,necessarily project onto a subspace of eigenvectors for A corresponding to A? 3.5.31. Let 'ul and 212 be Banach algebras with units Il and 12, respectively. Let cp be a homomorphism or anti-homomorphism (that is, cp(AB) = cp(B)cp(A))of 2I1 into 'u2 such that cp(Il)= I , .
230
3. BANACH ALGEBRAS
(i) Show that spu2(lp(A))E spu,(A) for each A in 211. (ii) Show, by example, that the inclusion of spectra described in (i) can be proper.
3.5.32. Let A! be a closed left ideal in a Banach algebra 2l, and let J1/' be a closed right ideal in 2l. Let 9 and % be the quotient Banach spaces % / A and '+I respectively. /& (i) Show that q ( A ) and $ ( A ) are bounded linear operators on 9 and ?iY, respectively, for each A in 2l, where q(A)(B
+ A) = AB + A,
$(A)(B
+ N)= BA + JK
(ii) Show that cp is a bounded homomorphism and $ is a bounded antihomomorphism of B into a(%) and 2l into a(%), respectively, and that q(l) and $(Z) are the respective identity operators on X and Y.
3.5.33. With Z the additive group of integers andf, g complex-valued functions on Z, letf* g be the function whose domain consists of those integers s for which CtSzf(t)g(s - t ) converges and whose value at s is the sum. (We call f * g the convolution off and 9 . ) Show that: (0 f * g = 9 *f; (ii) iff. fl(Z) and g E I,@)
(where 1 < p ) , then f*g E I,,@)
Ilf*s l l p < llflll
+ h) = a
llsllp
;
and a E C, then f * (a g
(iii) if f~I l ( Z ) , g and h are in I@'), a .f* g +f*h are in I,,@) and
f* (a * g
*
and
*
+ h ) and
f * g +f*h;
(iv) iff, g E I , @ ) and h E Ip(Z), then (f*g ) * h andf* ( g * h) are in I,@)
and
(f*g) * h =f* (9 *h). (v) Conclude that l,(Z), provided with the mappings (f,g ) +f* g and f + Ilflll, is a commutative Banach algebra with unit e, where e(0) = 1 and e ( t ) = 0 if r # 0.
3.5.34. (i) Show that for each z in Ulthe equation (,(t) = z' (t E Z) defines a homomorphism (, of the additive group Z into Tl . (We call such a homomorphism a character of Z.) (ii) Show that the set f of characters of Z provided with the multiplication (5 * ( ' ) ( t ) = ( ( t ) ('(t) for t in Z is a group. (We call f the dual group of Z.) (iii) Show that the mapping z + 5, is an isomorphism of Ul onto f.
23 1
3.5. EXERCISES
3.5.35. Using the notation of Exercises 3.5.33 and 3.5.34, let t (= tz)be a character of E. Show that: (i) the formula (*)
Pz(f)
=
Cf(t)tz(t) (=Rz>>
( f Eh ( Z ) )
tsz
defines a non-zero multiplicative linear functional on l1 (Z) ; (ii) each such functional on Il(Z) corresponds to a (unique) point z in Ul by means of (*); (iii) the functionfdefined in (*) is continuous on Ul,and the mapping z 4 pz of Ulonto the set A ( Z ) of non-zero multiplicative linear functionals on I,@) is a homeomorphism of Ul onto A ( Z ) with its weak* topology.
3.5.36. Let q(s) be exp is for s in [ - n, n), and let m(S) be the Lebesgue measure of tj-l(S) divided by 2n when S is a subset of Ul such that f1(S) is Lebesgue measurable. (i) With the notation of Exercise 3.5.35, show that the mappingf-fof
Il(Z) into C(Ul) is linear and satisfies
(ii) Show that the mapping f-fof tion of I2(E) onto L2(Ul,m).
(i) extends to a unitary transforma-
[Compare the results of (i) and (ii) with the Plancherel theorem (3.2.31).]
3.5.37. With the measure m on Ulintroduced in Exercise 3.5.36, define the convolution of m-measurable functionsfand g on Ulas the functionf* g whose domain consists of those w in Ulfor which the integral f(z)g(z- w)dm(z) converges and whose value at w is this integral. Show that (ib(v) of Proposition 3.2.22 hold for this convolution (where Ul replaces R in those assertions).
SB1
3.5.38. With t in Z and z in Ul,let tr(z) be z'. (i) Show that tris acontinuous homomorphism of Ulinto Ul(a character of Ul)and that each such homomorphism has the form t, for some tin Z. [Hint. Recall the form of the characters of R.] (ii) Show that the set T1of characters of Ul provided with pointwise multiplication is a group and that the mapping t + tris an isomorphism of Z onto T,.
232
3. BANACH ALGEBRAS
3.5.39. Using the notation of Exercises 3.5.37 and 3.5.38, let pf be given by the formula
pf(A= J f(z)tt(z)dm(z)( = ~ t ) )
(*)
(fE~1~1~m)).
TI
(i) Show that pf is a non-zero multiplicative linear functional on Ll(Ul, rn) for each t in Z. (ii) Show that each non-zero multiplicative linear functional on Ll(Ul,m) is pf for some tin Z. [Hint. Make use of the fact that (5, : r E Z}spans a dense linear suspace of Ll(Ul, rn) and 5, * 5, = 0 when r # s to give a shorter argument than the one patterned on the proof of Theorem 3.2.26.1 (iii) Show that the set A(U1) of non-zero multiplicative linear functionals on Ll(Ul ,m)is not weak* compact. Deduce that Ll(Ul,m) does not have an identity element. [Hint. Compute p,(t-J for r and t in Z.] (iv) Let d o ( U l ) be the Banach algebra obtained from Ll(Ul,m) by adjoining an identity Z (as in Remark 3.1.3). Let pm be the (non-zero) multiplicative linear functional on d o ( U l ) that assigns 1 to Zand 0 to eachfin Ll(Ul, m).Denote by pi the (unique) non-zero multiplicative linear functional on d o ( U l ) extending p f . Show that { p m ,p i : t e Z } (= d O ( U l ) ) is the set of all non-zero multiplicative linear functionals on d o ( U l ) and that the mapping A’ of d O ( U l )onto the one-point compactification {Z, oo} of Z that assigns 00 to pm and t to p: is a homeomorphism of Ao(U1) with its weak* topology onto
{Z,0 0 ) . (v) Show that lim
Id+
IT(r>I= 0
(fc L ~ ( um)). ~~
m
3.5.40. Let Sf be f (as defined in Exercise 3.5.39) for f in L2(U1, m ) ( G Ll(Ul, m)),and let T be the unitary transformation of Iz(Z)onto L2(Ul,rn)
described in Exercise 3.5.36(ii). (i) Show that S is a unitary transformation of Lz(Ul,m) onto 12(Z). (ii) Let U be the (self-adjoint) unitary operator on Lz(Ul,rn) that maps 5, onto for each t in Z. Show that T W f ) =f
(YE L2(U19 m))
sum)= 9
(9EW)).
and (iii) Deduce that T* = T-’
and
= S(J
3.5. EXERCISES
233
3.5.41. Let L,(R) denote the Banach algebra formed by providing Ll(R) with convolution multiplication. Withfin L,(R), denote byfi the “translate” off by the real number r (fi(t) =At - r)). Let 9 be a norm-closed linear subspace of Ll(R). (i) Show that if 9 is an ideal in L,(R), then 9 is invariant under translations (that is,f;E 9 whenfE9). [Hint. Use the approximate identity of Lemma 3.2.24 and (5) of Section 3.2.1 (ii) Show that if 9 is invariant under translations, it is a (norm-closed) ideal in Ll(R). [Hint. Use the Hahn-Banach theorem to support the “view” of y *fas Sg(t)f;dt and recall the identification of the dual of L1.] (iii) Show that if the span of the translates of a functionfin L,(R) is norm dense in L,(R),fvanishes nowhere on R. (With its converse, this is one of the main theorems in a body of work known as “Wiener’s Tauberian theorems.”) 3.5.42. Let Ll(Ul ,m)be the Banach algebra described in Exercise 3.5.37. denote byfw the “translate” offby w ( f w ( z )=f(zW)). Let Withfin Ll(Ul ,m), 9 be a norm-closed linear subspace of L1(T1,m). (i) Show that the sequence {u,} satisfies
as n + 00, where u, = 2n(u, tj- l ) , {u,} is the “approximate identity” described in Lemma 3.2.24 and tj is defined by q(s) = exp is. (ii) Show that if 9 is an ideal in Ll(Ul, m),then 9 is invariant under translation. (iii) Show that if 9 is invariant under translation, it is a (norm-closed) ideal in Ll(Ul ,m). (iv) Show that if the span of the translates of a functionfin L1(T1,m)is norm dense in Ll(Ul,m), thenf((t) # 0 for each t in Z. 0
3.5.43. Let 9I be a Banach algebra and A be an element of ‘2I such that sp,(A) E @ \ R - , where R - = { z : z E @ z, = - lzl}. (i) Show that there is an element A . in ‘2I such that (Ao)’ = A . (The element A . is said to be a square root of A in ’u.) (ii) Deduce that if B E 9I and 111 - BII < 1, then B has a square root in a. 3.5.44. Let A be an element of the Banach algebra 9I. (i) Show that if sp,(A) E {A: A E R, 0 < i}, then A has a squareroot in ’u with positive spectrum. (ii) If the hypothesis of (i) is weakened to
sp,(A) E {A:AER, 0 < A} (= R+),
234
3. BANACH ALGEBRAS
does A still have a positive square root in %? [Hint. Consider
in the Banach algebra of complex 2 x 2 matrices.] (iii) Show that if sp,(A) E R’ and IlBll = r,(B) for each Bin the Banach subalgebra 910 of 9l generated by A and I, then A has a square root in 910 with spectrum (relative to 910) in R + .
[Hint.Study the square roots of A applied to 9t0.]
+ n- ‘ I as n + co and use Remark 3.2.11
3.5.45. Let p be a hermitian functional on C(X),where X is a compact Hausdorff space. Suppose llpll = p(1). (i) Use Proposition 3.4.1 1 to show that p is a positive linear functional on C ( X ) . (ii) Show that p is a positive linear functional on C ( X ) without using Proposition 3.4.1 1. 3.5.46. Let Xbe acompact Hausdorff space and d be aclosed subalgebra of C(x> containing the constant functions and containing f when f~d. (i) Show that iffis a real-valued function in d,I € spd(f), and I is not real, then there is a real-valued function g in d such that iEspd(g). ( - ig)”/n! (= exp( - ig)) is an element (ii) With g as in (i), show that of d of norm 1. (iii) Use a multiplicative linear functional on d to show that e E sp&dexp( - i d l ’ (iv) Conclude that sp,(f) s R for each real-valued function f i n d. (v) Conclude that spd(f) E R, again, with the aid of Exercise 3.5.28(v). 3.5.47. Let C ( X ) and d be as in Exercise 3.5.46 and let the partial ordering of the (real) algebra d ,of real-valued functions in d be that induced by the partial ordering of C(X, R). (i) Show that each (non-zero) multiplicative linear functional p on d has an extension p’ to C(X)such that p’ is a state of C(X). (ii) Show that p is a pure state of d. (iii) Show that the set d of state extensions of p to C(X) is convex and weak* compact. (iv) Show that each extreme point of 6 is a pure state of C(X).
235
3.5. EXERCISES
(v) Conclude that p has an extension to C ( X ) that is a (non-zero) multiplicative linear functional on C ( X ) .
3.5.48. Let C ( X ) and d be as in Exercise 3.5.46. (i) Show that sp,(fl = sp,,,,(fl for eachfin d. (ii) Use Exercise 3.5.44to show that iffis a positive function in d,then the (unique) positive square root off in C ( X ) lies in d. (iii) Conclude that If1 ~d when f ~ d .
3.5.49. Let X be a compact Hausdorff space, and let 9 be a subset of C(X, R) such that, withfand g in 9,Y containsf v g andf
A
g (in this case,
9is said to be a sublattice of C(X, R)). Suppose that for each pair r, s of real numbers and each pairp, q of distinct elements of Xthere is anfin 9such that f ( p ) = r andf(q) = s. Show that 9 is norm dense in C(X, R). 3.5.50. Combine the results of Exercises 3.5.48and 3.5.49to give another proof of the StoneWeierstrass theorem (3.4.14).
CHAPTER 4 ELEMENTARY C*-ALGEBRA THEORY
In this chapter we study a special class of Banach algebras, termed C*algebras, the ones that have an involution with properties parallel to those of the adjoint operation on Hilbert space operators. With X a compact Hausdorff space and X a Hilbert space, C(X) and B ( 2 )are examples of C*-algebras, and so is each norm-closed subalgebra of B ( S )that contains the adjoint of each of its members. Two basic representation theorems (4.4.3 and 4.5.6) assert that, up to isomorphism, these are the only examples; every C*-algebra can be viewed as a normed-closed self-adjoint subalgebra of .93(2),for an appropriate choice of S, and every abelian C*-algebra is isomorphic to one of the form C(X). Earlier sections of the chapter are devoted to studying the spectral theoretic properties of certain special elements in C*-algebras and the order structure in such algebras and in their Banach dual spaces. These are basic tools, both for the proofs of the representation theorems just cited, and also for all the subsequent theory. 4.1. Basics
By an involution on a complex Banach algebra 'u, we mean a mapping
A
-,A*, from 2I into 'u, such that (i) (aS + bT)* = dS* (ii) (ST)* = T*S*, (5) (T*)* = T,
+ 6T*,
whenever S, T E'u and a, b E C and a, bdenote the conjugate complex numbers. A C*-algebra is a complex Banach algebra (with a unit element I ) with an involution that satisfies the additional condition (iv) IIT*TII = llTl12
(Tea).
This last condition ensures that the involution in a C*-algebra preserves norm (and is therefore continuous); for
IITIIZ= IlT*TII llT*11llTll7 236
4.1. BASICS
237
whence IlTll < IIT*ll, and we obtain the reverse inequality upon replacing T by T*. We have already encountered several examples of C*-algebras. If S is a Hilbert space, a(%)is a C*-algebra, with the adjoint operation as its involution; indeed, the defining conditions (i)-(iv) above are abstracted from the properties of adjoints of Hilbert space operators, as set out in Theorem 2.4.2 and the discussion that follows it. Several Banach algebras of complexvalued functions are C*-algebras, with an involution that assigns to an element f the conjugate complex function f,defined by Ax) =f(x). In this way, the Banach algebra C ( X ) , of all continuous functions on a compact Hausdorff space A’, becomes a C*-algebra. The same applies to the Banach algebra l,(X) of all bounded functions on an arbitrary set X (with pointwise algebraic operations and supremum norm), and to the Banach algebra L,(S, g m ) of all essentially bounded measurable functions (with pointwise algebraic operations and essential supremum norm), associated with a measure space (S,g m ) . We now introduce some terminology, concerning elements of a Banach algebra % with involution, and note certain immediate consequences of conditions (ib(iv) above. Motivated by the example of the algebra g ( Z ) ,we refer to A* as the adjoint of A (EB), and describe A as self-adjoint if A = A*, normal if A commutes with A*, unitary if A*A = AA* = I. With S = I* and T = I , it follows from (ii) and (iii) that I* = I ; so the unit element Iis both selfadjoint and unitary. The set of all self-adjoint elements of % is a real vector space, while the unitary elements form a multiplicative group, the unitary group of %. Each A in % can be expressed (uniquely) in the form H + iK, where H (= $(A + A * ) ) and K (= $i(A* - A ) ) are self-adjoint elements of %, the “real” and “imaginary” parts of A ;moreover, A is normal if and only if H a n d K commute. From (ii), A is invertible if and only if A* is invertible, and then ( A - ’)* = ( A * ) - ’ . By applying this result, withal - A and its adjoint iil - A* in place of A and A*, it follows that the spectra of A and A* satisfy sp(A*) = { a : U€SP(A)}. Accordingly, these elements have the same spectral radius, r(A*) = r(A). If ‘$ and I93 are Banach algebras with involutions, a mapping cp from B into is described as a * homomorphism if it is a homomorphism (that is, it is linear, multiplicative, and carries the unit of B onto that of 98)with the additional property that cp(A*) = cp(A)* for each A in a.If, further, cp is one-to-one, it is described as a * isomorphism. Although we impose no continuity condition in these definitions, we shall see later (Theorem 4.1.8) that * homomorphisms do not increase norm and * isomorphisms are norm preserving, when B and 9?are C*-algebras. If is a Banach algebra with involution, a subset 9of % is said to be selfadjoint if it contains the adjoint of each of its members. A self-adjoint subalgebra of % is termed a * subalgebra. If the involution is continuous (in
238
4. ELEMENTARY C*-ALGEBRA THEORY
particular, if 2€ is a C*-algebra), the closure of a * subalgebra is again a * subalgebra. It is clear that a closed * subalgebra 28 of '$ that Icontains the unit of CLI is itself a Banach algebra with involution ;if, further, 2l is a C*-algebra, then so is a. In this last case, we describe 9 as a C*-subalgebra of 2€. The proposition that follows extends, to appropriate elements of a C*algebra, the information concerning Hilbert space operators contained in Theorem 3.2.14, together with Proposition 3.2.15 and the comments following it. Suppose that A is an element of a C*-algebra 2€. 4.1.1. PROPOSITION.
(i) I f A is normal, r(A) = IlAll. (ii) If A is self-adjoint, sp(A) is a compact subset of the real line R,and contains at least one of the two real numbers f 11A11. (iii) I f A is unitary, IlAll = 1 andsp(A) is a compact subset of the unit circle { a E C : la1 = I}. Proof. (i) With H self-adjoint in 2l and n a positive integer, llH2"11= ll(H")*H"ll = llH"112.By induction on m, llHqll = llH114 when q has the form 2* (m = 1,2, . . .); so, by Theorem 3.3.3,
r ( H ) = lim
llHq11"q
= llHll.
4-
With A normal and H the self-adjoint element A*A, it follows from the preceding argument, together with Corollary 3.3.4 and the C* property of the norm, that llAllZ= IIA*AII = @*A)
< r(A*)r(A)= r(A)'
6 IIAl12;
so r ( A ) = IlAll. (ii) With A self-adjoint in 2l, sp(A) is compact (Theorem 3.2.3) and so contains a scalar with absolute value r(A);and r(A) = 114,11, from part (i) of the present proposition. Consequently, if suffices to prove that sp(A) G 02. For this, suppose that c ~ s p ( A ) ,where c = a + ib. For each integer n, let B,, = A - a I + inbl, and observe that i(n
+ 1)b
=a
+ ib - a + inb E sp(B,,).
= li(n
+ l)b12 < [r(B,,)]' < 11B,,112
Accordingly
+ + 1)b'
(nZ 2n
= IIB,*Bnll= " ( A - a l - inbI)(A - a l + inbI)ll = ll(A - a1)'
+ n2b2111< 11-4 - alllZ + n2bz.
Thus (2n + l)b2< IIA - aIllZ(n= 1,2,. . .); so b = 0 , and c = ~ E R .
239
4.1. BASICS
(iii) With A unitary in a,it results from the C* property of the norm that IlAll' = llA*All = 1 14 = 1,
so llAll = 1. With a in sp(A), we have a-'Esp(A-')
= sp(A*);
hence la1 < IlAll = 1,
1aI-l
< IIA*II = I ,
and thus Jal= 1. 4.1.2. COROLLARY. I f A isa normalelement of a C*-algebra a,andAk = 0 for some positive integer k , then A = 0. llAll
Proof. Since A" = 0 when n 2 k , it results from Proposition 4.1 .I(i) that = r(A) = lim (IA"((""= 0.
Suppose that A is a self-adjoint element of a C*-algebra a,and denote by C(sp(A)) the C*-algebra of all continuous complex-valued functions on the spectrum sp(A). We now introduce the (continuous) function calculus for A , a mapping that associates with each f in C(sp(A)) an element f ( A ) of a.The existence and properties of this mapping are the subject of Theorems 4.1.3, 4.1.6, and 4.1.8(ii) and Propositions 4.1.4 and 4.2.3(i) below. At a later stage (Theorem 4.4.5) we shall construct a similar function calculus for a normal element of a C*-algebra. For self-adjoint elements, the two methods lead to the same function calculus; Remark 4.4.6, Theorem 4.4.8, and Example 4.4.9 provide information that, even in the self-adjoint case, is not contained in the other results just cited. 4.1.3. THEOREM.I f A is a self-adjoint element of a C*-algebra a,there is a unique continuous mapping f -,f ( A ): C(sp(A)) -, such that (i) f ( A ) has its elementary meaning when f is a polynomial. Moreover, when f,g E C(sp(A)) and a, b E @, (ii) Ilf(A)II = llfll; (iii) (af bg)(A) = a f ( A ) bg(A); (iv) ( f g ) ( A )= f (A)g(A); (v) f(A) = [ f ( A ) ] * ,wherefdenotes the conjugate complex function; in particular, f ( A )is self-adjoint ifand only i f f takes real values throughout sp(A); (vi) f ( A ) is normal; (vii) f ( A ) B = Bf(A) whenever B E % and A B = BA.
+
+
Proof. By Proposition 4.1.1(ii), sp(A) is a compact subset of the real line. The Weierstrass approximation theorem shows that the set P of all
240
4. ELEMENTARY C'-ALGEBRA THEORY
polynomials with complex coefficients, considered as a subset of C(sp(A)), is everywhere dense. If p is such a polynomial, say p ( t ) = a,
+ a,t + a2t2+ . . + ant", *
then p ( A ) = a , l + a,A [ p ( A ) ] * = rioz
+ a2A2+ . . . + a , # ' ,
+ &A +
&A2
+ . + ii,A". * '
Hencep(A) and [ p ( A ) ] * commute; that is, p ( A ) is normal. From the spectral mapping theorem for polynomials (see Proposition 3.2. lo), together with Proposition 4.1. I(i),
IIP(A)ll = r ( p ( 4 ) = max{lsl:sEsP(P(A))) = max{Ip(t)l: tESP(41 = IIPII
(the norm o f p as an element of C(sp(A))). If two distinct polynomialsp and q are identically equal on sp(A), we can replacep byp - q in the above argument, and deduce thatp(A) = q ( A ) ;of course, this question arises only when sp(A) is finite. The preceding argument shows that the linear mappingp + p ( A ) : P + % is well defined and continuous (in fact, isometric). Since % is complete and P is everywhere dense in C(sp(A)), there is a unique extension to a continuous mapping f +f(A) : C(sp(A)) -+ 2I. We have now proved the existence of a unique continuous mapping f +f(A) satisfying condition (i) in the theorem. In view of the above argument, each of the remaining properties (ii)-(vii) is easily verified when f and g are polynomials, and by continuity remains valid for all f and g in C(sp(A)). H Clauses (i)-(v) of Theorem 4.1.3 amount to the assertion that the function calculusf+f(A): C(sp(A)) + 2I is an isometric * isomorphism that carries the identity mapping on sp(A) to the self-adjoint element A of %. Since C(sp(A)) is a complete metric space, the same is true of its image {f(A) :fE C(sp(A))j in N, so this set is an abelian C*-subalgebra %(A) of %, containing I and A. Since polynomials form an everywhere-dense subset of C(sp(A)), each element of %(A) is the limit of a sequence of polynomials in A. A closed subalgebra W of % that contains I and A necessarily contains all polynomials in A and therefore contains %(A). We have now proved the following result. 4.1.4. PROPOSITION. ZfA is a selfadjoint element of a C*-algebra 2I, the set { f( A ) :fE C(sp(A))} is an abelian C*-subalgebra%(A) of 2I, and is the smallest closed subalgebra of 'ill that contains Iand A . Each element of U ( A )is the limit of' a sequence of polynomials in A .
4.1. BASICS
24 1
Suppose that 2I is a complex Banach algebra, W is a closed subalgebra that contains the unit I of %, and BE99.If a E spa(B), then a l - B has no inverse in 2I; accordingly, it has no inverse in a,so aEspg(B). Hence spa(B) s spa(@, and Example 3.2.19 shows that strict inclusion can occur. By use of the function calculus described in Theorem 4.1.3, we now show that the two spectra coincide when 2I and $3 are C*-algebras. 4.1.5. PROPOSITION. r f % is a C*-algebra, W is a C*-subalgebra of 2I, and Bc.99, then spa(B) = spa(B). Proof. As noted above, spa(B) c spa(@. In order to establish the reverse inclusion, it suffices to prove the following result : if A E W,and A has an inverse A - ' in 2I, then A - ' E ~ . We consider first the case in which A is self-adjoint. Since O#spa(A), the equationf(t) = t - defines a continuous function on spa@). By means of the function calculus for A relative to a, we obtain an element f(A) of 2I, and deduce from Proposition 4.1.4 that f ( A ) e W . Since t f ( t ) = 1 for each t in spa(A), it follows from Theorem 4.1.3(i) and (iv) that Af(A) = I; so A-' = f ( A ) E B . Consider next a (not necessarily self-adjoint) element A of W that has an inverse Cin 2I. Then A* lies in W and has inverse C* in 9. Since A*A is a selfadjoint element of W,with inverse CC* in %, it follows from the preceding paragraph that CC* E B. Accordingly, A - = C = (CC*)A* E 3. In the circumstances considered in Proposition 4.1.5, we can now omit the suffices 2I and W,and denote by sp(B) the spectrum of B relative to either algebra. From the preceding proposition, if A is an invertible self-adjoint element of a C*-algebra a, then A - ' is the norm limit ofp,(A), with eachp, a polynomial (for this, in the proposition take for W the C*-subalgebra generated by A and I).This can be strengthened as follows. If A is self-adjoint and has inverse A - ' in 'u, then there is a sequence {p,} of polynomials without constant terms such that IIpn(A)- A - ' " -,0. To see this, extend t - ' on sp(A) to a continuous function f on an interval containing sp(A) and 0, so that f ( 0 ) = 0. From the Weierstrass approximation theorem, f is the norm (uniform) limit of polynomials q, on this interval. Of course, q,(O) +f(O) = 0. Thus the polynomials pn obtained from qnby omitting the constant terms (q"(0))tend tofin norm on this interval (hence on sp(A)). From (i) and (ii) of Theorem 4.1.3, IIp,(A) -f(A)II -,0, and from (i) and (iv) of that theorem,f(A) = A - ' . Our next result is another spectral mapping theorem (compare Theorem 3.3.6).
'
'
4.1.6. THEOREM.If A is a self-adjoint element of a C*-algebra 2I, and f E C(sp(A)), then SP(f(4) = { f ( t ) :tESP(4).
242
4. ELEMENTARY C*-ALGEBRA THEORY
Proof. In view of Propositions 4.1.4 and 4.1.5, we can interpret sp(f (A)) as the spectrum of f ( A ) relative to the C*-subalgebra %(A) = M A ) :9 E C(SP(A))}
of 9l. Since the function calculus g + g ( A ) : C(sp(A)) --f 9l is a * isomorphism from C(sp(A)) onto %(A), it follows that sp(f (A)) coincides with the spectrum off as an element of C(sp(A)); that is, by Remark 3.4.4, SP(f(4) = I f
(0:t E SP(A)}.
We conclude this section with some further applications of function calculus. 4.1.7. THEOREM.Each element A of a C*-algebra 2l is a finite linear combination of unitary elements of 9l.
Proof. It is sufficient to consider the case in which A is self-adjoint and IlAll < 1. In these circumstances, sp(A) is a subset of the interval [ - 1,1], and Since we can define f in C(sp(A)) by f (t) = t id=.
+
trf(0 +m1,
mf
f(t>T(t)= (0 = 1, for each t in sp(A), it follows that the element f(A) (= U ) of 9l satisfies t=
A=t(U+U*),
UU*=U*U=Z.
H
Suppose that 9l is a C*-algebra, A = A* ~ 9 l and , f is a continuous complex-valued function whose domain includes sp(A). We denote byf (A) the element of % that, in the function calculus for A, corresponds to the restriction flsp(A). This convention is used in Theorem 4.1.8(ii), where we refer tof (cp(A)) although the domain off may be strictly larger than sp(cp(A)). 4.1.8. THEOREM.Suppose that 9l and 9 are (?-algebras homomorphism from 9l into a.
and cp is a
*
(i) For each A in 9l, sp(cp(A)) E sp(A) and llcp(A)ll < IlAll; inparticular, cp is continuous. (ii) If A is a selfladjoint element of 9l a n d f e C(sp(A)), then cp( f ( A ) )= f (cp(A))* (iii) Zfcp is a * isomorphism, then llcp(A)ll = IlAll and sp(cp(A)) = sp(A)for each A in 9l, and cp(%) is a C*-subalgebra of 93.
Proof. (i) The unit elements of 9l and 9will both be denoted by Z,since the context in each case indicates which one is intended. With A in 9l, we prove first that sp(cp(A)) E sp(A). For this, if a $ sp(A), a1 - A has an inverse S i n %; since cp(Z) = Z, aZ- q ( A ) has inverse cp(S) in 9,so a$sp(cp(A)); hence SP(cp(4) E SPU).
243
4.1. BASICS
With A in 2l, it results from Proposition 4.1.1(i) that (IA1I2= IIA*AII
(1)
llcp(41I2
= r(A*A),
= Ilrp(A)*cp(A)II = lIcp(A*A)lI = r(cp(A*A))*
Since sp(cp(A*A)) E sp(A*A), we have r(cp(A*A))< r(A*A), and therefore IIcp(A)II < IlAll. (ii) If { p n }is a sequence of polynomials tending to f uniformly on sp(A) (hence, from (i), on s ~ ( d A ) ) ) ,then d P n ( A ) ) d f( A ) ) and p n ( d A ) ) f(cp(A)).Since cp(p,(A)) = p,(cp(A)) for each n (because cp is a homomorphism), (ii) follows. (iii) Suppose now that cp is a * isomorphism. With B self-adjoint in %, it follows from (i) that sp(cp(B)) G sp(B). If strict inclusion occurs, there is a nonzero element f of C(sp(B)), whose restriction to sp(cp(B)) is identically zero. From part (ii) of the theorem, we have +
f (4f 09
cp(f
+
(4) = f (cp(B))= 0,
contrary to the assumption that cp is one-to-one. Hence sp(cp(B)) = sp(B), and so r(cp(B))= r(B), for each self-adjoint B in 2l. With A in 2l and B = A*A, it follows from the preceding paragraph and (1) that IIAIIZ = r(A*A) = r(cp(A*A))= llcp(A)1I29
Ilcp(4II = IlAll.
Since 2l is a complete metric space and cp: % + W is an isometry, the * subalgebra cp(2l) of W is closed, contains I, and is therefore a C*-subalgebra of W. By Proposition 4.1.5, the spectrum of cp(A) in W is the same as its spectrum in cp(%); and sp(A) = spV($)(cp(A)),since cp is an isomorphism from 2l onto
cp(W The following result strengthens the final conclusion of Theorem 4.1.8(iii). 4.1.9. THEOREM.If% and W are C*-algebras and cp is a * homomorphism from 2l into W,then cp(2l) is a C*-subalgebra of W. Proof. Since cp(2l) is a * subalgebra of W (containing I),it suffices to show that cp(2l) is closed in W. Accordingly we must prove that, if B E B and IIB - cp(A,)II + 0 for some sequence { A n }of elements of 2l, then B~cp(%). By expressing B, A ,A 2 , . . . in terms of their real and imaginary parts, we reduce to the case in which B and the An's are self-adjoint. Upon passing to a subsequence of { A n } ,we may suppose also that llq(An+l)
- cp(An)II < 2-"
(n = 1,2,**.).
Let fn be a continuous function on R, with values in the real interval [-2-",2-"], such thatf,(t) = t when It1 < 2-". From Theorem 4.1.8(ii), and
244
4. ELEMENTARY C*-ALGEBRA THEORY
sincef, restricts to the identity mapping on sp(cp(A,+,) - &I,, we )) have , d A n t 1)
- d A n ) =fn(q(An+
1
- An)) = dfn(An+ 1 - An)).
+
SinceIlfn(An+l- An)ll < 2-", theseriesA, C,"=lfn(An+l - AJconverges to an element A of a; and by continuity of cp (Theorem 4.1.8(i)),
{
m- 1
c lim { d A d + c
c p ( ~ ) = lim
m- m
V ( A1)
+
p(fn(An+1 - An))}
n= 1
m- 1
=
m-r m
Ccp(An+l)
- r(A.)l}
n= 1
= lim cp(A,) = B. m-
00
Thus BE(P(%). H At later stages (Corollary 4.2.10 and Theorem 10.1.7) we shall show that a closed two-sided ideal X in a C*-algebra is automatically self-adjoint, and that the quotient algebra % / Xis a C*-algebra. With the aid of the latter result, Theorem 4.1.9 becomes a simple consequence of Theorem 4.1.8(iii) (see the proof of Corollary 10.1.8).
4.2. Order structure In Section 2.4 we introduced a concept of positivity for operators acting on a Hilbert space, and a partial order relation on the set of self-adjoint operators. In this section we study the corresponding notions for elements of a C*algebra. We recall that a bounded linear operator A, acting on a Hilbert space X, is said to be positive if (Ax, x) 2 0 for each x in X. By Proposition 2.4.6(i) and Theorem 3.2.l4(ii), a positive operator A is self-adjoint, and its spectrum is a subset of the non-negative half-line R + = { Z ER: t 2 0). Conversely, if A is a self-adjointelement o f g ( X ) , andsp(A) E R+, theequationf(2) = t l ' Z defines a real-valued continuous function f on sp(A). By means of the function calculus, for A as a member of the C*-algebra B ( X ) ,we obtain a self-adjoint operator H ( = f ( A ) ) such that H Z= A. Since ( A x , x ) = ( H x ,H x ) 2 0
(X€X),
A is a positive operator. Accordingly, the positive operators are precisely those that are self-adjoint and have spectrum contained in R+. Motivated by these considerations, we describe an element A of a C*algebra 2l aspositioe if A is self-adjoint and sp(A) E R+ ;we denote by at the set of all positive elements of a. From the preceding discussion, this definition
245
4.2. ORDER STRUCTURE
is consistent with our earlier conventions when 'u = 9-3(%). If 9-3 is a C*subalgebra of 'u, a self-adjoint element B of 9-3 is positive relative to 9-3 if and only if it is positive relative to 'u (that is, 9?+ = 9n a+),since it has the same spectrum in 9-3 as in 2l. If cp is a * homomorphism from 'u into a C*-algebra V and A €a+,then c p ( A ) ~ % ? +for ; cp(A) is self-adjoint, and sp(cp(A)) c sp(A) G IW', by Theorem 4.1.8(i). From Proposition 4.1.1(ii), IIAIIES~(A> when A€%+. With X a compact Hausdorff space, andfin the C*-algebra C(X),fis selfadjoint if and only iffis real valued throughout X ; moreover, sp(s) = { f ( x ) : x € X } . Accordingly,fis positive (in the C*-algebra sense just defined) if and only if f ( x ) 3 0 for each x in X . 4.2.1. LEMMA. I f A is a self-adjoint element of a C*-algebra 'u, a E R, and a 2 llAll, then A € % + ifand only ifllA - all( < a. Proof. Since sp(A) c [ - a, a ] , and
IIA - all( = r(A - a l ) = sup It - a1 = sup ( a - t), fesp(A)
tesp(A1
it is apparent that IIA - all1 6 a if and only if sp(A) E R+. Clauses (ii), (iii), and (v) of the following theorem tell us that 'u+ is a (positive) cone (in the sense explained in the discussion preceding Definition 3 . 4 3 , in the real vector space of all self-adjoint elements of %. 4.2.2.
(i) (ii) (iii) (iv) (v)
THEOREM.
Suppose that 'u is a C*-algebra.
2l+ is closed in 'u. aAE'u+ $ A € % + and a E R + . A + B E % +i f A , B ~ ' u + . ABg'u' i f A , B ~ ' u +and A B = BA. I f A ~ 2 l +and - A € % + , then A = 0.
Proof. (i) From Lemma 4.2.1, 'u+ = { A € ' u : A = A* and [ [ A- llAlllll G 11A11},
whence '$isI+ closed (since the norm is continuous on 2l). (ii) If A E 'u+ and aE R+, then aA is self-adjoint, and sp(aA) = {at:tEsp(A)} G R+. (iii) If A , B E % ' , it results from Lemma 4.2.1 that IIA - llAll4l G llAll9
IIB - IlBlllll G
IlBll.
246
4. ELEMENTARY C'-ALGEBRA THEORY
Thus IIA
+ B - (IlAll + IIBll>Zll < ll4l + 11J41;
and from the same lemma (with a = IlAII + ( ( B (2( ((A + Bll), it follows that A +BE%+. (iv) With A, Bcommuting self-adjoint elements of %+, AB is self-adjoint since (AB)* = BA = AB. Since each of A, B, AB has the same spectrum in 2l as in the commutative C*-subalgebra generated by { I , A, B ) , it follows from Proposition 3.2.10 that sp(AB) E (st:s~sp(A),t~sp(B)} E R+. =
(v) If A, - A € % + , then A is self-adjoint and sp(A) E R+ n - R t ( 0 ) ;so JJAll= r(A) = 0.
4.2.3. PROPOSITION. Suppose that A is a self-adjoint element of a C*algebra %, and fE C(sp(A)).
(i) f(A) E %+ if and only i f f ( 1 ) 2 0 for each t in sp(A). (ii) ((AllZf A E %+. (iii) A can be expressed in the form A + - A -, where A', A - E %' and A + A - = A - A + = 0. These conditions determine A + and A - uniquely, and IlAll = max(llA+II,IP-II). Proof. (i) By Theorem 4.1.6, f(A) has spectrum { f ( t ) :t E sp(A)} ; so f takes non-negative values throughout sp(A) if f(A) E a'. Conversely, if f(t) 2 0 for each t in sp(A), then f(A) is self-adjoint (sincefis real valued) and has spectrum a subset of R+. (ii) Withfin C(sp(A)) defined byf(t) = J(Allk t, f takes non-negative values throughout sp(A). By (i), f(A) E %+ ; that is, IlAllZ k A E a+. (iii) With u, u + , u- the continuous real-valued functions defined, for all real t, by u(t) = I,
u ' ( t ) = max{t,O},
u - ( t ) = maxi-
t,O},
we have u = u+
- u-,
u+u- = u - u + = 0.
Since u(A) = A, we have A = A + - A-,
A + A - = A-A'
= 0,
where A + = u+(A) and A - = u-(A); moreover, A + , A - c % + , by (i). The supremum norms of u, u + , and u - , as elements of C(sp(A)), satisfy
llull = max{llu+ll,Ilu-lll~ so IIAII = max{llA+lI,IlA-113.
247
4.2. ORDER STRUCTURE
To prove the uniqueness clause of (iii), suppose that A = B - C , where B, C E N + and BC = CB = 0. Then A" = B"
+ (-
( n = 1,2,3,...),
C)"
and therefore p ( A ) = p(B) + p( - C) whenever p is a polynomial with zero constant term. There is a sequence {p,,} of such polynomials that converges to U + uniformly on sp(A) u sp(B) u sp( - C); and u + ( A ) = limp,(A) = lim[p,(B)
+ p,( - C ) ] = u+(B) + u + ( - C ) .
Since u + ( s )= s (sEsP(B)),
u+(t) = 0
(tEsp(- C)),
we have u+(B) = B , u + (- C) = 0. Thus B = u + ( A )= A + ,
C= B- A
=A+
- A =A-.
4.2.4. COROLLARY. Each element A of a C*-algebra combination of at most four members of N+.
is a linear
Proof. From Proposition 4.2.3(ii) or (iii), the real and imaginary parts of A can each be expressed as a difference of elements of a+.
Our next objective, achieved in Theorem 4.2.6, is to give a number of conditions equivalent to positivity for elements of a C*-algebra. For this purpose, we require the following preliminary result. 4.2.5. LEMMA.If% isa C*-algebra,A ~ N , a n d- A*AE%I+,then A = 0. Proof. Let A = H + iK, with Hand K self-adjoint in a. Since sp(H) c [w and sp(H2) = {Iz: tEsp(H)} G R+, it follows that H2 (and similarly P)is positive. Since AA* is self-adjoint, and sp( - AA*) E sp( - A*A) u ( 0 ) c R + by Proposition 3.2.8, - AA* is positive. Now A*A
+ AA* = ( H - iK)(H + iK) + ( H + iK)(H - iK) = 2H2 + 2 K 2 , A*A = 2H2 + 2K2 + (- AA*).
Since all three terms on the right-hand side of the last equation are positive, A*A (as well as - A*A) is positive and so A*A = 0, by Theorem 4.2.2(iii) and (v). Thus I(A1(2= JIA*AII= 0, and A = 0. 4.2.6. THEOREM.If% is a C*-algebra and A E a, the following conditions are equivalent: (i) A € % + . (ii) A = H 2 ,for some H in a+. (iii) A = B*B, for some B in a.
248
4. ELEMENTARY C*-ALGEBRA THEORY
When these conditions are satisfied, the element H occurring in (ii) is unique. If X is a Hilbert space and 2l is a C*-subalgebra of a(%)), the preceding three conditions are equivalent to
(iv) ( A x , x ) 2 0, for each x in X. Proof. If A E%+, the equation f ( t ) = t 1 / 2 defines a non-negative realvalued continuous function f on sp(A) ( E R+). With Hdefined asf(A), HE%+ and H2 = A. This shows that (i) implies (ii), and it is apparent that (ii) implies (iii). Suppose next that A = B*B, for some B in 3 .Since A is self-adjoint, it has the decomposition A + - A - described in Proposition 4.2.3(iii). With C defined as B A - , C*C = A - B * B A - = A - ( A + - A - ) A - = - ( A - ) 3 . Since A - E%+ and ( A - ) 3 has spectrum { t 3 :tEsp(A-)}, it follows that - C * C = (A-)3~21+.Fr~mLemma4.2.5,C= O ; S O ( A - )=~ Oand,sinceAis self-adjoint, A - = 0 by Corollary 4.1.2. Thus A = A E 2l+, and (iii) implies (0. Having proved the equivalence of (i)-(iii), we show next that, when A E a+, the element H in (ii) is unique. For this, suppose that K is any element of 2l+ satisfying K 2 = A, while (as above) H =f(A), wheref ( t ) = t1l2( t E sp(A)). Let { p n }be a sequence of polynomials converging tofuniformly on sp(A), and let qn(t)= pn(t2).Since sp(K) c R + and +
sp(A) = sp(K2) = { t2 :t E sp(K)}, it follows that lim qn(t)= lirn pn(t2)= f ( t 2 ) = t , n+ m
n+m
uniformly for t in sp(K). Hence
K = lim qn(K)= lim p n ( K 2 ) n-. m
n+ m
=
lim p,(A) =f ( A ) = H, n+ m
and the uniqueness assertion is proved. With 2l a C*-subalgebra of a(%), %+ = 2l n a(%)+. Accordingly, in proving the equivalence of (i) and (iv) in this situation, it suffices to consider the case in which 2l = a(&). The required result then amounts to the following assertion, already proved in the introductory discussion of the present section : if A E ~ ( % ) ,then ( A x , x ) 2 0 for each x in X if and only if A = A* and sp(A)s R+. When A E 2l+, the element H occurring in condition (ii) of Theorem 4.2.6 is A similar procedure called thepositive square root of A , and is denoted by
4.2. ORDER STRUCTURE
249
can be used to introduce an element A" of a+,for other real values of a. Withf, defined byf,(t) = t", is a continuous non-negative real-valued function on sp(A) when a > 0 (for all real a, if A is invertible). Note thatfa(t)fa(i) = f a + B ( t ) , fl(t) = t , andfo(t) = 1 when A is invertible, for all t in sp(A). With A" defined as fa(A), we have A"€%', A"AB= A"'B, A ' = A, and A' = Zif A is invertible. It follows easily that this definition of A" agrees with the elementary one when a is an integer; in particular, if A is invertible, its inverse is the positive element A (=f-I(A))of 2I. 4.2.7. COROLLARY. If 2I is a C*-algebra, A€%', B* A B E 2I'.
and BE%, then
Proof. This follows from Theorem 4.2.6, since B*AB = (A1/2B)*A1/2B.
rn Suppose that a,, is the real linear space consisting of all self-adjoint elements of a C*-algebra CU. Since the adjoint operation is norm continuous, %,,is closed in 2l, and is therefore a real Banach space. From Theorem 4.2.2, it is a partially ordered vector space with a closed positive cone a+.In the partial ordering on a,,, A < B if and only if B - A E 2l' ; and, of course, %+ = { A E ( U h : A 2 0 ) .
From Proposition 4.2.3(ii), - llAlll< A < IIAl(Iforeach A in a,,;in particular, therefore, I is an order unit for ah. Moreover
2 0, - a1 < A < aZ}, since in its function calculus A corresponds to the identity mapping i on sp(A) (c R), while I corresponds to the constant function 1, whence llAll
= inf{a: a
llAll = 1 1 ~ 1 1= sup{ldt)l: t€SP(A)J =inf{a: - a . 1 < z < a . 1).
Just as in the case of real-number inequalities, one can add inequalities between self-adjoint elements of 2I (because sums of positive elements are positive), multiply through by positive scalars (because positive multiples of positive elements are positive), and take limits (because %' is closed in a), while multiplication by a negative scalar reverses inequalities. Since a product of commuting positive elements is positive, it follows that AC < BC whenever A < B, C E ~ I ' and , C commutes with both A and B. This last condition is essential, since without it, AC and BC are not self-adjoint. The corresponding non-commutative result, that C*AC < C*BC whenever A < Band C E2I, is a consequence of Corollary 4.2.7. An element A of 2I is invertible if and only if A 2 alfor some positive real number a. Indeed, A 2 a1 if and only if A - ale a ', and since sp(A - a l ) = { t - a : t E sp(A)}, this occurs if and only if sp(A) E [a, m). Since sp(A) is a +
250
4. ELEMENTARY C'-ALGEBRA THEORY
compact subset of R + , sp(A) G [a, co) for some positive a if and only if 0 $ sp(A) (equivalently A is invertible). Suppose that A and B are self-adjoint elements of a 4.2.8. PROPOSITION, C*-algebra %.
(i) r f - B < A < B, then llAll < IlBll. (ii) ZfO < A < B, then A'/' < B'". (iii) If0 < A < B and A is invertible, then B is invertible and B - ' < A Proof.
'.
(i) Since -
lIB((Z< - B < A
< B < IIBllZ,
(i) follows. (ii) and (iii) Suppose that 0 < A < Band A is invertible. Then A >, a l , for some positive real number a ; hence B >, aZ, and so B is invertible. Moreover 0 B - 1/2AB- 1 / 2 < B - 1/2gg- 112 = 1, and IJB-'/ZAB-'/211< 1 by (i). Thus (1) IIA '/2B- 11 = II(A1/2B- 1/2)*A1/2B-1/2111/2 = JIB-1/ZAB- 112 IIl / z
< 1.
From this, JIAI/ZB- 1A1/211 = IIA1/2B-I/2(A1/2B-1/2)Il.1, * <
whence A 1 / 2 B - ' A 1 / < Z Z, and therefore B-' < A - ' / z Z A - ' / 2 = A - l . Furthermore, from Proposition 3.2.8 and (l), IIB- 1/4A1/2B-1/4II = r ( B - 1 / 4 A 1 / 2 ~1/4 -1
.
) - r ( ~ 1 / 2 ~ - 1 / 4 ~ - 1 4IIA1/ZB-1/2 I1 < 1.
Thus B - 1 / 4 ~ 1 / 2 ~ - 1 41, and A1/2 < B1/4IB1/4 = B'/2. This proves (iii), and (ii) in the case in which A is invertible. Given only that A,BE%andO < A < B,wehaveO < A + & I < B + EIandA + Elisinvertible, for every positive real number E. From the preceding argument, ( A + E Z ) ~ / ' < ( B + EZ)'/'. (2) With T in a', and A in C(sp(T)) defined by f c ( t ) = ( t + &)'I2, we have ~ , ( T ) E %and [ + [fe(T)]' = T + EZ. Thus (T + &I)'/' =fe(T);sincefe(t) t l / Z as E + O , uniformly on sp(T), it follows that (I(T+d ) l / ' - T'/211-0. Accordingly, from Theorem 4.2.2(i), when E + 0, (2) gives A l l z < B 1 / 2 .
Suppose thatfis a continuous real-valued function defined on a subset S of the real line. We say thatfis operator-monotonic increasing on S if f (A) 0),g and h are
4.2. ORDER STRUCTURE
25 1
operator-monotonic increasing on their respective domains of definition, by Proposition 4.2.8(ii) and (iii). It is clear that an operator-monotonic-increasing function on Sis monotonic increasing (in the elementary sense) on S. However, the converse is false; for example, iff ( t ) = t’, then f is not operator-monotonic increasing on R + (consider
as operators on C’). For further information on this subject, see [6]. We conclude this section with some results concerning ideals in C*algebras, proving first that closed one-sided ideals are “positively generated” in a strong sense. Since the involution induces a one-to-one correspondence between left and right ideals, it suffices to formulate this result for left ideals only. 4.2.9. PROPOSITION. ZfX is a closed lefi ideal in a C*-algebra a,each element S of X can be expressed in the form S = AK, with A in and K in Xn%+.
Proof. Note first that, if T E Xn a+, then T “ ’ E X n a[+. Indeed, X contains all positive powers of T, and so contains p ( T ) whenever p is a polynomial with zero constant term. There is a sequence { p , } of such polynomials, with { p , ( t ) }converging to t’/’ uniformly for t in sp(7‘). Indeed, the Weierstrass approximation theorem asserts that t’l’ is the uniform limit of a sequence {q.(t)} of polynomials, and it suffices to take p,(t) = q,(t) - qn(0). Since X is closed and p,( Z‘) E X, it follows that TI/’ = limp,( T )E X. With S in X, let H = (S*S)’/’ and K = H”’. Then S * S E X n a+, and thus H , K E X n a[+. For n = 1,2,. . . , define A , = S(n-’Z so that
(3)
+ H)-’/’,
252
4. ELEMENTARY C*-ALGEBRA THEORY
where f,, is the continuous function defined on the positive half line by
f,,,(t) = t[(rn-'
+ t)-'/'
- (n-'
+ t)-'/'].
Thus
(4) Now
,/-
llAm - An11 = SUP{lfmn(t)l:~ E ~ P ( H ) ) -
&
+ as n 00, uniformly for t in sp(H). Thus fm,,(f) -+ 0 as and min(rn, n ) -+ oc,, uniformly for t in sp(H) ; from (4), {A,,}is a Cauchy sequence in 2l. With A defined as lim A,,, it now follows from (3) that -+
S = AH''2 = A K .
4.2.10. COROLLARY. Each closed two-sided ideal X in a C*-algebra 2l is self-adjoint. A closed two-sided ideal F in X is a two-sided ideal in 2l. Proof. Since X is a closed left ideal in a, each S in X has the form S = AK, with A in % and K ( = (S*S)'/4) in X n 2l+,by Proposition 4.2.9 and its proof. Since X is a right ideal in W,S* = KA* E X, so X is self-adjoint. Suppose next that SEZT Then S E X ,the preceding paragraph shows that S* E X ; since F is a left ideal in X, it now follows that S*SE9- n a+.Upon approximating the square-root function by polynomials with zero constant term, as in the proof of Proposition 4.2.9, we deduce that F n 2l+ contains the positive square root of each of its members. Successive application of this shows that K'/' (= ( S * S ) ' 1 8 ) ~ Z T With Bin 3,BAK"' and K ' / * B are in X, since K I / ' E X and X is a twosided ideal in %. Since 9- is a left ideal in X, and K E %
BS = BAK
=BAK"K"2~F;
so 9- is a left ideal in %. From this, A K l i 2E 9- ;and since 9- is a right ideal in X , SB = AKB = AK"'K'/'BEX This proves that F is a right (and hence two-sided) ideal in '$I.
4.2.11. LEMMA. Suppose that 93 is a closed * subalgebra (not necessarily containing the unit element) of a C*-algebra %, and let A = (BE93n9I':llBll < 1).
253
4.2. ORDER STRUCTURE
(i) I f B l , B2 E A , there is an element B of A such that B1 6 B and B2 6 B, (ii) I f S E and ~ E > 0, there is an element Bo of A with the following property: i f B EA and B b Bo, then [IS - SBI( < E . Proof. (i) Given B1 and B2 in A , we can choose 6 ( > 0) so that "(1 + 6)Bjll 6 1 ( j = 1,2). For each positive integer n, t 6 tll" when 0 < t 6 1, and thus (1 + 6)Ej ,< [( 1 + 6)Bj]I/". When n has the form 24 for some positive integer q, repeated application of Proposition 4.2.8(ii) shows that
+
+
+
+
[(l 6)(Bl BJ]"" 2 [(I 6)Bj]I'"2 (1 6)Bj. From the foregoing argument, Bl ,< B and B2 6 B, where B = (1
+ 6)-'[(1 + 6)(Bl + B2)l""
for a positive integer n of the form P. Now
+
+
+
+
(1 6)(B1 B Z ) E ~ , 0 ,< ( I 6)(Bl B2) < 21; the function tlln is the uniform limit, on the interval [0,2], of polynomials without constant term. Since g is closed, it now follows that B e g ; moreover,
0 < B 6 (1 + S)-121'nI. When n is sufficiently large, (1 + 6)-121/" < 1; and then, B E A . (ii) G i v e n S i n B a n d & ( >O),letS= aTwherea > l[Sll(sothat T e a a n d JJTJJ < 1). Let B, be the element (T*T)"" of A, where n is a positive integer sufficiently large to ensure that t(1 - t"") < c 2 a P 2
(0 < t 6 1).
If B E A and B 2 Bo, we have [IS - SBll = allT(I - B)ll = allT(I - B)2T*111/2. Now O < I - B < I - Bo < I, and thus o < ( I - B ) <~ I - B 6 I - B,, = z - (T* 7 - p ;
so 0 6 T(I - B)'T*
< T [ I - (T*T)l'"]T*.
Hence JJT(/- B)2T*JJ < IIT[I - (T*T)l'"]T*ll = r( T [ I - (T* T)""] T*)
= r( T* T [ I - ( T* T)""])
< sup(t(1 - t1'"):O < I <
,< 1)
254
4. ELEMENTARY C’-ALGEBRA THEORY
from Propositions 4.2.8(i), 4.1 .l(i), and 3.2.8, together with Theorem 4.1.6 (noting that sp(T*T) G [0, I]). Hence IlS - SBll = allT(Z - B)2T*1]”2< E .
If W contains the identity l o f 8,both Bin (i) and Bo in (ii) of the preceding lemma can be taken to be appropriate positive multiples of I. Of course, the interest of the lemma is precisely in the case where Z$B. In Lemma 3.2.24, it was found helpful to introduce an “approximate identity” in the Banach algebra L,(R). Similar devices are useful in studying ideals and * subalgebras (not containing the unit element) of C*-algebras. Let 9’ be a subset of a C*-algebra 8.We say that a net (V,, L E A , 2 )(= { Va}) of self-adjoint elements of 9’is an increasing right approximate identity for 9’if [IS - SV,il +a 0, for each Sin g while 0 < Va < V , < Zwhenever A, p E A and A < p. With obvious modifications, we formulate similar definitions for increasing, left, or two-sided approximate identities. Suppose that A3 is a closed * subalgebra of a C*-algebra 2t. The set A occurring in Lemma 4.2.1 1 is directed by the usual partial order relation on self-adjoint elements of 8,by part (i) of that lemma. By defining VBto be B, when B E A , we obtain an increasing net { VB}of positive elements in the unit ball of W.From Lemma 4.2.1 l(ii), (IS - SVBll + B 0, for each Sin g ;and, since W is self-adjoint, we also have
11s- VfjSll = IIS* - S* Vfjll+
0.
B
Thus { VB}is an increasing two-sided approximate identity for B, Suppose next that 3- is a closed left ideal in %. With X * the closed right ideal {S*: S E X } ,X n X * is a closed * subalgebra of 8,and contains the selfadjoint elements of X. By the preceding paragraph, X n X * has an increasing two-sided approximate identity { Va}. From Proposition 4.2.9, each Sin X has the form AK, with A in 2t and K in 3- n 8’ ( E X n X * ) .Since
11s - SVAll = Il4K - KV,)ll < IlAll IIK - KVall 0, +
a
it follows that { Va} is an increasing right approximate identity for X. We have now proved the following result.
A closed left ideal in a C*-algebra has an increasing 4.2.12. PROPOSITION. right approximate identity, and a closed * subalgebra has an increasing two-sided approximate identity. Of course, the roles of “left” and “right” can be reversed in Proposition 4.2.12. Note, however, that a closed left ideal in a C*-algebra does not always have a left approximate identity (see Exercise 4.6.37).
255
4.3. POSITIVE LINEAR FUNCTIONALS
4.3. Positive linear functionals
In this section, we study linear functionals on a subspace of a C*-algebra. In most applications of the results described below, it suffices to consider the case in which the subspace is the whole C*-algebra; but occasionally, the more general setting is required. We assume throughout that A is a self-adjoint subspace of a C*-algebra 8, and contains the unit I of 8.The set A n 8 +of all positive elements of A is denoted by A + .If A c g c 8, where 93 is a C*-subalgebra of 8, then A += d i n % += Anan%+ = AnnW'; so A?+ is unchanged if A is viewed as a subspace of % . f instead of 8. Since A contains the real and imaginary parts of each of its members, while each self-adjoint A in A is the difference of two elements, t(llAllZ k A), of A +,it follows that 4is the linear span of A +. With p a linear functional on A, the equation p*(A) = p(A*) (A E A ) defines another such functional p*. We describe p as hermitian if p = p * ; that __ is, if p ( A * ) = p ( A ) for each A in A. (Compare the discussion of Remark 3.4.9, where this construction is applied to the C*-algebra C(X).) By expressing elements of A in terms of their real andimaginary parts, it follows that p is hermitian if and only if p ( H ) = p * ( H ) (= p ( H ) )whenever H = H* E A ;so p is hermitian if and only if p ( H ) is real, for each self-adjoint H in A. Each linear functional p on A can be expressed, uniquely, in the form p1 i p 2 , where p1 (= +(p + p * ) ) and p2 (= $(p* - p ) ) are hermitian. If p is a bounded hermitian functional on A,
+
llpll = sup{p(H):H= H*EA,llHll< 11;
that is, the norm of p is the same as the norm of its restriction to the (real linear space of) self-adjoint elements of A?. Indeed, if E > 0, we can choose A in the unit ball of A' so that Ip(A)I > llpll - E . For a suitable scalar Q with (QI = 1, llpll - E < Ip(A)I = p ( a 4 = p ( a 4 = p((aA)*).
With Ho the real part of aA, llHoll < 1 and p(Ho) > IIp[I - E . Thus [lpll < sup{p(H): H = H * E A ,llHll s l}, and the reverse inequality is evident. A linear functional p on A is said to be positive if p ( A ) 2 0, for each A in A + ;if, further, p(Z) = 1, p is described as a state of A. A positive linear k A) 2 0 since functional p is hermitian; for if A = A* E A, then p(((A((Z IJAllZk A E +,and p ( A ) is real because p ( A ) = :Cp(llAIII
+ A ) - p(llAlll- 41.
The real vector space Ah,consisting of all self-adjoint elements of A, is a partially ordered vector space, with positive cone A and order unit I. A linear functional p on A is hermitian if and only if its restriction p I A h is a +
256
4. ELEMENTARY C'-ALGEBRA THEORY
linear functional (of course, real-valued) on A,,;and each linear functional on Ahextends, uniquely, to a hermitian linear functional on A. Moreover, p is positive (or a state of A)if and only if p l A his positive (or a state of A,,)in the sense of Definition 3.4.5. The positive linear functionals on A form a cone P, in the real vector space consisting of all hermitian linear functionals on 4 (9n - 9 = {O},because A is the linear span of A +).Hence there is a partial order relation on the hermitian linear functionals; p1 < p 2 if and only if p 2 - p1 is positive. With X a Hilbert space and x in 2,the equation w x w = (Ax,x )
( A € -%XN
defines a linear functional w, on a(%). In view of the equivalence of two concepts of positivity for Hilbert space operators (conditions (i) and (iv) in Theorem 4.2.6, with 8 = a(X)), w,(A) 2 0 whenever A E W ( X ) + .Since, also, w,(I) = 11x112,it follows that w, is a positive linear functional on a(%), and is a state if llxll = 1. If % is a C*-subalgebra of a(#),and (as usual) JZ is a selfadjoint subspace of % that contains I, the restriction w x l Ais a positive linear functional on A.The states of A that arise in this way, from unit vectors in 2, are termed vector states of A. 4.3.1. then
PROPOSITION.
I f p is apositive linear functional on a C*-algebra %,
Ip(B*A)I2 < p(A*A)p(B*B)
( A , B E 8).
Proof. With A in 8,we have A*A E 8 +and , therefore p ( A * A ) b 0. From this, and since p is hermitian, the equation ( A , B ) = p(B*A)
( A ,B E % )
defines an inner product ( , ) on 8, and we have the Cauchy-Schwarz inequality
I ( 4 B>I2< ( A , A ) ( & B ) ; that is, )p(B*A)I2< p(A*A)p(B*B). W We refer to the inequality occurring in Proposition 4.3.1 as the Cauchy-Schwarz inequality for p. This inequality appeared, in the case of the C*-algebra C(X), at the end of Remark 3.4.9 and, again, in Remark 3.4.12. 4.3.2. THEOREM. If A is a selj-aa'joint subspace of a C*-algebra % and contains the unit I of 8,a linear functional p on A is positive if and only if p is bounded and llpll = p ( l ) . Proof. Suppose first that p is positive (and therefore hermitian). With A in A,let a be a scalar of modulus 1 such that ap(A) 2 0, and let H be the real part
257
4.3. POSITIVE LINEAR FUNCTIONALS
b(4I= p ( a 4 =
= p(dA*)
+ dA*)) = p ( H ) 6 p(l)llAll. This shows that p is bounded, with (IpIJ< p ( l ) ; and the reverse inequality is = p(f(aA
evident. Conversely, suppose that p is bounded and llpll = p(Z); it suffices to consider the case in which l(pll = p ( I ) = 1. With A in & +,let p ( A ) = a + ib, where a and b are real. In order to prove that p is positive, we have to show that a 2 0 and b = 0. For small positive s, sp(Z - sA) = {l - s t : tESP(A)} E [ O , 11, since sp(A) E Iw ; so (II - sAll = r(I - sA) < 1. Hence +
1 - su < 11 - S(U
+ ib)( = ( p ( I - sA)I < 1,
and therefore a 2 0. With B, in A! defined as A - a I + inbl, for each positive integer n,
11Bn,112= IlS,*B,((= ll(A - al)'
+ n2b2Z((< (IA - a1(I2+ n2b2.
Hence (n2
+ 2n + l)b2 = Ip(Bn)12,< JIA - aIllZ+ n2b2
(n = 1 , 2 , . . .),
and thus b = 0. H From Theorem 4.3.2, each state p of A is a bounded linear functional on A!, with llp[l = 1. Accordingly, the set Y ( M )of all states of A! is contained in the surface of the unit ball in the Banach dual space A'. It is convex and weak* closed, since Y ( A )= { p E & " : ( l )
= 1 , p(A) 2 0 (A€&+)},
and is therefore weak* compact, by Corollary 1.6.6. It follows that Y(A!),with the weak* topology, is a compact Hausdorff space, the state space of A!. 4.3.3. PROPOSITION. rf'u is a C*-algebra with unit I, A is a self-adjoint subspace of Cu containing I , A E .A',anda E sp(A), then there is a state p of A such rhar p ( A ) = a . Proof. For all complex numbers b and c, ab + cEsp(bA + c l ) , and therefore lab c ( < ((bA clll. Accordingly, the equation po(bA + c l ) = ab + c defines (unambiguously) a linear functional p o on the subspace
+
+
258
4. ELEMENTARY C*-ALGEBRA THEORY
+
{bA cI: b, C E C } of A, and p o ( A ) = a , p o ( I ) = 1 , llpoll = 1. By the Hahn-Banach theorem, po extends to a bounded linear functional p on A, with llpll = 1 (= p ( I ) ) . From Theorem 4.3.2, p is positive (and is therefore a state); and p ( A ) = a.
4.3.4. THEOREM.Suppose that W is a C*-algebra with unit I, A? is a sevadjoint subspace of '9l containing I, and A E A . (i) I f p ( A ) = 0, for each state p of A, then A = 0. (ii) I f p ( A ) is real, for each state p of A, then A is self-adjoint. (iii) If p(A) 2 0, for each state p of A, then A E A + . (iv) I f A is normal, there is a state p of &'such that Ip(A)I = IlAll. Proof.
(i) Suppose first that A is self-adjoint and p ( A ) = 0 for each state
p of A. From Proposition 4.3.3, sp(A) = {0}, so IlAll = r(A) = 0, A = 0.
Next, let A = H + iK, with Hand Kself-adjoint in A. If p ( A ) = 0, for each state p of A, then p(H) = p(K) = 0, since p ( A ) = p(H) + ip(K)and p(H) and p ( K ) are real. From the preceding paragraph, H = K = 0, whence A = 0. (ii) If p ( A ) is real, for each state p of A, then
p ( A - A*) = p ( A ) - p ( A ) = 0, and A - A* = 0 by (i). (iii) If p ( A ) > 0, for each state p of A, then A is self-adjoint by (ii), sp(A) E R + by Proposition 4.3.3, and so A E A + . (iv) If A is normal, r(A) = 11A11, so sp(A) contains a scalar a such that la1 = IlAll. By Proposition 4.3.3, a = p(A), for some state p of A, and then Ip(A)I
=
IlAll.
Our next objective is to prove a non-commutative analogue of the Hahn-Jordan decomposition for linear functionals on C(X) (Proposition 3.4.11 and Remark 3.4.12). We show in Theorem 4.3.6 that every hermitian functional on A can be expressed (in a unique optimal manner, when A is the whole of '9l) as a difference of positive linear functionals. For this purpose, we require the following lemma, which will be needed again later when we characterize those subsets of the state space Y ( A )that retain some of the properties of Y ( A )set out in Theorem 4.3.4. With X a subset of the Banach dual space A', we write E ( X )for the weak* closed convex hull of X.
4.3.5. LEMMA.Suppose that W is a C*-algebra with unit I, A? is a seuadjoint subspace of W'containing I , and % is a set of states of A. If IlHll = sup{lp(H)I: P E % } ? for each self-adjoint H in A, then =(Yov - Yo)is the set of all hermitian functionals in the unit ball of An.
259
4.3. POSITIVE LINEAR FUNCTIONALS
Proof. The set of all hermitian functionals in the unit ball (An)l is convex and weak* closed, and contains You - Yo;so it contains Co(.u?, u - Yo). We have to show that the two sets coincide. Suppose the contrary, and let po be a hermitian functional on A, such that llpoll < 1, po#C5(% u - %). By the Hahn-Banach theorem (Corollary 1.2.12), and since the weak* continuous linear functionals on A narise from elements of Jt (Proposition 1.3.5), there is an A in .I, and a real number a, such that
Re p ( A ) < a
Re p o ( A ) > a,
( p ~ E 5 ( %u - %)).
With H the real part of A, p(H) = : C P W
+ P(A*)l
=
Re P ( 4 9
for every hermitian functional p on A ; so p(H) < a
p o w > a,
(PE=(%
u - %)).
Thus Ip(H)I ,< a ( p E %), and a
a contradiction.
-= p 0 ( W < IlHll = sup{lp(H)I: p€%I < a, H
4.3.6. THEOREM.If% is a C*-algebra with unit Z and A is a seljladjoint subspace of CU containing I , each bounded hermitian functional p on A can be expressed in the form p - p - , where p + and p - are positive linearfunctionals = )Jp+JI llp-11. If A is the whole of %, these conditions on Jll and JJp1I determine p + and p - uniquely. +
+
Proof. We may assume that llpll = 1. With Y the state space of A,
l l 4 = suPo7(4l: .r E 9 1 for each self-adjoint A in A, from Theorem 4.3.4(iv) and since 11711 = 1 when ~ €By9 Lemma 4.3.5, p ~ E 5 ( 9 u- 9). A straightforward calculation shows that the subset { a o - b 7 : o , t ~ Y : a , b ~ R + , a + b1)=
ofC5(Y u - 9) is convex. It contains Y u - Y: and is weak* compact since it is the range of the continuous mapping (o,7 , a ) -,ao - (1 - 4 7 :
9 x 9'x [O,l]
+ An.
Accordingly, it is the whole of C5(Y u - 9). From this, and the preceding paragraph, p has the form ao - b7, with p and 7 in Y: a and b in R and a + b = 1. With p + and p - the positive linear functionals ao and b7, respectively, p = p + - p - and +
IIP+II + IIP-ll
=a
+ b = 1 = llpll.
260
4. ELEMENTARY C*-ALGEBRA THEORY
Suppose now that A = a. To prove the uniqueness of the decomposition of p, we assume that p = p - v = p‘ - v‘, where p, p’, v, v’ are positive linear functionals on 9l and
llpll + llvll = 11P:Il + llv’ll
=
llpll = 1.
Given E (> 0), choose a self-adjoint H in the unit ball of ‘illfor which p ( H ) > llpll - $ E ~ ,and let K = :(I - H).Then 0 < K < I, p(I)
+ v(l)
= 11pIl
+ llvll = IIPII < p(H) + y
p(I - H )
= p(H)
- v(H) + fEZ,
p ( K ) + v(I - K ) < iEi.
+ v ( l + H ) < $&Z,
Since K , l - K Ea+,while p and v are positive linear functionals, 0 d v(I - K ) < $&? 0 < p ( K ) < $&2, With A in a, the Cauchy-Schwarz inequality gives
Ip(KA)12 = Ip(K’l2 K’/2A)IZ< p(K)p(A*KA) < $ E ~ ( I A ~ ( ~ , 9
Iv((l- K)A)12 < v ( I - K)V(A*(I - K ) A ) < $&211AJI2. From this, and a similar argument for p’ and v‘, we have
< tEllAll, I W - K)4I < t&IIAll.
< ;&ll~ll, Iv((l - KM)I < ;&ll4l,
IP’(K4
lP(KA)I
Since p - p‘ = v
- v’,
p ( A ) - $(A) = p(KA) - p’(KA)
+ v((Z - K ) A ) - v’((I - K ) A ) ,
and so Ip(A) - $(A)[ < 2&llAIJ.Since the last inequality has been proved for each positive E , it follows that p = p’, whence v = v’. 4.3.7. COROLLARY. Zf9l is a C*-algebra with unit I and A is a self-adjoint subspace of N containing I, each bounded linear functional on A is a linear combination of at most four states of A. Proof. Each bounded linear functional z on A has the form p (T bounded hermitian functionals. Hence
+ io, with
p and
z = p+ - p-
+ ia+ - ia-,
and each term on the right-hand side is a scalar multiple of a state of A. We shall later give an alternative proof of the existence of the decomposition p = p + - p - , by reduction to the case in which rU is abelian, and appeal to Proposition 3.4.11 (see Remark 4.3.12). The uniqueness clause of
26 1
4.3. POSITIVE LINEAR FUNCTIONALS
Theorem 4.3.6 fails, in general, if one deletes the assumption that A = % (see Exercise 4.6.22). Since the state space 9(A)of is convex and weak* compact, it has extreme points; indeed, by the Krein-Milman theorem, Y(&)is the weak* closed convex hull cO(9(A)) of the set 9(A)of its extreme points. Elements of 9(A)are termed pure states of A,and the weak* closure 9(A)is called the pure state space of A. In general, 9(A)is not a closed subset of An, and the pure state space then has elements that are not pure states. When X is a compact Hausdorff space, the pure states of the C*-algebra C ( X ) are precisely the non-zero multiplicative linear functionals (Theorem 3.4.7); and the set 9 of pure states is therefore weak* compact (Proposition 3.2.20). Thus 9 coincides with the pure state space, in this case. A linear functional p on A is a pure state if and only if its restriction PI&, , to the partially ordered vector space Ahconsisting of all self-adjoint elements of A, is a pure state of A,,in the sense of Definition 3.4.5. Indeed, this follows from similar assertions, concerning hermitian linear functionals and states, occurring in the discussion preceding Proposition 4.3.1. 4.3.8. THEOREM. Suppose that % is a C*-algebra with unit I , A is a sewadjoint subspace of % that contains I, and A E A . (i) I f p ( A ) = 0, for each pure state p of A, then A = 0. (ii) I f p ( A ) is real, for each pure state p of A, then A is seif-adjoint. (iii) If p(A) 2 0, for each pure state p of A, then A E A . (iv) I f A is normal, there is apure state po of A such that Ipo(A)I = IlAll. +
Proof. If p ( A ) = 0 (or p ( A )is real, or p ( A ) 2 0) for all p in 9(A), then the same is true for all p in 9(A),since every state is a weak* limit of convex combinations of pure states. In view of this, the first three parts of the theorem follow, at once, from the corresponding assertions in Theorem 4.3.4. Suppose now that A is normal. By Theorem 4.3.4(iv), there is a scalar c and a state t of A such that 4 4 )= c, (c( = 11A11. Let be the (weak* continuous) linear functional on Asthat takes the value p ( T ) at p, and let a be a complex number of modulus 1 such that t ( a A ) = IcI = IIAJJ.From Corollary 1.4.4,there is a po in 9(A)such that IlAll 2 Ipo(A)I 2 Re ape>
2 sup{Re a ( p ) : ~ E Y ( A ) } 2 Re
a(t)Re t ( a A ) =
=
IlAll.
4.3.9. THEOREM. If% is a C*-algebra with unit I , A is a self-adjoint ' is a subset of the state space 9(A),the subspace of % thal contains I, and 9 following four conditions are equivalent:
262 (i) (ii) (iii) (iv)
4. ELEMENTARY C*-ALGEBRA THEORY
I ~ A E and A p(A) > 0 for each p in 9& then A E A ' . IlHll = sup{lp(H)I: p € Y 0 } ,for each self-adjoint H in A. cO(Yo)= Y ( A ) . 9(A)E (%)- the weak* closure of % in A'.
Proof. With H self-adjoint in A, define a (< IlHll) by
a = sup{lp(WI : ~ € 3 3 , and note that
p(aZ k H ) = a
kp(H)2 0
(p~g).
If (i) is satisfied, then aZ k H E & + , - aI < H < aZ, hence IlHll < a, and so IlHll = a. Thus (i) implies (ii). Suppose next that (ii) is satisfied. With Y; defined a s s ( % ) , and (A')1the unit ball in A', we have YoE Y; c_ Y ( 4 )c_ (A')l,so (ii) remains true when Yois replaced by From Lemma 4.3.5, cO(Y; u - .yI) is the set of all hermitian functionals in (A')l;in particular, Y(&) E E(Y;u - Y;). The set
x.
{ao - b z : o , z ~ Y ; ,a , b E R + , a
+ b = 1)
contains Y; u - Y ; , inherits convexity and weak* compactness from Y;, and so coincides with cO(Y; u - 3 )(compare this with the proof of Theorem 4.3.6). Accordingly, each state p of 4has the form ao - br, with o and z in Y;, a and b in R +,and a + b = 1. Since 1 = p(Z) = ao(Z) - bz(Z) = u - b = 1 - 2b, we have b = 0, a = 1 and p = o E Y;. Hence Y ( A )E Y;, and since the reverse inclusion has already been noted, Y ( A )= Y; = =(%Po). Thus (ii) implies (iii). If =(Yo)= 9'(A), it follows from Theorem 1.4.5 that 9(A)E (Yo)-;so (iii) implies (iv). E (sP,)-. If A E 4 and p ( A ) 2 0 for each p in Finally, suppose that 9(4) Yo,the same is true for each p in (Yo)-(in particular, for each p in 9(A)), by weak* continuity of the mapping p -+ p(A). By Theorem 4.3.8(iii), A E A +,so (iv) implies (i). W 4.3.10. COROLLARY. I f H is a self-adjoint operator acting on a Hilbert space X, then IlHll = suP(l(Hx,x)l: XEX, llxll = 11.
If& is a self-adjoint subspace of a(#)containing Zand Yois the set of all vector states of A, then 9(A)c (Yo)-and Y ( A )= cO(Yo). Proof. If A E A and p(A) 2 0 for each p in Yo,then
4.3. POSITIVE LINEAR FUNCTIONALS
263
4.3.9, it now follows that iZ(Yo)= Y ( A ) ,9(A)c (Yo)-, and
IlHll = sup{lp(H)I: P
E W =
suP{l(Hx,x>l: XE3E4 llxll = I > ,
for each self-adjoint H in A. Since the last conclusion applies, in particular, when A = a(%), the corollary is proved. W The formula for IlHll, in Corollary 4.3.10, can also be proved without reference to C*-algebra theory, by combining Lemma 3.2.13with Proposition 3.2.15. By a function representation of A on a compact Hausdorff space X , we mean a linear mapping (P: A -,qAfrom A into the C*-algebra C ( X ) ,such that qr is the constant function with value 1 throughout X , and qAE C ( X ) +if and only if A E A + . If, in addition, given any two distinct points x and y in X , there is an element A of A such that qA(x)# ( P ~ ( Y we ) , describe (P as a separating function representation. Two function representations, (P: A + C ( X ) and t,b: A -,C( Y ) ,are said to be equivalent if there is a homeomorphismffrom X onto Y , such that ( P ~ ( x= ) t,bA( f ( x ) ) , for each A in A and x in X . If (P: A?+ C ( X ) is a function representation of A, and XEX, the evaluation mapping p x : A + (P~(x) is a state of A, since it is a positive linear functional and p x ( l ) = q I ( x )= 1. Hence I(PA(x)l = lpx(A)I 6 llAll
( A E A ,
XEX)
and ll(PAl1
= SUP I(PA(x)l 6
llAll
( AE d ) *
*EX
For each self-adjoint Hin A,q H is a real-valued function, since ( P ~ ( x=) px(H) and px is hermitian. With c defined as II(PHII (PclfH(4
=c
* (PH(X)2 0
(XEX);
so qEI f H E C ( X ) +; hence c l 2 H E A +,- c l 6 H 6 cl, and therefore
IlHlI 6 c = ll(PHll 6 l l H l l . Thus (P maps self-adjoint elements of A, isometrically, onto real-valued functions. By expressing an element A of A in the form H + iK, with Hand K self-adjoint in A, it follows that (P preserves adjoints; moreover, since (PA
= (PH + I'(PK,
llAll 6
llHll + llKll
= ll(PHll
+ II(PKI1 6 211(PAll*
Accordingly,
:ll4 6 II(PAll 6 IlAll,
II(PHII =
IlHll
( A E A , H = H*E.m.
The set Jf = ((P, :A E A} is a self-adjoint subspace of the C*-algebra C(X), and contains its unit. Since (P is a one-to-one bicontinuous linear
264
4. ELEMENTARY C*-ALGEBRA THEORY
mapping from the normed space A onto the normed space Jy; its Banach adjoint cp': 7 + cp," is a bicontinuous linear mapping from N' onto A'. Since cpA is self-adjoint, or positive, if and only if A has the same property, while cp:(A) = 7(cpA) ( A E A , T E N 'it) ,follows that cpf is hermitian, or positive, if and only if the same is true of z. Since cp is isometric on self-adjoint elements of A,while the norm of a hermitian functional is unchanged by restriction to selfadjoint elements, cp' is isometric on the hermitian functionals in N'. We now exhibit some canonical function representations associated with a C*-algebra. For each A in A', the equation =
( P E Y(&)
defines acontinuous complex-valuedfunction A on the state space Y ( A )If. SP, is a closed subset of Y(&) and contains the pure state space P ( A ) - ,it is apparent that the restriction A 1% takes non-negative values throughout % if A E A +,and the converse assertion follows from Theorem 4.3.9. Since p(aA we have
n
+ bB) = ap(A) + bp(B),
( a A + b B ) = a2
+ bB
( A , B E A , a,bE@).
If p1 and p2 aredistinct elements of Yo,then we have p l ( A ) # p 2 ( A ) for some A in A ; that is, &pl) # A ( p 2 ) . Moreover, j ( p ) = p ( l ) = 1, for each p in SP,. Accordingly, the mapping A+A(SP,: A+C(%) The following theorem is a separating function representation of A on goPo. shows that every separating function representation of 4 is equivalent to one that arises in this way, by appropriate choice of %. The most important function representations of A are the two "extreme" ones, obtained from the above construction when Yois either the state space Y ( M )or the pure state space &'(A)-. 4.3.11. THEOREM. r f '2.l is a C*-algebra with unit I, A is a self-adjoint subspace of '2.l containing I, Xis a compact Hausdorffspace, and cp :Jf + C(X) is a separatingfunction representation of A, then there is a unique closed subset Yo of Y ( A ) ,such that &'(A)c SP, and cp is equivalent to the function representat ion
A - * A l S : A+C(%). Proof. For each x in X,the equation p,(A) = cpA(x)defines a state pr of A.Given two distinct points x and y of X , there is an element A of J2 for which cpA(x) # cpA(y), whence p,(A) # py(A) and so px # p y . If { x a } is a convergent net of elements of X , with limit x, it follows from the continuity of the function
4.3. POSlTlVE LINEAR FUNCTIONALS (PA
265
that
so pxm+ px in the weak* topology. From the preceding paragraph, the mapping
f: x + p x :
X+Y(A)
is one-to-one and continuous. Since Xis compact, the same is true of its range, X onto Yo.If A E A and p ( A ) 2 0 for each p in z,then
Yo= { px : x E X), and f is a homeomorphism from
(XEX); 20 so cpA€ C ( X ) + ,and therefore A E A ' . Since Yois closed, it now follows from Theorem 4.3.9 that P ( A ) - c Yo.Finally, (PAW
=p x ( 4
(XE X, A E 4, W ( x , >= 4 P x ) = P x ( 4 = c p A ( 4 and therefore cp is equivalent to the function representation
A-+C(%).
A+ai$:
To prove the uniqueness clause of the theorem, suppose that cp is equivalent also to the function representation A+&q
A+C(%),
hhere 3 is a closed subset of Y(&) containing 9(A)-.Let g : x + g x be a homeomorphism from X onto g,such that (PA(x) = a
(XE X ,
A E A).
Then g x is a state of A, and ( AEM).
g x w = &L> = (PA@) = P X W
Hence g x = p x for each x in X,and
y; = { g x :X E X }
=
{ p x :X E X }
=
z. rn
4.3.12. REMARK.We illustrate the use of function representations by giving an alternative proof of the existence of a decomposition of a bounded hermitian functional as a difference of positive linear functionals (Theorem 4.3.6), by reduction to the abelian case (Proposition 3.4.11). With cp : A + cpA a function representation of A on a compact Hausdorff space X, and Af the subspace { q A :A E A ) of C(X), we recall that cp has a Banach adjoint operator cpI: 7 -+ cp: from N aonto .A!'; that cp: is hermitian, or positive, if and only if 7 has the same property; and that cp' is isometric on hermitian elements of N 8 . In order to show that a bounded hermitian
266
4. ELEMENTARY C*-ALGEBRA THEORY
functional p on A can be expressed as p + - p - , where p + and p - are positive linear functionals and JIp+II+ IIp-ll = llpll, it now suffices to prove the corresponding statement for J1/: With p a bounded hermitian functional on N, p extends without increase of norm to a bounded linear functional 7 on C(X). We can suppose that 7 = 71 iz2, where 71, 7 2 are hermitian, and I(zjJId llzll ( j = 1,2). Since p and the restrictions 71 IJV and .rZ(JV are hermitian, while Upon replacing z by 71, we may p = (ti + i.t2)IXit follows that p = 71 assume that 7 is hermitian. By Proposition 3.4.1 1, 7 can be expressed as 7 + - 7 - , where 7 + and 7 - are positive linear functionals on C ( X ) , and Il~+ll Ilr-ll = IITII. With p+ and p - defined as 7 + 1N . and 7 - 1.V;respectively, p + and p - are positive linear functionals on Jv; and p = p + - p - . Moreover,
+
+
IIP+II + IIP-ll d 117+11 + 117-11 = 11711 = IlPll = IIP+ - P-ll SO
d IIP+II + IlP-II,
IIP+II + lip-11 = llpll.
We conclude this section with-some further results concerning states and pure states. 4.3.13. THEOREM. Suppose that 'u is a C*-algebra with unit I , JZ is a selfadjoint subspace of % that contains I, and p is a state of 4. For each self-adjoint H in 'u, defne I H = S U ~ { ~ ( B ) : B = B * E AB ', < H } , uH = inf{p(B) : B = B* E A, B > / H }.
(i) - IIHII d 1" d U H < IIHll ( H = H * E a). (ii) p extends to a state 7 of 'u. I f c is a real number and H is a self-adjoint element of 'u, the extended state 7 can be chosen so that 7 ( H ) = c if and only iJ 1H
< c < uH.
(iii) p extends uniquely to a state 7 of 'u ifand only $1, = uHfor each selfaa'joint H in %. (iv) I f p is apure state of A, then p extends to apure state of 2I. I f , further, p has only one extension as a pure state of 'u, then p has only one extension as a state of a. Proof. (i) With H self-adjoint in 'u,
l l H l I I ~ 4 , - IIHIII d H d IIHIII. If B = B * E A and B < H , then B d ((HI(Zand therefore p(B) = IlHll. Hence the set
< llHllp(I)
{p(B): B = B * E A , B < H } is bounded above by IlHll and (with B = - IIHllZ) contains - IlHll.
267
4.3. POSITIVE LINEAR FUNCTIONALS
Accordingly, its supremum lH satisfies - IlHll 6 lH 6 11Hl1; and a similar argument shows that (IHll< uH 6 IIHII. If B1 and B2 are self-adjoint elements of & and B1 6 H 6 B2, then p ( B , ) < p(B2). By allowing B1 and B2 to vary, subject only to the conditions just stated, it follows that I , 6 u H . (ii) With H self-adjoint in 'Lz, 1, 6 uH by (i), so we can choose a real number c such that 1, 6 c 6 uH. We shall prove that p has an extension to a state t of 8 such that t ( H ) = c. As a first step, we show that the equation
-
to(aH + A ) = uc + p ( A )
(1)
defines a positive linear functional A? = {uH
t oon
(a€@, A E A )
the self-adjoint subspace
+ A : U E C ,A E A }
generated by Hand A.This is evident when HE4, since in this case Jlr = A, 1, = uH = p(H), hence c = p(H); and to,as defined by (l), is p . We assume henceforth that H$ A, whence each element T of Jlr is uniquely expressible as aH A , with a in C and A in A.Accordingly, (1) defines a linear functional t o on J,'to(H) = c and 70 extends p ; in particular, zo(l) = p(Z) = 1. In order to show that t o is positive, suppose that T E N ' , and let T = aH + A , as above. Since
+
0 = T* - T = (U - a ) H
+ A* - A ,
(ii - a)H = A - A*€&!, and it follows that a is real. If a = 0, then T = A E A ' and t O ( T )= p ( A ) b 0. If a > 0, then H + a - ' A = a - ' T a 0, so - u - ' A E A ,- a - ' A 6 H ; from the definition of lH, - u-'p(A) 6 lH
( 6 c), and therefore
t,(T) = a[c + a-'p(A)] 2 0.
If a
-= 0, then - H - a - ' A = (-
a)-'T 2 0, so
- a-'A 2 H; -a-'AEA, from the definition of uH, - a - ' p ( A ) 3 uH ( 2 c), and therefore to(T) = - a [ - c - a - ' p ( A ) ] 2 0. The preceding enumeration of cases shows that zo(T)2 0 whenever T EN + , so t ois a positive linear functional on M. From Theorem 4.3.2, t o is bounded, and I(tOll= to(Z) = 1. By the Hahn-Banach theorem, t o extends without change of norm to a bounded linear functional t on 8 ;moreover, t is positive, again by Theorem 4.3.2, since T(l) = fo(1) = 1 = [ ( T o ( ]=
Accordingly, t is a state of 8, t(H)
= to(H) = c,
Il'Tll. and
t
extends p .
268
4. ELEMENTARY C'-ALGEBRA THEORY
Conversely,suppose that t l is any state of % that extends p. If B1and B2 are self-adjoint elements of A, for which B1< H < B2, then tI(Bl) < t l ( H ) < t l ( B 2 ) ;that is, p(B,) < tl(H) < p(B2). By allowing B1 and B2 to vary, subject only to the restrictions just stated, it follows that 1, < t l ( H ) < uH. (iii) This is an immediate consequence of (ii). (iv) Suppose now that p is a pure state of A, and define Yoto be the set {tEY(%)
: T(B) = p ( B ) ( B E d ) }
of all states of 2I that extend p. Then % is a closed convex subset of Y(U),is therefore weak* compact, and is non-empty by (ii). From the Krein-Milman theorem, = =(Po),the closed convex hull of the set Poof all extreme points . Pois not empty, and consists of a single element if and only if Yo of 9,Hence has just one element. It now suffices to show that each T in Po is a pure state of a. For this, suppose that z = a t l + (1 - a)z2, where tl, t 2E Y(%) and 0 c a c 1. Since the restrictions z1 IA, t 2! Aare states of A, while p is a pure state of d and p=TlA=a(Tll~@
+(I
-U)(TzId),
+
it follows that z l l A = z 2 l A = p . Thus t l , t 2 ~ Yand 0 ; since t ( = a t , (1 - a ) t 2 )is an extreme point of Yo,t 1= t 2 = T . Hence T is an extreme point of Y(9l);that is, T is a pure state of a. The results on extensions of states and pure states, set out in Theorem 4.3.13, remain valid in the context of a partially ordered vector space V with order unit I, and a subspace of V that contains I (see Exercise 4.6.49).
If% is a C*-algebra with center V and p is a pure 4.3.14. PROPOSITION. state of a, then p ( A C ) = p ( A ) p ( C )for all A in % and C in V. Moreover, the restriction plV is a pure state of V. Proof. In order to show that p ( A C ) = p ( A ) p ( C )when A E % and CEV, it suffices (by linearity) to consider the case in which 0 < C < I. In this case, for each H in %+, we have 0 < HC < H , and thus 0 < p ( H C ) < p ( H ) , since H commutes with C (see the discussion following Corollary 4.2.7). Hence the equation po(A) = p ( A C ) defines a positive linear functional p o on %, and p o < p. Since p is a pure state, so is its restriction PI%, to the partially ordered vector space a,,of all self-adjoint elements of (see the paragraph preceding Theorem 4.3.8). Since polah < PI%,,, it follows from Lemma 3.4.6 that pol%h= a(pl2I,J,for some scalar a. Hence p o = u p ; and P(AC) = P O W
for each A in
a.
= a p ( 4 = w ( O p ( A ) = PO(I)P('4 = p(C)p(A),
269
4.4. ABELIAN ALGEBRAS
From the preceding paragraph it follows, in particular, that the non-zero linear functional ~ 1 % ' is multiplicative on V; so the final assertion of the proposition follows from Proposition 4.4.1 oust below). H 4.4. Abelian algebras With X a compact Hausdorff space, C ( X ) is an abelian C*-algebra. Our main purpose in this section is to show that every abelian C*-algebra 2I is * isomorphic to one of the form C ( X ) . As a first step, we prove that the pure states of 2I are precisely the multiplicative linear functionals on 2I, a fact already noted in Theorem 3.4.7 for the algebra C ( X ) . 4.4.1. PROPOSITION. A non-zero linear functional p on an abelian C*algebra 2I is apure state ifand only i f p ( A B ) = p ( A ) p ( B )for all A and B in 2l.
Proof. The first assertion of Proposition 4.3.14 includes, as a special case, the fact that pure states of an abelian C*-algebra are multiplicative. Conversely, suppose that p is a multiplicative linear functional on 2I. By Proposition 3.2.20, p is bounded and llpll = p(1) = 1, so p is a state of 2l. In order to prove that pis pure, suppose that p = apl bp2,where pl, pz E 9(2I), a > 0, b > 0, and a + b = 1. With C self-adjoint in 2I,
+
Cpj(C)12= Cpj(IC)IZG pj(Opj(C2)= pj(C2) by the Cauchy-Schwarz inequality. Accordingly,
(i= 1,2),
0 = P(C2)- ")I2
+ bpz(C2)- Cap1(C) + bPZ(C)l2 >, a(a + b>CP1(C>I2 + b(a + b)CP2(C)l2- Cap1(C)+ bPZ(C)l2 = ap1(C2)
= abCp1(C) - PZ(C)l2. From this, p l ( C )= p2(C)for each self-adjoint Cin 2I;so p1 = p z , whence p is a pure state. H
The argument just given shows that a multiplicative linear functional on a (not necessarily abelian) C*-algebra is a pure state. This can also be proved by reduction to the case of algebras of the form C ( X ) and an application of Theorem 3.4.7. Indeed, if p = (1 - a)pl + ap2, we can show that p ( A ) = p l ( A ) = p 2 ( A ) ,for a given self-adjoint A in 2l,by restricting p, p l , and p2 to the C*-subalgebra generated by I and A , and identifying this algebra with C(SP(A)). 4.4.2. COROLLARY. The set 9(%) ofpurestates of an abelian C*-algebra 2I is a closed subset of the state space Y(2I).
270
4. ELEMENTARY C*-ALGEBRA THEORY
Proof. By Proposition 4.4.1,
9(%) = { p E Y ( % ) : p ( A B )= p(A)p(B) ( A , B E % ) } .
m
4.4.3. T H E O R E M . Suppose that % is an abelian C*-algebra, P('9l)is the set of allpure states of 'u, andfor each A in %, a complex-valuedfunction A is defined by &p) = p ( A ) . Then 9(%) is a compact Hausdorfs space, throughout 9(%) relative to the weak* topology, and the mapping A 4A is a * isomorphismfrom % onto the C*-algebra C(P(%)). Proof. A substantial part of the argument required to prove this theorem is already contained in the discussion of function representations of (not necessarily abelian) C*-algebras, preceding Theorem 4.3.1 1. Two new features that are special to the abelian case, those set out in Proposition 4.4.1 and Corollary 4.4.2, suffice to complete the proof. For the sake of clarity, the argument is presented below in unified form, even though this involves some repetition of the earlier discussion. From Corollary 4.4.2, P(%)is weak* compact. With A in %, it is apparent from the definition of the weak* topology that A is a continuous complexvalued function on P(%).For all A and B in %, a and b in @, and p in 9(%) n (aA + b B ) ( p ) = p(aA bB) = ap(A) + bp(B) = a&) + b&),
+
I&)l
= IP(A)I G
- -
IlAll,
A*(P)
= P(A*) = p
(4
=
m;
and ( m P ) = d A B ) = P(A)P(B)= since p is multiplicative, by Proposition 4.4.1. Since % is abelian, A is normal, so by Theorem 4.3.8(iv) there is a pure state p o of % such that Ipo(A)I = llAll. From this, &mP)9
IlAll = I 4 P o ) l
<
SUP
I m l G IlAll,
P€@(rn)
IlAll = SUP
lm= ll-4l.
P€P('U)
The function 1 is the unit of C(9(%)), since
b)= P ( 0 = 1 ric)
( P E 9(W.
The preceding discussion shows that the mapping cp: A * isomorphism from '$ into I C(9(%)).Its range
-,A is an (isomet-
$ =i{ A : A € % ) is therefore a * subalgebra of C(9(%)),contains the constant functions, and is closed since % is complete and cp is an isometry. Given distinct pure states p1
271
4.4. ABELIAN ALGEBRAS
and p2 of 2l, we can choose A in 2l so that pl(A) # p2(A), equivalently & p l ) # & p 2 ) ; so separates the points of P('u).By the Stone-Weierstrass theorem, @ = C(P(2Q). With % an abelian C*-algebra, the * isomorphism described in Theorem 4.4.3 isjust the function representation of 2l on its pure state space, constructed in the discussion preceding Theorem 4.3.11, and is by far the most important example of a function representation of a.For this reason, we shall frequently refer to it as the function representation of %. 4.4.4. PROPOSITION. I f A is a normal element of a C*-algebra %, and a E sp(A), there is a pure state p of 2l such that p(A) = a.
Proof. The set of all polynomials in I, A, and A* is an abelian * subalgebra of %, and its closure is an abelian C*-subalgebra 1.Now A has the same spectrum relative to 2l or d .From Remark 3.2.1 1 and Proposition 4.4.1, there is a pure state of d whose value at A is a. From Theorem 4.3.1 3(iv), this pure state of d extends to a pure state p of 'u; and p(A) = a. In the remainder of this section, we make use of the function representation for abelian C*-algebras in establishing the existence and properties of the function calculus associated with a normal element of a (not necessarily abelian) C*-algebra. 4.4.5. THEOREM.I f A is a normal element of a C*-algebra 'u, C(sp(A)) is the abelian C*-algebra of all continuous complex-valued functions on sp(A), and I in C(sp(A)) is defined by r ( t ) = t (t~sp(A)),rhen there is a unique * isomorphism cp: C(sp(A)) -, %such that q ( i ) = A. For eachfin C(sp(A)), cp( f)is normal, and is the limit of a sequence of polynomials in I, A, and A*. The set
Icp(f): .fE
C(SP(AN1
is an abelian C*-algebra, and is the smallest C*-subalgebra of % that contains A. Moreover, A is self-adjoint if and only if sp(A) c IF!, positive if and only if sp(A) E IF!', unitary if and only if sp(A) c C1 (= { t e e : It1 = I}), and a projection ifand only ifsp(A) G (0,I}. Proof. Let d be any abelian C*-subalgebra of % that contains A; for example, d could be the closure of the set of all polynomials in I, A, and A*. From Theorem 4.4.3, there is a compact Hausdorff space X and a * isomorphism I) from d onto C(X). With u the element $ ( A ) of C(X), SP(4 = SPdV) = SPC(X)(l(l(A))= SPC,X)(U) = {
U W :X E W .
For each f in C(sp(A)), the composite function f u is continuous throughout X ; the mapping f +fa u is a * isomorphism from C(sp(A)) into C(X). From this, and since I)- : C ( X ) + d ( c 'u) is a * isomorphism, the mapping 0
'
212
4. ELEMENTARY C*-ALGEBRA THEORY
cp: f+ t , - ' C f ~ u )is a l + - y I O u )
*
isomorphism from C(sp(A)) into
a;and
~ ( i = )
= l+-l(u) = A .
Suppose also that cp': C(sp(A)) + is a * isomorphism, and @ ( I ) = A. Then cp and cp' are linear, multiplicative, and adjoint preserving, c p ( r ) = cp'(~) = A, and cp(1) = cp'(1) = I, where 1 denotes the unit of C(sp(A)). From this, q ( f )= cp'(f) wheneverfis a polynomial in 1 , 1 , and the conjugate complex function i. Since polynomials of this type form an everywhere-dense subset of C(sp(A)), by the Stone-Weierstrass theorem, while cp and cp' are isometric (see Theorem 4.1.8($), it follows that ~ ( f=)cp'(f) for eachfin C(sp(A)). Since C(sp(A)) is an abelian C*-algebra, it results from Theorem 4.1.8(iii) that its image {cp(f):f E C(sp(A))} under cp is an abelian C*-subalgebra of a, and contains A (= cp(r)). From this, is normal for each f in C(sp(A)). Moreover, ~ ( f is) the limit of a sequence of polynomials in I, A, and A*, sincef is the uniform limit on sp(A) of polynomials in 1, I, and i. If 9? is a C*subalgebra of W,and A ~ 9 ?then , 93 contains I, A, A*, and hence contains all limits of polynomials in I, A, and A*; so cp(f)~B, for each f in C(sp(A)). Since cp is a * isomorphism and p(i) = A, it follows that A is self-adjoint (or positive, or unitary, or a projection) if and only if the same is true of the element I of C(sp(A)). Now I is self-adjoint if and only if it is real valued on sp(A); that is, if and only if sp(A) E R. Similarly I is positive if and only if it takes nonnegative values throughout sp(A) (equivalently, sp(A) E R'). Also I is unitary (or a projection) if and only if i ( t ) l ( t ) = 1 (or [ i ( t ) I 2 = i ( t ) = I(t))for all t in sp(A), and this occurs if and only if sp(A) E C1 (or sp(A) E {0,1}). H
cp(n
The * isomorphism cp: C(sp(A)) + W described in Theorem 4.4.5 is called thefunction calculus for the normal element A of the C*-algebra a. Withfin C(sp(A)), we usually denote byflA) the element cp(f) of U. Note that, iffhas the form
c m
At) =
n
1
ajktj(?)k
(fEsp(A))
j=O k=O
(that is, f = 11 ajkl'(i)k),then m
f(A)=
1
c n
ajkA'(A*)k.
j = O k=O
4.4.6. REMARK. Suppose that 93 is a C*-subalgebra of a C*-algebra W and A is a normal element of $3. We can consider two function calculi for A,
f-+fn(A) relative to
a,and
C(SPn(4)
+
a
273
4.4. ABELIAN ALGEBRAS
relative to a. Since sp,(A) = sp&i) and 98 c CU, both function calculi can be considered as mappings from C(sp(A)) into %, and it is clear from the uniqueness clause in Theorem 4.4.5 that they coincide. In this sense, the function calculus for a normal element is independent of the containing C*algebra. If A is self-adjoint, the function calculus described in Theorem 4.4.5 coincides with the one considered in Theorem 4.1.3, by the uniqueness clause in either of those theorems. W Suppose that A is a normal element of a C*-algebra CU andfis a continuous complex-valued function whose domain of definition includes sp(A). Just as in the case of self-adjoint elements, we denote byf(A) the element of CU that, in the function calculus for A, corresponds to the restriction fIsp(A). 4.4.7. PROPOSITION. If% and 9 are C*-algebras, cp is a * homomorphism from CU into 93,A is a normal element of CU, and fE C(sp(A)), then q(A) is a normal element of sp(cp(A)) G sp(A), andf(cp(A))= cp( f ( A ) ) .
a,
Proof. Since cp(A)cp(A)* - cp(A)*cp(A)= cp(AA* - A * A ) = 0, cp(A) is normal. By Theorem 4.1.8(i), sp(cp(A)) G sp(A), and the mappings
f cp(fl4):
f -+f(cp(4L being
-+
C(SP(A))
+
a,
* homomorphisms, are both continuous. For m, n = 0, 1,2,. . . , cp(A"(A*)") = cp(A)"Ccp(A)*ln,
so cp(p(A)) = p(cp(A)) whenever p (in C(sp(A))) has the form r n n
p(t)=
C C
ajktj(T)k.
j = O k=O
Accordingly, the mapping
f llf(cp(A)) -+
- cp(AA))tl: CbP(A))
+
is continuous throughout C(sp(A)), takes the value zero on the everywheredense subset consisting of polynomials p of the type just described, and so vanishes throughout C(sp(A)).
f
4.4.8. THEOREM. If A is a normal element of a C*-algebra CU and C(sp(A)), then spCf(A)) = M i ) : tESP(A)j.
Zfg E C(sp(f(A))), the composite function g 0flies in C(sp(A)), and ( g of)(A) =
& ( A ) ) , where g ( f ( A ) ) denotes the element of CU that corresponds to g in the function calculus for the normal element f ( A ) .
214
4. ELEMENTARY C*-ALGEBRA THEORY
Proof. Since the mappingf + A A ) is a * isomorphism from C(sp(A)) onto a C*-subalgebra $3 of %, we have SP(A4) = SP,(AAN
= sPc,,,,,,,(f)
= {At): t E sP(A)}.
If g E C(spCf(A))), g is continuous on the range o f f , so g o f is continuous throughout sp(A). The mappings 9
+
9 of:
C(spCf(A)))
+
C(sp(A)),
h + h(A): C(sp(A)) + 2I are
* isomorphisms, and hence so is $:9
+
(9 o n ( A ) : C(SP(A4))
+
a.
With I the identity mapping on sp(AA)), z of =f, so $ ( I )= f ( A ) . From the uniqueness clause in Theorem 4.4.5, $ coincides with the function calculus g + gCf(A)) for AA); that is, g(A-4)) = 4%)
= (9 of)(A),
for each g in C(sp(AA))). 4.4.9. EXAMPLE.If X is a compact Hausdorff space and 2I is the abelian C*-algebra C(X),each g in 2I is normal. We assert that the function calculus for g is given by
As) =f
9
(fE C(SP(9))).
For this, note first that, since sp(g) = {g(x) : x E X},the composite function f o g is continuous throughout X , when f E C(sp(g)). Accordingly, the mapping * : f +fog is a * isomorphism from C(sp(g)) into C ( X )(= %). With z the identity mapping on sp(g), I g is g, and thus $ ( I ) = g. From the uniqueness clause in Theorem 4.4.5, $ is the function calculus for g; that is, Ag) =fog for each f in C(SP(9)). With % a C*-algebra and H self-adjoint in %, the equationflt) = exp it defines a continuous function f on sp(H), and we denote the corresponding element AH) of 2I by exp iH. Since ~-
A M t ) =flt)At)= 1 (tESP(HN, we have AH)AH)* = f ( H ) * f ( H ) = I, so exp iH is unitary. Since the series 1(it)”/n ! converges to At), uniformly on sp(H), it follows by considering its partial sums that (iH)”
-.
exp iH= n=O
n!
4.5. STATES AND REPRESENTATIONS
275
In some C*-algebras 8,every unitary element has the form exp iH,with Hselfadjoint in 8 (see, for example, Theorem 5.2.5). In a general C*-algebra, there may be unitaries not of this form (see Exercise 4.6.5), but we have the following result. 4.4.10. PROPOSITION. r f U is a unitary element in a C*-algebra 8, and sp(U) is not the whole unit circle, U = exp iH for some seljhdjoint H in 8. Proof. Since sp(U) is a proper subset of the unit circle, there is a real number a such that sp(U) 5 {exp is: u < s < a
+ 2nJ.
In order to use the function calculus for U , we define a continuous functionfon SP(U) by Aexp is) = s ( a < s < a + 2n). Sincefis real valued on sp(U), and exp fit) = t
( t E sp( U ) ) ,
flu) is a self-adjoint element H of 8 ;and exp iH = exp IXU)= U , by Theorem 4.4.8.
H
4.4.11. REMARK.If A and B are normal operators whose spectra are contained in the domain of a continuous function g, and if g has a continuous inverse functionf, then g(A) = g(B) if and only if A = B ; for if g(A) = g ( B ) , then, from Theorem 4.4.8,
= M A > ) =f(g(B))= (f.g)(B) = B. As an application of this comment, we note that if A" = B" with A and B positive operators and n a positive integer, then A = B, so that a positive operator has a unique positive nth root. H A
=(f.d(A)
Bibliography : [4]
4.5. States and representations By a representation of a C*-algebra '2I on a Hilbert space A?, we mean a * homomorphism cp from 8 into B(X).If, in addition, rp is one-to-one (hence, a * isomorphism), it is described as afuithfil representation. Our main purpose in this section, achieved in Theorem 4.5.6 (the Gelfand-Neumark theorem) is to show that every C*-algebra has a faithful representation on some Hilbert space.
276
4. ELEMENTARY C*-ALGEBRA THEORY
Suppose that cp is a representation of a C*-algebra %on a Hilbert space 2. In view of our convention that * homomorphisms preserve units, and from Theorem 4.1.8, cp(1)= I , IIcp(A)II < IlAll for each A in 2l (whence cp is continuous), and IIcp(A)II = llAll if cp is faithful. The set {A E%: cp(A) = 0) is a closed two-sided ideal in U,the kernel of cp. If there is a vector x in X for which the linear subspace cp(9l)x = {cp(A)x: A E 2l}
is everywhere dense in X, cp is described as a cyclic representation, and x is termed a cyclic vector (or generating oector) for cp. It turns out that there is an intimate connection between states of 2l and cyclic representations ; and the proof of the existence of a faithful representation depends on the construction, from states, of an abundance of cyclic representations. We give a number of examples to illustrate the concepts just introduced. With X a Hilbert space and 9l a C*-subalgebra of g ( X ) ,the inclusion mapping from 2l into g ( 2 )is a faithful representation of 2l on X. Suppose that X is a closed subspace of X, and is invariant under each operator in 2l. When A E 2l, the restriction A I X can be viewed as a bounded linear operator on X, and coincides with the compression of A to X, as described in Section 2.6. Since compression is an adjoint preserving process, it is easily verified that the mapping A + A I X : 2l + B ( X )is a * homomorphism, and is therefore a representation of 9l on X. With A in a, X is invariant under both A and A*, and so reduces A ; equivalently, A commutes with the projection E from H onto X. Accordingly, the orthogonal complement X is invariant under each operator in a, and gives rise to a representation A + A I X ' of 2l on X ' . When X E 2,F 5 %?(X), and x E %,' as in Section 1.2 we denote by [ X I the closed subspace of X generated by 3; we write S X for the set {Ax: A E x E X}, and define S x to be 9 { x } .Since 2lX is invariant under each operator in 2l,the same is true of the closed subspace [2lX], so the mapping A + A I [%XI is a representation of 2l on [%X]. The representation A + A I [ax] of 2l on [ax] is cyclic, having x as a cyclic vector. When 2l is a C*-subalgebra of a(#),it is evident (as noted above) that 2l has a faithful representation on X, the inclusion mapping from 2l into B(,%). We shall see in Chapter 10 that it is nevertheless important, even in this case, to study other representations of 2l, on different Hilbert spaces. For the present, however, our main concern is to construct representations of a C*-algebra that is not at the outset presented as a self-adjoint algebra of Hilbert space operators. As an example of this type, consider the C*-algebra L , of all essentially bounded complex-valued measurable functions on a a-finite measure space (S,Zm),with pointwise algebraic structure, complex conjugation as involution, and the essential supremum norm. Eachfin L , gives rise to a multiplication operator M , acting on the Hilbert space L 2 , and it is apparent from the discussion in Example 2.4.1 1 that the mapping f -,M , is a
277
4.5. STATES AND REPRESENTATIONS
faithful representation of L , on L 2 .When the measure is Lebesgue measure on a compact interval X (cR), C ( X ) is a C*-algebra, and the mapping
f + M , : C ( X )+ g ( L * ) is a faithful representation of C ( X ) on L z . Suppose that cp is a representation of a C*-algebra % on a Hilbert space X‘ and x is a unit vector in X. With o,the corresponding vector state of @(X‘), the composite function o,ocp is a state p of 2l. Indeed, since q ( I ) = I, cp(%+) E g(X‘)+, and ( A E 2%
p ( A ) = wx(cp(A))= (cp(A)x,x>
it is evident that p is a positive linear functional on %, and p(Z) = w,(I) = 1. We prove, in Theorem 4.5.2, that each state of a C*-algebra arises in this way, from a vector state in an appropriate representation. We first need an auxiliary result. 4.5.1. PROPOSITION. If p is a state of a C*-algebra %, the set
9’= { A E % : p(A*A) = 0 ) is a closed left ideal in 2l, and p(B*A) = 0 whenever A E 2’and B E %. The equation (A
+ Y’, B + Yp)= p(B*A)
( A ,B E % )
defines a definite inner product ( , ) on the quotient linear space %/Yp, Proof. Since p is positive (and hence, also, hermitian), we can define an inner product ( , )o on 2l by (A,B)o
= p(B*A)
(A,BEW;
and YP= { A € % : ( A , A ) , = O } .
From Proposition 2.1.1(ii), Ypis a linear subspace of (A
+
Y p ,
B +2
p )
a, and the equation
= ( A , B)o = p(B*A)
defines a definite inner product on 2l/Y’. If A E Ypand B E2l, Ip(B*A)12 < p(B*B)p(A*A)= 0, so p(B*A) = 0. Upon replacing B by B*BA, it follows that
p((BA)*BA)= p((B*BA)*A) = 0,
BA E Y P ,
whenever A E 64,and B E8.Hence 2’’ is a left ideal of %, and is closed since p is continuous. W
278
4. ELEMENTARY C*-ALGEBRA THEORY
We refer to 9, as the left kernel of the state p.
If p is a state ofla C*-algebra 2I, there is a cyclic 4.5.2. THEOREM. representation np of’% on a Hilbert space S,, and a unit cyclic vector xpfor np, such that p = oxp 0 np ; that is, ( A E a).
p(A) = (n,(A)x,, xp>
the left kernel of p , the quotient linear space 2I/=Y, is a Proof. With 9, pre-Hilbert space relative to the definite inner product defined, as in Proposition 4.5.1, by (A
+ 9,B,+ zp)= p(B*A)
( A ,B E % ) .
Its completion is a Hilbert space X p . If A, B1,B2 E 2I, and B1 + 9, = B2 + 9, then , B1 - B2 E YP, A B I - AB2 €9, since 9,is a left ideal in 2l, and therefore AB1 9, = AB2 9,. Accordingly, the equation n(A)(B + LYP)= A B + 64, defines, unambiguously, a linear operator n(A) acting on the pre-Hilbert space %/9,,. Now
+
I/AII2Z -
A*A
=
+
IIA*AIIZ - A*A € % +;
hence B*(I(AI12Z- A*A)BE%+, and therefore llA1I211~ + -YPl12 -
Il4w +
9P)l12
= llAl1211B+ 9 , 1 1 2- llAB = IIAI12(B
+ .-qI2
+ Y , , B + 9,) - ( A B + 9 , , A B + 9,)
= IIA1I2p(B*B) - p(B*A*AB) = p(B*(I(AII2Z- A*A)B) 2
0,
for all A and B in 2I. Thus n(A) is bounded, with 11n(A)ll< llAll; and n(A) extends by continuity to a bounded linear operator np(A)acting on Sp. Since n(Z)is the identity operator on 2I/9,, np(Z)is the identity operator on X p .When A, B, CE%and a, b e @ , n,(aA
+ bB)(C + YP)= (aA + bB)C + 9,, = a(AC + YP) + b(BC + 9,) = (an,(A)
n,(AB)(C
+ bnp(B))(C+ 9,),
+ 9,) = ABC + LYP = n,(A)(BC + 9,) = np(A)np(B)(C
(n,(A)(B
+
9p),
+ YP),c + -Yp)= ( A B + 9,c,+ gp>= p(C*AB) = p((A*C)*B) = ( B + Y , , A * C + Yp) =(B
+ pp,n,(A*)(C + -Yp)>.
279
4.5. STATES A N D REPRESENTATIONS
From these relations, and since %/9, is everywhere dense in X,, it follows that n,(aA
+ bB) = an,(A) + bn,(B),
xp(AB) = x,(A)n,(B),
",(A)* = n,(A*). Accordingly, xp is a representation of 2I on X p . With xp the vector Z + 9, in U / Y p , xp(A)xp= x,(A)(Z
+ 9,) = A + LYp
( A E 2I).
Hence n,(%)x, is the everywhere-dense subset %/Yp of X,, and xp is a cyclic vector for x,. Moreover, (n,(A)x,,x,)
in particular,
=
(A
= p(1) =
+
-qp,z+
Yp> =p ( 4
(A€%);
1.
The method used to produce a representation from a state, in the proof of Theorem 4.5.2, is called the Gelfand-Neumark-SegaI construction, or GNS construction, and provides one of the basic tools of C*-algebra theory. The associated notation will be used frequently, sometimes without comment ; when p is a state of a C*-algebra %, the symbols Xp ,x,, and xpalways bear the meaning attached to them in the theorem. In applications, the properties of X,, x,, and x,, set out in the theorem, are more important than the details of the construction used to produce them. In a sense made precise in the following proposition, the Hilbert space X,, the cyclic representation x,, and the unit cyclic vector x, are (essentially) uniquely determined by the condition p = 0,. IT,. 0
4.5.3. PROPOSITION. Suppose that p is a state of a C*-algebra % and x is a cyclic representation of U on a Hilbert space X such that p = w, 0 n for some unit cyclic vector x for x . Zf X,, x,, and xp are the Hilbert space, cyclic representation, and unit cyclic vector produced from p by the GNS construction, there is an isomorphism U from X, onto X such that x = ux,,
x ( A ) = Ux,(A)U*
(A€%).
Proof. For each A in U, 11x(A)x112 = ( x ( A ) x , x ( A ) x ) = ( x ( A * A ) x , x ) = p(A*A) =
(n,(A*A)x,, x p ) = l
l~,~~~~,l12.
If A , B E % and x,(A)x, = x,(B)x,, it follows from the above equations (with A - B in place of A ) that n(A)x = n(B)x. Accordingly, the equation Uoxp(A)xp= n(A)x ( A E 2I) defines a norm-preserving linear operator from xp(21)xponto x(2I)x. Since [xp(%)xp] = X, and [.(%)XI = X, U , extends by
280
4. ELEMENTARY C*-ALGEBRA THEORY
continuity to an isomorphism U from Xponto 2,and u x , = uonp(z)xp= n(l)x = x . With A and B in
a,
Un,(A)n,(B)x,
=
Un,(AB)x,
= n(AB)x = n(A)n(B)x = n(A)Un,(B)x,,
Since vectors of the form np(B)xp(BE2I) form an everywhere-dense subset of 2,,it follows that Un,(A) = n(A)U, and thus n(A) = Un,(A)U*. I Suppose that 2I is a C*-algebra and that cp and $ are representations of 2I on Hilbert spaces 2 and X, respectively. We say that cp and $ are (unitarily) equivalent if there is an isomorphism U from 2 onto X such that $(A) = Ucp(A)U* for each A in 2I. If p is a state of 2I, n is a cyclic representation of 2I, and p = w, o n for some unit cyclic vector x for n, it follows from Proposition 4.5.3 that n is equivalent to the representation np obtained from p by the GNS construction. In addition, the isomorphism U can be chosen so that Ux,, = x. 4.5.4. COROLLARY. I f x is a unit vector in a Hilbert space 2,2I is a C*subalgebra of a(%), and p is the vector state mxI2I, the representation n, obtained from p by the GNS construction is equivalent to the representation A + A I[2Ix] of 2I on the Hilbert space [%XI. The isomorphism U : 2, + [ a x ] that implements this equivalence can be chosen so that Ux, = x . Proof. This follows from Proposition 4.5.3, since x is a unit cyclic vector for the representation n: A --t AI[%x], and p = w, o n. I
We prove next that the set of all representations of a C*-algebra 2I, obtained from (pure) states of 2I by the GNS construction, is large enough to “separate” the elements of 2I. 4.5.5. PROPOSITION. I f A is a non-zero element of a C*-algebra 2I, there is a pure state p of 2I such that np(A)# 0, where np is the representation obtained from p by the GNS construction. Proof. By Theorem 4.3.8(i) there is a pure state p of 2I such that p(A) # 0, equivalently (n,(A)x,, x,,) # 0, whence n,(A) f 0. I
In order to complete the proof that every C*-algebra has a faithful representation, we need the concept of a “direct sum” of representations. Suppose that 2I is a C*-algebra, (Xb)b.B is a family of Hilbert spaces, and (Pb is a representation of 2I on Xb for each b in B. When A € % , Ilfpb(A)II 6 IlAll
4.5. STATES AND REPRESENTATIONS
28 1
( b E El), so the direct sum C b & @ %(A) is a bounded linear operator acting on the Hilbert space C b e B @ X b . From the results set out at the end of the subsection on direct sums, in Section 2.6, it is apparent that the mapping
1@ c p b ( A )
9: A
is a representation of 8 on C @ z b . We call cp the direct sum of the family (fpb)bpB of representations of 8,and write cp = 1 @qb. 4.5.6. THEOREM(The Gelfand-Neumark theorem). Each C*-algebra has a faithful representation.
Proof. With 8 a C*-algebra, let Yobe any family of states of 8 that contains all the pure states. Let cp be the direct sum of the family {n,: p €Yo}, where n, is the representation obtained from p by the GNS construction. If A E and ~ cp(A) = 0, then n,(A) = 0 ( ~ €since 9~ cp(A) ) is C @ x,(A); in particular, x,(A) = 0 for each pure state p of 8,and A = 0 by Proposition 4.5.5. Hence cp is a faithful representation of 8. 4.5.7. REMARK.If cp is a faithful representation of a C*-algebra 8 on a Hilbert space X, then cp is isometric and q ( 8 )is a C*-subalgebra of a(%'), by Theorem 4.1.8(iii). Accordingly, the Gelfand-Neumark theorem can be restated as follows: if 8 is a C*-algebra, there is a Hilbert space X such that 8 is * isomorphic to a C*-subalgebra of g ( X ) . 4.5.8. REMARK.Suppose that 8 is a C*-algebra with state space Y and that 9is the set of pure states of 8.In proving the Gelfand-Neumark theorem, we showed that the representation
c 0%
,€YO
of 8 is faithful whenever 9 E YoE 9 When Yo= represen tation @=
we obtain a faithful
c en,, PSY
the universal representation of 8,which will be studied in more detail in Section 10.1. With T a state of 8,T = w, o n, for a suitable unit vector x (= x,) in Z,; thus T = w Y o @ , where y is the vector C p e Y @ y p ,in the Hilbert space Xm= CpEY@ X,, defined by y, = x, y , = 0 ( p # T). Accordingly, each state of 8 has the form wyo @, with y a unit vector in X'. Since the mapping T T o @ - carries the state space of 8 onto the state space of the C*subalgebra @(%) of B(,&,), it now follows that each state of@(%) is a vector state. This last is the most basic fact about the universal representation, from which its other properties will be deduced in Section 10.1. --f
282
4. ELEMENTARY C’-ALGEBRA THEORY
When Yo= 9, we obtain another faithful representation, $=
c on, Peg
of %. While this representation has useful properties, it is more convenient to work with a “reduced” form of $, known as the reduced atomic representation, which will be studied further in Section 10.3. W In Section 3.2, we studied characters of R and their relation to the operator algebras d t ( R ) and 210(R). A character 5 of R may be viewed as a (continuous) homomorphism of R into the unitary group of the one-dimensional Hilbert space @, where t corresponds to multiplication by <(t) on @. There are more general homomorphisms of R into the group 9 ( X )of unitary operators on a Hilbert space X. If &(#) is provided with the strong-operator topology and the homomorphism of R into 9 ( X )is continuous, we refer to the homomorphism as a one-parameter unitary group (also, as a unitary representation of R). The complete analysis of one-parameter unitary groups (Theorem 5.6.36, Stone’s theorem) must await our development of the spectral theory of unbounded self-adjoint operators in Section 5.6. In the present discussion, we shall relate one-parameter unitary groups to certain representations of go@). For this purpose, it is convenient to extend our concept of “representation on X ” to self-adjoint algebras of operators that may not be norm closed and may not contain I (as, for example, dl(R)). Again we require that our representation be a * homomorphism into B ( X ) .Those representations for which the union of the range projections of operators in the image is I are said to be essential. When the algebra does not contain Z, the assumption that a representation is essential replaces the requirement that the image of Z is the identity operator. 4.5.9. THEOREM.Zft + U, is a one-parameter unitary group on z? there is a representation cp of 910(rW) on X such that
and cp restricts to an essentialrepresentation of for eachf in Ll(R) and x, y in 8 ; d , ( R ) . If cp is an essential representation of d , ( R ) on X, there is a oneparameter unitary group t + U, on JV such that (1) is sutisfied- in particular, cp extends to u representation of a0(R). Proof. The integral in (1) defines a conjugate-bilinearfunctional on X for each f in LI(R). As
4.5. STATES AND REPRESENTATIONS
283
from Theorem 2.4.1 there is an operator cp(Lf) on JP such that Ilcp(L,-)II d llflll and (1) holds. The mapping cp of d l ( R ) into a(*)is linear. Since (cp(L,*,)X9Y) =
s
=
(f*d(t)(UIX,Y)dt
(cp(Lf )Y, x> = ( x , cp(Lf).Y);
so that cp(L,*)= cp(Lr)*. Thus cp is a representation of d l ( R ) on $.' To prove that cp is an essential representation of d l ( R ) , we note that if (cp(L,)x,y) = 0 for allfin Ll(R) and all x in H,then y = 0. For such a vector y , ( U , x , y ) = 0 for almost every t, from (1). Since t -+ U, is strong-operator continuous, t -, ( U r x , y ) is continuous and vanishes identically. Thus, ( x , y ) = 0 for all x in H , y = 0, and cp is essential on d l ( R ) . We prove next that IIcp(Lf)ll < llLfll, from which it will follow that cp has a (unique) bounded extension to 'ul(R) and from 'ul(R) to %,(R) (by assigning the identity operator to Zand extending the resultingmapping linearly). Let '$Io be the norm closure of the algebra generated by Iand cp(dl(R)). Then 'u, is an abelian C*-algebra on 2.From Theorem 4.3.8(iv), there is a pure state p of 'u,
284
4. ELEMENTARY C'-ALGEBRA THEORY
such that (p(cp(L,))I= llcp(Lf)ll. Proposition 4.4.1 implies that p is multiplicative on 910. Thus the equation pl(f) = p(cp(L,)) defines a non-zero multiplicative linear functional p Lon L,(R). From Theorem 3.2.26, there is a real number r such that p l ( n =T(r). From Theorem 3.2.27, there is a po in A o ( R ) such that p o ( L f )=I@). Thus
IIcp(Lf)ll = IP(cp(Lf))I
= lPl(nl = IT(r)l = IPO(Lf)l
IlLfll?
since Ao(R) is contained in the unit ball of 910(R)'. Suppose now that rp is an essential representation of d s , ( R )on %'. The argument set out in the preceding paragraph shows that IIcp(L,-)II < llL,-ll for all f i n L,(R). The heuristic discussion preceding Theorem 3.2.26 suggests that rp(Lfr) should be U,(p(L,), for eachfin Ll(R), and that we should define U,cp(L,-)x to be cp(L,-,)x for eachfin L,(R) and x in %' We must show that if cp(Lf,)Xl
+
* *
+ rp(Lfn)X"
= 0,
then cp(L,/,)h
+
*
. +d4f&n
= 0.
This last will follow if we establish that the norms of the sums on the left-hand sides of these equalities are equal. More generally, we show that the inner products of two such sums and of their "translates" by r (that is, their images under U,) are equal. For this it suffices to note that
( c p ( L f hcp(LgrlY) = (cp(Lg:*JX7Y) and that
= js*(r)f(s- t)dt = (s* *f>(s),
so that g: * f i = g* *f.It follows that U, is unambiguously defined and extends Since . to a unitary operator on &' (which we denote, again, by 17,) U r + s d L ~ )= x V ( L ~ ~= + CP(L(J& ~)X = urq(Lf,)X = urUsq(LJ)x,
r -, U, is a homomorphism of R into the unitary group of X. To establish the strong-operator continuity of this mapping, it will suffice to prove that, given x in some set of vectors generating a dense linear manifold in &' and a positive E, there is a positive 6 such that 11 U,x - XI[ c E when It1 < 6. For our set of vectors, we may choose {cp(Lf)u}where u E %' andfis a continuous function on R with (compact) support in a finite interval (since such functions are dense in L,(R)).
285
4.6. EXERCISES
As f is uniformly continuous and its support is a finite interval, there is a positive 6 such that I\f -J;II1 < ~/llullprovided It1 < 6. Writing x for cp(L,)u, when It1 < 6, we have that IlU,x - XI[ = llcp(LJt- L,)ull < E , for llcpll < 1, from the preceding paragraph; and t -, U, is strong-operator continuous. To complete the proof, we must show that cp, as constructed, satisfies (1). It will suffice to prove (1) with vectors u of the form cp(L,)x in place of x, where gEL1(R). Now, the mapping f -,(cp(L,)x,y) is a bounded linear functional on L,(R). From Theorem 1.7.8, there is an h in L,(R) such that (cp(L,)x,y) = j f(r)h(r)drfor all f in L,(R). If u = cp(L,)x, then (cp(LJ)u,
v> =
Y>
r r
=
{ f (f)(
/ r
U A Y )df,
where the Fubini theorem applies since (Igl* If l)lhl E Ll(R). Bibliography: [4, 191 4.6. Exercises Sz, T1,T2,and A are elements of a C*-algebra 2I, 4.6.1. Suppose that S1, and 0 < S 2 < T2. O < S1 < Ti,
Prove that llS:’2All G llT:’2AlI,
and deduce that llS;’ZAS;’211 < IIT;’2AT;’21(.
4.6.2. Suppose that 2I and 9are C*-algebras and cp is a * homomorphism , B self-adjoint and K positive, and from 2I onto L’d.Suppose that B, K E ~with
286
4. ELEMENTARY C*-ALGEBRA THEORY
let V be the exponential unitary expiB. Show that there exist A, H, U (= exp iA) in W, with A self-adjoint and H positive, such that cp(A) = B,
cp(H) = K ,
q(U)= V.
[See also Exercises 4.6.3 and 4.6.59.1
4.6.3. Suppose that D is the unit disk { Z E a3 :1zI < l}, U is its boundary {ZEC: JzI = l}, and S is the union {exp itl :tlE R, n/4 < 101 ,< 3 4 4 ) of two closed arcs in U. Consider the C*-algebras C(D), C(B), C ( S ) and the * homomorphisms cp (from C(D) onto C(U)) and 9 (from C(T) onto C ( S ) ) defined by restriction; that is,
0=flT
9(d = glS
(fEC @ ) , 9 E C(W*
Find
(i) a unitary element u of C(T) that is not of the form ~ ( ffor ) any invertible element f o f C(D); [Hint. Use the fact (from elementary algebraic topology) that there is no continuous mapping of D onto B that leaves each point of U fixed-that is, T is not a retract of D.] (ii) a projection q in C ( S )that is not of the form $ ( p ) for any projection p in C(U). [See also Exercise 4.6.59.1
4.6.4. Determine whether the following assertion is true or false : if 2I and
a are abelian C*-algebras, cp is a * homomorphism from 'illonto W,and B is an invertible self-adjoint element of 99, there is an invertible self-adjoint element A of 2I such that q ( A ) = B.
4.6.5. Use the results of Exercises 4.6.2and 4.6.3(i)to provide an example of an abelian C*-algebra 99 and a unitary element Vof W that is not of the form expiB for any self-adjoint B in 99. 4.6.6. Let 42 be the (multiplicative) group of all unitary elements in a C*algebra a. (i) Show that, if U ~ 4 and 2 111 - Lrll < 2, then U = exp iH for some selfadjoint H in 2I. (ii) Show that, if V, WE% and IIV- WII < 2, then V = WexpiH for some self-adjoint H in 2I. (iii) Let ( E %) be the set of all products of the form (exp iH,)(exp iH2) * . * (exp iHk), where { H I ,H 2 , .. .,H k } is a finite set of self-adjoint elements of 21. Show that
287
4.6. EXERCISES
is open, closed, and arcwise connected, in the (relative) norm topology on 9. 4.6.7. Suppose that 9I is an abelian C*-algebra, 4 is its unitary group (considered as a topological group with the norm topology), 4, is the connected component of 9 that contains the identity I, and U E ~Use . the results of Exercise 4.6.6(iii) to show that the following three conditions are equivalent.
(i) U = expiA for some self-adjoint A in 2l. (ii) U is connected to I by a continuous arc in 4 . (iii) U E ~ , . 4.6.8. Show that, in both of the following cases, each unitary element in the C*-algebra 2l has the form expiA for some self-adjoint A in a.
(i) 2I is the C*-algebra L , associated with a o-finite measure space. (ii) 2l is the C*-algebra C(X), where the compact Hausdorff space X is contractible (that is, there is a point xo in X and a continuous mapping f : X x [0,1] + X such thatf(x,O) = x, f ( x , 1) = xo for each x in X).[Hint. Use the result of Exercise 4.6.7.1 4.6.9. (i) Show that, if a, b are complex numbers and P, Q are projections with sum I in a C*-algebra, then the function calculus for the normal element UP+ bQ (= N ) is given by
f ( N ) =f ( 4 P + f(b)Q. (ii) Suppose that U is the unit circle {ZEC: IzI = 1) and A is the algebra of all 2 x 2 complex matrices, so that & becomes a C*-algebra when identified in the usual way with the set of all linear operators acting on the twodimensional Hilbert space @'. The Banach space C(U,&) (see Example 1.7.2) becomes a C*-algebra when products and adjoints (as well as the linear structure) are defined pointwise. Let E and F be the projections in 2l given by E(eie)=
[i ]
1 -cos8 -sin0
,
-sin8 1 +cos8
1
(0 < 8 Q 2x), and let U be the unitary element (exp ixE)(exp ixF) of 2l. Show that U(e")
+ e-"Q
= eioP
(0 Q 8 Q 2x),
where P and Q are the projections in & given by p=!['2 i
-:I,
a=![2
-i
1i].
288
4. ELEMENTARY C*-ALGEBRA THEORY
Deduce that U , a product of two exponential unitary elements of a, is not itself an exponential unitary. 4.6.10. Let 'u be a C*-algebra and 9 be a norm-closed (though not necessarily self-adjoint) subalgebra of 2l. Let A be a self-adjoint element of 2l in 39. Show that spd(A) = sppI(A).[Hint. See Exercise 3.5.28.1 4.6.11. Suppose that A is a positive element of a C*-algebra a, E and F are orthogonal projections in 2l, and EAE = 0. Show that EAF = 0. 4.6.12. Suppose that a C*-algebra 2l has a maximal abelian * subalgebra XI that is finite dimensional.
(i) Show that d is the linear span of a finite orthogonal family { E l ,. . . ,En} of projections in 2l with sum I (the identity of 2l). (ii) By considering the family { E , ,. . . , En, E j A E j } ,where A = A* E 2l, show that Ej21Ej = {aEj :U E C} for j = 1,. . . ,n. (iii) Suppose that j , k E { 1,. . . ,n} and j # k. Given A and B in 55, let a,b, c, d be the scalars determined by EjA*EkAEj= aEj, EjA*EkBEj= bEj, EjB*EkBE, = CEj, EkBEjA*Ek = dEk. Prove that a 2 0, c 2 0. By considering suitable expressions for bdE,, bdEk, and acEj, show that b = d and ac = JbI2. Deduce that (SEkAEj
+ tEkBEj)*(SEkAEj + tEkBEj) = 0
for suitable scalars s and t (not both 0). Deduce that Ek21Ej is at most one dimensional. (iv) Prove that 2l is finite dimensional. 4.6.13. Suppose that 2l is an infinite-dimensional C*-algebra. By using the result of Exercise 4.6.12, show that there is an infinite sequence { A , ,A 2 , .. .} of non-zero elements of a+such that AjAk = 0 when j # k. 4.6.14. By using the result of Exercise 4.6.12, show that, in an infinitedimensional C*-algebra, there is a positive element with infinite spectrum. 4.6.15. Let p be a state of a C*-algebra 2l. We say that p isfaithfulif A = 0 when A E W and p(A) = 0. (i) Show that Y,,,the left kernel of p , is (0) when pis a faithful state of 2l. Deduce that Xp is the completion of 2l relative to the inner product ( A , B) .+ p(B*A) ( = ( A , B)), and that np is faithful. (ii) Let p be a state of 2l such that n,,is faithful. Must p be faithful?
4.6. EXERCISES
289
4.6.16. Let A be a self-adjoint element in a C*-algebra 2I. Let p be a state of 2I such that p(A2) = P ( A ) ~(We . say that p is definite on A in this case.) Show that p(AB) = p(BA) = p(A)p(B) for each B in A. 4.6.17. Suppose that '$I, d,and { E l ,. . . ,En}are as in Exercise 4.6.12. Let pj be a state of 2I that extends the state of d assigning 1 to Ej and 0 to Ekwhen k # j. Let p be n-' pj.
c;=l
(i) Show that p is a faithful state of 2I. (ii) Choose Ajk ill 'u such that (EjAjk&)*(EjAjkEk) = nEk for those k a n d j for which Ej21Ek is one dimensional. Show that the set of EjAjkEkforms an orthonormal basis for Xp. 4.6.18. Let X be a Hilbert space of finite dimension n, { e l , .. . , e n ] be an orthonormal basis for H,Ejkbe the element of g ( X )that maps ek to ej and ek, to 0 when k' # k,and p be an element of the Banach dual space @(X)'.Define A to be the element of W ( X ) with matrix representation [ajk] relative to { e l , .. . , e n } , where ajk = p(Ekj), and t to be the normalized trace ( t ( B ) = n-' Cj"= bjj = n-' tr(B), where B has matrix [bjk] relative to { e l , .. . ,e,,]). Show that: (i) z is a faithful state of a(%); (ii) p(B) = tr(A B) for each Bin g ( X ) ,and p is a state if and only if 0 < A and tr(A) = 1; (iii) A is independent of the basis { e l , .. .,en] ; (iv) p is a pure state of W ( X ) if and only if A is a projection with onedimensional range (in which case p = ox,where x is a unit vector in the range of A); (v) p is a faithful state of g ( 2 )if and only if tr(A) = 1 and A 2 aZ for some positive scalar a. 4.6.19. With the notation of Exercise 4.6.18, suppose p is a state and E is the projection in W ( X )with range the range of A. Show that the left kernel Y of p is ,%?(X)(Z - E). 4.6.20. Suppose that phism from 2I onto 58.
'$I and
B are C*-algebras, and cp is a
* homomor-
(i) Let B 1 and B2 be elements of 9' such that B I B l = 0. By considering B1 - B 2 ,show that there exist elements A l and A 2 of 2I' such that A l A 2 = 0, cp(A1) = B1, and cp('42) = B2. (ii) Suppose that { B 1 ,B 2 ,B 3 , . . . .} is a sequence of elements of 2 ' such that BjBk = 0 whenj # k. Prove that there is a sequence {A,, A 2 , A 3 , . . .} of elements of '$I' such that AjAk = 0 when j # k, and cp(Aj)= Bj for each
290
4. ELEMENTARY C*-ALGEBRA THEORY
j = 1,2,3,. . . . [Hint. Upon replacing Bj by bjBj for a suitable positive scalar
b j , we may assume that the series C Bj and C B ; / 2 converge to elements of a. Prove, by induction on n, the following statement: there exist elements A 1 , A 2 , . . . , A , , A', of 2l+ such that AjAk = A j X , = 0 for allj, k = 1,. . ,n with j # k, and v(A 1) = BI . . ., V(An) = Bn V(Xn) = Bn + 1 9
9
*
-1
are C*-algebras, cp is a * homomorphism
4.6.21. Suppose that 2l and from 2l onto 9, and AEN',
+ Bn + 2 +
BE^,
0 6 B 6 cp(A).
(i) Show that there is a self-adjoint element R of 2l such that R 6 A and cp(R)= B. Deduce that there exist positive elements S and T of 2I such that
cp(S) = B, (ii) For n
=
cp(T)= 0,
S6T
+ A.
1,2, . . . ,let
U, = S"'(n-'Z
+ T + A ) - ' ( T + A)1/2A1/2.
Show that U,*U,,< A ,
cp(U,) = B'I2[n-'Z
+ cp(A)]-'cp(A).
(iii) By using the result of Exercise 4.6.1, show that
llu,,,- u , ~ ~ < -~~~l () ~( t- ~1l 1
,-1
T+A)-~(T+A)~/~II
z~ ++ ~ ) - 1 ( n - 1 1 +
1/2
I , llB1l2- cp(U,,)ll 6 Iln-1cp(A)1'2[n-11+ cp(A)]-'II
< in-1/2,
(iv) Deduce that the sequence { U,*U,} converges to an element A . of 2l such that 0 < A0
< A,
q ( A 0 ) = B.
4.6.22. Let 2l be the C*-algebra la(A), where A is the set { 1,2,3,4}, and define a self-adjoint subspace 4 of 2l (that contains the identity of 2l) by
4 = {fE%
+f(2) =f(3) +f(4)1.
Find a hermitian linear functional p on 4that has more than one expression in the form p = p1 - p 2 , where p1 and p2 are positive linear functionals on A and IIp(J= llplll IIp211. [See the discussion following Corollary 4.3.7.1
+
4.6.23. Let cp be a * homomorphism of one C*-algebra 2l onto another C*-algebra 93,and let p be a linear functional on 9.
29 1
4.6. EXERCISES
(i) (ii) (iii) (iv)
Show that p o cp is a state of % if p is a state of 3. Show that p is a state of if p cp is a state of %. Show that p o cp is a pure state of % if p is a pure state of 3. Show that p is a pure state of 3 if p cp is a pure state of %, 0
0
4.6.24. Let 9’be the set of states of a C*-algebra %. With A in %, let b be s U P M 4 l : PE9’4p). (i) Show that there is a pure state p of 2l such that (p(A)(= b. (ii) When A is a normal element of %, b = llAll, from Theorem 4.3.4. Find an example of a C*-algebra % and an element A of % for which b < llAll. 4.6.25. Suppose that U is a unitary element of a C*-algebra 2l and p is a state of %. Show that the equation P u ( 4 = P(UA U * )
( AE defines a state pa of % and that pa is pure if and only if p is pure. 4.6.26. Let 2l be a C*-algebra. Show that: (i) if % is abelian, then llpl - p211 = 2 whenever p1 and p2 are distinct pure states of 2l; (ii) if there is a positive real number 6 such that I1pl - p211 2 6 whenever p1 and p2 are distinct pure states of %, then % is abelian. [Hint. Let H be a selfadjoint element of %, and define U(t) = exp itH for all real t. With the notation of Exercise 4.6.25, show that p = pact)(and, hence, that p(AU(t)) = p(U(t)A) for all A in 2l) whenever p is a pure state of % and It( is sufficiently small. Deduce that p(AH - HA) = 0.1 4.6.27. Suppose that 9(%) is the set of all pure states of a C*-algebra 2l, and cp: A
+A
: % + c(9(%)-)
is the function representation of % on its pure state space 9’(2l)- (see the discussion preceding Theorem 4.3.1 1). (i) Show that cp(%) = C(9(%)-) if and only if 2l is abelian. [Hint. Use the result of Exercise 4.6.26(ii).] (ii) Show that % is abelian if and only if cp(2l) is a subalgebra of C(%W - 1. 4.6.28. Let 2l be a C*-algebra. (i) Show that if A E 2l and I Esp(A*A), then A*A - IZ has neither a left nor right inverse, (ii) Show that the intersection of the maximal left ideals in % is (0).
I
292
4. ELEMENTARY C*-ALGEBRA THEORY
4.6.29. Suppose that 2l is a C*-algebra, and each closed left ideal in 2I is a two-sided ideal. Prove that : (i) each maximal left ideal in 2l is also a maximal right ideal; (ii) if 9 is a maximal left ideal in 2l (and, hence, a two-sided ideal), then each non-zero element of the quotient Banach algebra a/$ is invertible; (iii) each maximal left ideal in 2l is the kernel of a multiplicative linear functional on 2l; (iv) if A E % and a E sp(A), then a = p ( A ) for some multiplicative linear functional p on %. Deduce that 2l is abelian.
4.6.30. Suppose that 2l is a C*-algebra with the following property: if A E N and A 2 = 0, then A = 0. (i) For each positive integer n, letf, : R + [0,1]be a continuous function such thatf,(r) = 0 when (rl < 1/2n andf,(t) = 1 when It1 l/n:Prove that f , ( A * A ) =fin(A*A)fn(A*A),
IIA - Af,(A*A)II
< n-1'2,
f,(A*A)B =fn(A*A)Bf2in(A*A),
for all A and B in 2l. (ii) By using the result of Exercise 4.6.29,show that % is abelian.
4.6.31. Suppose that A is a self-adjoint element of a C*-algebra 'u and , l ~ s p ( A ) Show . that there is a pure state p of 2l such that p ( A ) = ,l and p is definite on A (see Exercise 4.6.16). [Hint. Show that I = p o ( A ) for some pure state po of the C*-subalgebra of 2l generated by A and I , and use Theorem 4.3.13(iv).] 4.6.32. Let A be a self-adjoint element of the C*-algebra %. Suppose that for each non-zero self-adjoint Bin % there is a state p of %, definite on A, such that p(B) # 0. Show that A lies in the center of 'u. 4.6.33. Suppose that A is a self-adjoint element of the center of a C*algebra 'u. Show that for each non-zero self-adjoint element B of % there is a state p of %, definite on A, such that p(B) # 0. 4.6.34. Let A, B, and C, be elements of a C*-algebra a. Suppose that A is self-adjoint, C is in the center of 3, and AB - BA = C. Show that C = 0. 4.6.35. Suppose that Yois a (not necessarily closed) left ideal in a C*algebra %. Given a finite subset F = {A,, . . . ,A,} of Yo,define HF and VF in 2 0 by HF=AYAl + +A,*A,, VF=HF(HF+n-'I)-'. *
a
*
4.6. EXERCISES
293
Show that, with the family 9of all finite subsets of Yodirected by the inclusion relation 2 ,the net ( VF,F E E2 )is an increasing right approximate identity for 90. 4.6.36. With the notation of Exercise4.6.35, let Y be the closure of 90, so that Y is a closed left ideal in 'u. Show that { V F } is an increasing right approximate identity for 9. Prove also that, if 2 ' is a two-sided ideal in 'u, then { V F } is a two-sided approximate identity for 9. 4.6.37. Suppose that A? is a Hilbert space with dimension at least 2, y is a unit vector in X, and Y is the closed left ideal in the C*-algebra 9 ( X )defined by Y
=
{ A E a ( X ) : A y = O}.
Show that Y has no left approximate identity. 4.6.38. Provide examples, as indicated below, to show that the statements in Corollary 4.2.10 concerning closed two-sided ideals in a C*-algebra 2I are not in general valid for two-sided ideals that are not closed, even when 2I is abelian. (i) Find an ideal that is not self-adjoint in the abelian C*-algebra C(D), where D is the unit disk { Z E C : IzI < 1). (ii) Suppose that Xis the unit interval [0,1], 'u is the abelian C*-algebra C ( X ) , and X is the ideal u2I in 2I, where u( t ) = t (0 < t < 1). Find an ideal in X that is not an ideal in 2I. 4.6.39. Let 9 be a closed left ideal in a C*-algebra a.Suppose that A € 4 < 1, and AA* < P.By considering the sequence {C,,},where C, = (B + n - ' I ) - ' A , show that A = BC for some C in Y with llCll < 1. B E Y + , IlBll
4.6.40. Let 9 be a closed two-sided ideal in a C*-algebra a.Suppose that
A l , A 2 , .. .EX and I,"= I IIA,,IIZ < 1. Show that there exist elements B, C1,C2,.. . of 9 such that B 2 O,llC,ll < 1, and A, = BC,,. [Hint.Use the
result of Exercise 4.6.39.1 Y
4.6.41. Suppose that Y is a closed left ideal in a C*-algebra 'u, and 9'. Prove that
=
(i) (ii) (iii) (iv) result of
Y is a closed subset of a', A B E Y whenever A, B E % a A E Y whenever A E Y and a 2 0, A E 9whenever A E 'u and 0 < A < B for some Bin 9[Hint. Use the Exercise 4.6.39.1
+
294
4. ELEMENTARY C*-ALGEBRA THEORY
Show also that the closed left ideal Y is a two-sided ideal if and only if (v) U A U * E Y whenever A E Y and U is a unitary element of 2I.
4.6.42. Suppose that a subset Y of a C*-algebra ‘ill satisfies conditions (ib(iv) of Exercise 4.6.41. Show that there is a unique closed left ideal 9in ‘ill such that Y += g and that 2 = {A e2I :A*A €9’0). 4.6.43. Suppose A is an element in a C*-algebra 2I, IAl V,, = A(A*A + n-’Z)-”* for each positive integer n.
= (A*A)l/’, and
(i) Establish the following two inequalities :
+ n-11)-1’2 1 II .-=n - l , (A*A)”Z(A*A + n - 1 Z ) p Z - J ~i ~n - ” 2 .
IIA*A[I- (A*A)”Z(A*A
11 lAl[Z-
Use these inequalities to show that : (ii) [ [ A- V,,lAl 11 -+ 0 as n -, 0 0 ; (iii) 11 IAl - V,*All -+ 0 as n + co. A
4.6.44. Suppose 9 is a closed left ideal in the C*-algebra ‘ill.Show that E if and ~ only if ( A * A ) 1 / 2 ( =IAI)EY. 4.6.45. Suppose A is an invertible element of a C*-algebra a.
(i) Show that A = U H for some unitary element U in 2I and some positive element H in 2I. (ii) Show that the elements U and H of 2I occurring in the (“polar”) decomposition of A described in (i) are unique.
4.6.46. For each positive real number a, let f.:R f + R + be the continuous function defined by f.(t) = fa. Prove that f. is operator-monotonic increasing if 0 c a < 1 but not if a > 1. [Hint.Let X be the set of all positive real numbers for which fa is operator-monotonic increasing. Then 1 E X , and (from the discussion following Proposition 4.2.8), E Xand 2 $ X. Show that X is closed in R+\{O}. Given a,b in X,prove that abEX and (by an argument similar to the proof of Proposition 4.2.8) that %a + 6 ) ~ X . l 4.6.47. Let { u l ,u z }be an orthonormal basis in a two-dimensional Hilbert space M,and let { u l , u z } be the orthonormal basis given by u1 = 2-”Z(u1
+ uz),
uz = 2-’/Z(u1 - 242).
Define A and B in a ( M ) ’ by AX = ( x , u l ) u l ,
BX = A(x,
Ul)Ul
+ P ( X , UZ)UZ
(XE
X),
295
4.6. EXERCISES
where I and p are positive real numbers. Show that, for each continuous function f:R + + R withf(0) = 0,
f ( 4 x =f(l)(x9u1)u19
f(&
=f(wG4>h+ f ( P ) ( X ,
U 2 h .
By considering the matrix off(@ - f ( A ) , relative to the basis { u l , u,}, show thatf(A)
f(4 +AP>2 max{2f(l),0>9
f(l>Cf(J-) +f(PL)I < 2f(J-)f(P).
By considering the case in which I = 1 + E and p = 1 - E + 2c2 for a sufficiently small positive real number E , show that the function f,:t+ta:
R + j R
is not operator-monotonic increasing when a > 1 (thus re-proving a part of the result of Exercise 4.6.46).
4.6.48. Suppose that Y is a partially ordered vector space with positive cone Y and with an order unit I, and let Y be the set of all states of K (The relevant definitions are given in the discussion following Remark 3.4.4.) Prove that : +
(i) each element of Y can be expressed as the difference of two positive elements ; (ii) each positive linear functional on Y is a non-negative multiple of a state of Y ; (iii) the set of all positive linear functionals on Y is a cone in the algebraic dual space Y ‘ of Y ; the subset { p ( H ): p E Y } of R is bounded; (iv) for each H in (v) Y is a convex subset of Y ’ and is compact in the weak topology a(Y’,Y) obtained by considering Y as a separating family of linear functionals on Y ’ .
4.6.49. With the notation of Exercise 4.6.48, suppose that A is a subspace of V that contains I, so that A is a partially ordered vector space with positive cone A n Y and I is an order unit for A. Let po be a positive linear functional on A, and for each H in Y define +
1,
= SUp{po(B) : B E & ,
uH = inf{po(B) : B
B
E A ,B 2 H } .
(i) Prove that fH and uH are real numbers satisfying 1, < u H . (ii) Show that, if H EK C ER, and I , < c < u,, the equation p,(aH B) = ac po(B) (UER, B E A )
+
+
296
4. ELEMENTARY C*-ALGEBRA THEORY
defines a positive linear functional p1 on the subspace A1=
{aH+ B:aER,BEA}
of K (iii) Let C be the set of all pairs (&T) in which M is a subspace of Y that contains A? and z is a positive linear functional on M that extends po . By use of Zorn’s lemma, applied to C with the partial ordering in which “(&,zl) 6 (M,,,,)” means “4E M2 and z1 = z2 I&”, prove that po extends to a positive linear functional p on V. Prove also that, if H EY and c E R, the extension p can be chosen so that p ( H ) = c if and only if lH 6 c 6 uH. (iv) Show that each state of A extends to a state of %and each pure state of A! extends to a pure state of V.Prove also that, if a pure state of A has only one extension as a pure state of %then it has only one extension as a state of K [Hint. Note the analogy with Theorem 4.3.13.1 4.6.50. With the notation of Exercise 4.6.48, define
llHll,
= inf{aE R+ : - a l
< H < al}
for each H in V. (i) Prove that llHllr = sup{lp(H)l: ~ E Y }[Hint. . Use the result of Exercise 4.6.49(iii), with A the subspace { a l : U E R} of K] (ii) Prove that (1 1, is a semi-norm on K then (iii) Show that, if p is a positive linear functional on Ip(H)I < p(l)llHllI
( H EY).
(iv) Show that the subset
g = {api - b p , : p l , p , ~ ~ ~ : a , b ~ I Wb += , 1)~ + of the algebraic dual space Y ’ is convex, and is compact in the topology o ( Y ’ , Y ) .By means of a Hahn-Banach separation theorem, show that
a = { T E V ‘ : I T ( H ) I < llHll,(HEf)}. (v) Show that a linear functional T on ‘ 9 can be expressed as the difference of two positive linear functionals on Y if and only if there is a real number k such that
lW)I6 kllHllr
(HE
4.6.51. In the partially ordered vector space C([O, 11, R), let u be the order unit defined by u(t) = 1 (0 < t < l), and let A? be the subspace (containing u) that consists of all polynomials with real coefficients. WhenfE A, letfbe the unique extension offas a real polynomial defined throughout R. Show that the
297
4.6. EXERCISES
equation
=m
p(f) ( Y E Jo defines a linear functional p on A' that cannot be expressed as the difference of two positive linear functionals on A!. 4.6.52. Suppose that Y is a partially ordered vector space with positive cone Y +and with an order unit I, and let 11 1 , be the semi-norm defined in Exercise 4.6.50. We say that Y is archimedian if the following condition is satisfied: if H E Y and H 6 EIfor every positive real number E , then H 6 0. (i) Prove that, if Y is archimedian, then 11 1, is a norm on K V in the associated norm topology on % and
+
is closed
Y += { H E Y : ~ ( H 2 )0 for each state p of Y > .
(ii) Show that the real vector space W2 ( = Y )becomes a partially ordered vector space, and has an order unit (1,O) ( = I ) , when the positive cone V"+ is defined by ^Y'+ = { ( x , y ) e W 2 : x> 0 or x = 0 and y 2 0).
Show also that Y is not archimedian, I( 1 , is not a norm on K and Y \ Y + contains elements H such that p ( H ) 2 0 for each state p of K
4.6.53. By a Banach lattice, we mean a partially ordered vector space Y (with positive cone Y and an order unit I ) that is archimedian, is a lattice with the partial ordering induced by Y +,and is a Banach space with the norm 11 1 , defined in Exercise 4.6.50. When Xis a compact Hausdorff space, C(X,R) is a Banach lattice; the present exercise shows that every Banach lattice is isomorphic to one of the form C(X, R). Suppose that Y is the state space of a Banach lattice Y and 9-is the closure in Y of the set 9of all pure states of K When A E %define a real-valued function on 9-by d ( p ) = p(A). +
a
(i) Prove that ~ E C ( ~ - ,for R each ) A in K (ii) Show that the mapping A -,d : Y + C ( 9 -,W) is a linear isometry, with range a closed subspace A! of C ( 9 - , W ) that contains the constant functions and separates the points of 9 - .Show also that A 2 0 (in Y )if and only if d 2 o (in c(Y-,w)). (iii) Show that each pure state po of A' extends uniquely to a pure state of C(Y -,R). By using the results of Exercise 4.6.49, deduce that for all f in C ( 9 - , R) ~
p = )inf{d(p) : A E % ~>f} = sup{&)
: A E K d
(PEP).
298
4. ELEMENTARY C'-ALGEBRA THEORY
(iv) Let A, B E and define C = A v B, D the lattice operations in V ) .Show that Q p ) =max{'w
&)I,
=A A
B (where v,
b(P)= minI&4,
A
denote
&p))
c a
c
for all p in 8 - .[Hint. Since C 2 A and C 2 B, we have 2 and 2 B ;so 2 J wheref(in C ( 9 - , R)) is defined byf(p) = max{&), B(p)}. Use (iii) to show that we cannot have c ( p ) f(p) when p E 8.1 (v) Use the result of Exercise 3.5.49 to show that A = C ( F ,R) (and deduce that 8 = F ) .
c
=-
4.6.54. Suppose that % is a C*-algebra and, with the usual partial ordering, the set a,, of all self-adjoint elements of 'illis a lattice. Prove that is abelian. [Hint. Use the results of Exercises 4.6.53 and 4.6.27.1 4.6.55. Show that, if x and y are unit vectors in a Hilbert space a? and the corresponding vector states of a(%)satisfy I(w, - wy(lc E , then IIx - cyll < c l i 2 for some complex number c for which IcI = 1. Deduce that if a state w of a(X)is the norm limit of a sequence of vector states of &I(%'), then w is a vector state. [This is a special case of a result proved in Theorem 7.3.1 1.] 4.6.56. With the notation of Exercises 1.9.19 and 3.5.4, observe that I , becomes an abelian C*-algebra in which c is a C*-subalgebra and co (cc) is a closed ideal when the involution in I, is pointwise complex conjugation (that is, {x,,}* is {X,,}for each bounded complex sequence {x,,}). Note also that the equation po({xnl) = lim xn
({xnl EC)
n-r w
defines a pure state po of c. (i) Let p be any pure state of I, that extends po (Theorem 4.3.13(iv)). Show that p is a multiplicative linear functional on I,, and that lim inf x,, < ~({x,,})< lim sup x, for every bounded real sequence {x,}. (ii) Show that the multiplicative linear functionals p satisfying the conditions set out in (i) are precisely the elements of p(N)\N (see Exercises 3.5.5 and 3.5.6). 4.6.57. Let a? be a Hilbert space and let Yobe the set of all sequences {ul ,u 2 , .. .} of elements of 2 that are weakly convergent to 0. Let p be a pure state of the C*-algebra I, , with the properties set out in Exercise 4.6.56(i).
299
4.6. EXERCISES
(i) Show that Yobecomes a complex vector space when a{ u,} + b{u,} is defined to be {au, + bun} for all {u,} and { v , } in Yo. (ii) Show that, if {u,}, {u,} €To, then the complex sequences {(u,,, u , ) } , { Ilu,ll} are in 1,. Show also that the equation
u,>>) defines an inner product ( ,)o on Y o and , the corresponding semi-norm 11 on Yois given by ({%}?
Iu,}>o
= P(I(Un3
Il{un}llo = P({IIunll}).
(iii) Let Nobe the subspace {{u,} €Yo : II{u,}llo = 0) of Y o With . TI the quotient space Yo/No, let ( , be the definite inner product on Yl , and 11 the corresponding norm, derived (as in Proposition 2.1 .l) from ( , )o. Let 9be the completion of the pre-Hilbert space Yo/No so obtained, and use the same symbols, ( , and 11 I l l , for its inner product and norm. Show that, for each T in a(%), the equation no(T>{un>= {Tun}
(Iun}EzO)
defines a linear operator no(T)acting on Yo,and Il.o(T){un}llo
G IITII Il{un>llo.
Deduce that the mapping { u,)
+ Mo
+
{Tun]
+ Jlro :
2%
+9 1
is well defined and extends uniquely to a bounded linear operator n(7')acting on 9 with lln(T)ll < IITI(. (iv) Show that the mapping 71: T-,
n(T) :
a(%) a(9) -b
is a representation of @(H). (v) Show that the kernel of z is the ideal 3? consisting of all compact linear operators acting on H.[Hint. Use condition (ii) in Exercise 2.8.20 as the defining property of a compact linear operator.] is the 4.6.58. Suppose that Jf is a separable Hilbert space, X (ca(%)) ideal consisting of all compact linear operators acting on %, and {e, : q E Q} is an orthonormal basis of % indexed by the (countable) set Q of all rational numbers. Let cp be a representation of g(%)that has kernel X (see Exercise 4.6.57). For each real number t , choose a sequence { q ( l ) ,q(2), . . .} of rational numbers (with no repetitions) that converges to t, and let El be the projection from YF onto the subspace generated by {e,(l),e,,2,, . . .}. Show that: (i) {El : t E W} is a commuting family of projections such that El 4 X, EsElE X ,whenever s, t E W and s # t ;
300
4. ELEMENTARY C*-ALGEBRA THEORY
(ii) the Hilbert space on which c p ( W ( 2 ) )acts is not separable; (iii) the result of Exercise 4.6.20(ii) cannot be extended to the case in is which { B , ,B 2 , .. .} is replaced by an uncountable family, even when abelian. 4.6.59. Suppose that {eo,el, e 2 ,. . .} is an orthonormal basis in a separable Hilbert space X , and Wis the isometric linear operator on X defined (as in Example 3.2.18) by Wej = ej+ ( j = 0,1,2,. . .). Let X be the ideal in g ( X ) that consists of all compact linear operators acting on X, and let 7c be a representation of g ( X )that has kernel X (see Exercise 4.6.57). .
(i) Show that W* W = I and WW* = I - E o ,where Eo is the projection from 2 onto the one-dimensional subspace containing eo . (ii) Show that n(W) is a unitary operator U . (iii) Show that there is no invertible operator T in g ( X ) such that n(T) = U. [Hint. If K EX and T = W + K, use the relation T* W = I + K* W and the result of Exercise 3.5.18(iv) to show that T is not invertible.] (iv) Show that there is no normal operator N in B ( H ) such that n ( N ) = U . [Hint. Suppose that K E X and W + K is a normal operator N . Let A?be the null space of N, so that .At is also the null space of N * (Proposition 2.4.6(iii)). Use the relation W*N = I + W*K and the properties of compact linear operators to show that .& is finite dimensional. Prove also that N + E (= W + K + E ) is normal and one-to-one, where E is the projection from X' onto A. Hence reduce to the case in which Jt = {O}. In this case, use the relation N* W = I + K* W and the properties of compact linear operators to show that N is invertible in W ( X ) , contradicting the conclusion of (iii).] 4.6.60. Suppose that 9 is a proper closed two-sided ideal in a C*-algebra 9I, ( V L } is an increasing two-sided approximate identity for 3, and cp: 9I ---i %/9is the quotient mapping from 9I onto the Banach algebra %/Y (Proposition 3.1.8). Prove that: (i) 9I/9has an involution defined by cp(A)* = cp(A*) ( A EJU); (ii) the usual quotient norm on 2l/Y satisfies ((cp(A)((= lim ( ( A- A VL(l= lim ( ( A- VLAJ( 1
( A €91);
1
(iii) with the quotient norm and the involution defined in (i), %/Y is a C*algebra. [This important result will be proved by another method in Theorem 10.1.7.1 4.6.61. Show that, if 9 is a proper closed two-sided ideal in a C*-algebra 2l, then there is a representation of % that has kernel 9.
30 1
4.6. EXERCISES
4.6.62. Suppose that 2I and 93 are C*-algebras, cp is a from CU into a, and 4 is a closed two-sided ideal in CU.
* homomorphism
(i) By adapting the proof of Theorem 4.1.9, show that cp(Y) is closed in &l. (ii) Let % be the C*-subalgebra { c I + S : CEC,SEY}of %, and let X be the kernel of the * homomorphism cp I V: % -+ &l. By considering the induced * isomorphism $ from the C*-algebra %/Xinto B, give a second proof that ~(9 is closed ) in B. 4.6.63. Suppose that 4 and 9are closed two-sided ideals in a C*-algebra %. By using the results of Exercises 4.6.6O(iii) and 4.6.62, show that the ideal
Y
+ 9 is closed in 2I.
4.6.64. Suppose that 9 and 9are closed two-sided ideals in a C*-algebra = B + CE%+,where B E Y and C E ~ .
a, and A
(i) Show that A = S + Tfor suitably chosen self-adjoint elements S of 4 and T of f . (ii) Suppose that E > 0, and define H = IS1 + IT1 + EI, D = A”2H-”2, Ti
S1 = DISID*,
= DITID*,
where IS1 and IT1 denote the positive square roots of S 2and T2, respectively. Prove that D*D Q I, and deduce that A - EIQ S1
+ Ti < A .
Prove also that S1 E
Y+,
TIE$+,
IlSlll Q IlAll,
IlTlll Q IlAll,
and 0 Q A l 6 EI,where A,
= A - S1
- Ti = ( S - S , ) + ( T - T 1 ) ~ ( +9 9)’.
(iii) By repeated application of the result of (ii), show that A can be expressed in the form X + Y , with X in 4 and Y in 9+ . [This exercise shows that (Y+ 2)’= Y 4 when Y and 9 are closed two-sided ideals in a C *-algebra.] +
+
+
+
4.6.65. Suppose that CU is a C*-algebra and 6: 2I+ 2I is a linear mapping such that 6(AB) = Ah(B)
+ 6(A)B
( A , BE%).
(Such a mapping 6 is called a derivation uf2I.) Let Y be the set of all elements A
302
4. ELEMENTARY C*-ALGEBRA THEORY
in '% for which the linear mapping
T+d(AT) :
%4%
is continuous. (i) Show that if A € % , then A €9if and only if the linear mapping
T + A 6 ( T ) : %4% is continuous. (ii) Show that 9 is a closed two-sided ideal in %. (iii) Show that the restriction 819 is continuous. [Hint.If d l 3 is discontinuous, there is a sequence { A l , A 2 , .. .} in 9 such that C llAjllZ 1 and Ild(Aj)ll -+ co.Use the result of Exercise 4.6.40 to obtain a contradiction.] (iv) Show that the quotient C*-algebra %/9 is finite dimensional. [Hint. Suppose the contrary, and deduce from Exercises 4.6.13 and 4.6.20(ii) that the unit ball of % contains a sequence {Sl,S 2 , .. .} of positive elements not in 9 such that SjSk = 0 whenj # k. Prove that there is a sequence { T1, T 2 , .. .} in % such that ( j = 1,2,. . .). IId(S;Tj)II a j IIS(Sj)II IITjII < 2-j,
-=
+
Obtain a contradiction by considering Sj6(C),where C = 1SjTj.] (v) Deduce that 6 :'% + % is continuous. 4.6.66. Suppose that % is a C*-algebra and 3 is a Banach space. We describe 3 as a Banach %-module if there are bounded bilinear mappings (A,x)+Ax,
(A,x)+xA : % x 34%
such that Zx = x l = x for each x in %, and the associative law holds for each type of triple product A l A 2 x , A l x A 2 ,x A I A z .By a derivation from % into a Banach %-module 3, we mean a linear mapping 6: % + 3 such that
6(AB) = A6(B)
+ d(A)B
( A ,B E % ) .
Adapt the program set out in Exercise 4.6.65 to prove that every derivation from a C*-algebra '% into a Banach '%-module is continuous. 4.6.67. Show that the set B of pure states of B ( 2 )is weak* closed when
4 is finite dimensional. 4.6.68. Let J? be a Hilbert space. Show that each vector state oxof g ( 2 ) is pure. 4.6.69. Suppose 2 is an infinite-dimensional Hilbert space, X i s the ideal ofcompact operators in g ( X ) ,P i s the set of pure states of a(#),and Yois the set of vector states of W ( 2 ) .
4.6. EXERCISES
303
(i) Use Exercises 4.6.57 and 4.6.23 to show that there is a pure state p of g(%)that is 0 on X. (ii) With a in [0,1], x a unit vector in &‘,and p in 8- and 0 on X, let w be the state am, + (1 - a)p of B(&‘).Show that w is in 8 - ,the pure state space of 9(2), and that o is in 9’;[Hint. . Use Corollary 4.3.10 to approximate p by a vector state w,,, of B ( X ) .With A l ,. . . ,A , self-adjoint operators in ( B ( J V ) ) ~ and E the projection with range [x, A l x , . . . ,A,x], estimate I(w - wz)(Aj)l where z = a1I2x + (1 - u)li2yand y = ll(I - E)y’ll- ‘ ( I - E)y’.] (iii) Conclude that 8 is not weak* closed (that is, 9 # 8 - )when &‘ is infinite dimensional. 4.6.70. Let CU be a C*-algebra and 3 be a self-adjoint subalgebra of containing I. Suppose that for each pair p l , p2 of distinct states of ‘$ there I is a B in B such that p l ( B ) # p2(B)(that is, B separates the states of a).Show that B is norm dense in CU.
CHAPTER 5 ELEMENTARY VON NEUMANN ALGEBRA THEORY
Those C*-algebras (von Neumann algebras) that are strong-operator closed in their action on some Hilbert space play a fundamental role in the subject. Historically they were the first class of such operator algebras introduced. Their study will occupy us in this and the following four chapters. In the present chapter we develop the elements of the subject. The strengthened closure assumption on the algebra entails significant structural changes. On the technical level, the strong-operator closed algebras abound in projections; while the general C*-algebra may contain no projections other than 0 and I. In a less technical (and deeper) sense, the passage from the general to the strong-operator closed C*-algebra corresponds to the passage from the algebra of continuous functions to the algebra of (bounded) measurable functions. This correspondence can be made precise in the commutative case and lends force to the interpretation of the theory of von Neumann algebras as “non-commutative measure theory.”
5.1. The weak- and strong-operator topologies Recall that the strong-operator topology on B ( 2 ) has a base of neighborhoods of an operator To consisting of sets of the type V(To:x,) . . . )x m ; &= ) { T € a ? ( X ) : l l ( T -T0)Xjll < & ( j = 1,..., m)},
where xl,. . .,x, are in 2 and E is positive. Thus the net {Ti} is strong-operator convergent to To if and only if {[I( Tj - To)xll}converges to 0 for each x in X, that is, if and only if the net { Tjx}of vectors in &‘converges to Toxfor each x in 2. (See the discussion following Proposition 2.5.8 and the comments in Remark 2.5.9.) Another topology on a?(*) will be important for us. 5.1.1. DEFINITION. The weak-operator topology on A?(&) is the weak topology on B(#)(in the sense described in Section 1.3) induced by the family 304
5.1. THE WEAK- AND STRONG-OPERATOR TOPOLOGIES
305
flW of linear functionals w , , ~ :W ( X )-+ C defined by the equation
w,,,(A) = ( A 4 . Y )
(X,YEXO, A E2wf)).
rn
If o,,,(A) = 0 for all x and y in X, then A = 0, whence & is a separating family of linear functionals for B(X).It follows that the weak-operator topology on 3?(X)is a locally convex topology determined by semi-norms (w,,,(A)I.The family of sets of the form V(TiJ: w,,,,,
Y
.
*
3
Wxm,ym
;4
= { T e B ( X ) : I ( ( T -T o ) x j , y j ) J < ~ (= j1,.
. . ,m)},
where E is positive and x , , . . . ,x,, y,, . . .,y, are in X, constitutes a base of convex (open) neighborhoods of To in the weak-operator topology. Since ( ( ( T- To)x,y ) ( < E when Il(T - To)xll < ~ ( 1 ~ ~ y ~ each ~ ) - open l , set relative to the weak-operator topology is open relative to the strong-operator topology. Hence the weak-operator topology is weaker (coarser) than the strong-operator topology. (See Exercise 5.7.2 where it is noted that this relation is “strict.”) As a consequence, the requirement that a subset of B ( X ) be strong-operator closed is less stringent than the requirement that it be weakoperator closed. An important exception to this occurs in the class of convex sets of operators.
+
5.1.2. THEOREM.The weak- and strong-operator closures of a convex subset X of W ( X ) coincide.
Proof. An operator in the strong-operator closure of X is in the weakoperator closure of X. Suppose A, in the weak-operator closure of X, and vectors x l r . .. ,x, in X are given. Let 9 be the direct sum X @. . . @ Xof X with itself n times. For Tin g ( X ) ,let T(y,, . . . ,y,) be ( T y l , .. . ,Ty,) (that is, T = T g . . @ T). Then { T : Tin X } is a convex subset 2 of a(*); and 2iis a convex subset of 2,where 1= ( x ~ , . ,.x,). As A’ is in the weak-operator closure of .$,A ’ i is in the weak closure of 21(in 9).From Theorem 1.3.4,21 is in the norm closure of 2 2 (in 2). Thus for some Kin X, llKxj - Axjll is small for eachj in { 1,. . . ,n}. It follows that A is in the strong-operator closure of X and that the weak- and strong-operator closures of X coincide. W By polarization (see 2.4(3)) the span of the functionals wx,x (= w,) coincides with the span of Fw, so that the semi-norms defined by ( ( A x , x ) l determine the weak-operator topology on W ( X ) . In fact, restricted to (.B(X)), ,the unit ball in g ( X )(or, equally, any bounded subset of &?(if)the ), weak-operator topology is determined by the semi-norms ( ( A x j ,xk)l where ( x j )spans a dense linear manifold in X. For this, note that AX, x ) ( is small (with A in (g(#)),) provided ( ( A y ,y)l is small with y (in the span of ( x j ) ) sufficiently near x .
306
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
Since ( B A x , y ) = ( A x , B*y), the mappings A .+ BA and A AB ( B e g ( # ) ) of B ( 2 )into g ( H )are weak-operator continuous. That is, left and right multiplication by B are weak-operator continuous. We note, too, from Theorem 1.3.1, that each weak-operator continuous linear functional on g ( H ) lies in the linear span of Fw. Another useful aspect of the weak-operator topology resides in a special compactness property it possesses.
5.1.3. THEOREM.The unit bail compact.
of B ( 2 ) is weak-operator
Proof. Let D I , be the closed disk of radius IIxJJ* llyll in the plane C of complex numbers. The mapping which assigns to each Tin ( W ( 2 ) ) , the point { ( T x , y) : x, y in 2')of IDx,, is a homeomorphism of with the weak-operator topology, onto its image Xin the topology induced on X by the product topology on ID,,, (from the very definition of these topologies). As D, is a compact Hausdorff space in the product topology (Tychonoff's theorem), Xis compact if it is closed. If b is a point in the closure of X and x l , y l , x z , y z are elements of 2, then, for each positive number E , there is a Tin (a(&)), such that each of
n,,, n,,,
nx,,
la ' b(xj,yk) - a(Txj,yk)l,
Ib(ax1 + xz,yj) - (T(ax1 + x A y j ) l , is less than
E,
Ib(xj,yk) - (Txj,yk)l, Ib(xj,ayl
+ YZ) - ( T x j , a y , + ~ 2 ) l
wherej, k = 1, 2. It follows that Ib(ax1
+ X2rY1) - a . &l,Yl)
- b(X2rYl)I
< 3&
and
I&,
yay1
+Y2) - a
*
K X l ,Yl>
- 4 x 1 9YdI c 3 E .
Thus b(ax1 + xz,y1) = a * b ( x 1 , y A + 4 X 2 , Y l ) and b ( x , , a y , + y z ) = a . b ( x , , y , ) b(xl,yz). In addition lb(x,y)l < llxll . Ilyll, since b ( x , y ) I~D,,,. Hence b is a conjugate-bilinear functional on H bounded by 1. From the Riesz representation (Theorem 2.4.1) of such bilinear functionals, there is an operator Toin (B(H))lsuch that b(x,y ) = (Tax, y) for all x and y in H.Thus b E X,X is closed, Xis compact, and (a(&')), is weak-operator compact.
+
The weak-operator topology and the Riesz representation of bounded conjugate-bilinear functionals on a Hilbert space appear once again in establishing a key order-topological property of g ( 2 ) . It concerns nets { I f a , A, < } of self-adjoint operators If, for which the operator-ordering and the partial ordering of the directed index set A agree (that is, Ha < Ha, if a < a'). We say that such nets are monotone increasing (decreasing if the operator-ordering reverses the ordering of A). Although it will prove useful to
5.1. THE WEAK- AND STRONG-OPERATOR
TOPOLOGIES
307
have the result that follows for nets, for present purposes the simpler circumstances of sequences would suffice. 5.1.4. LEMMA.If { H a } is a monotone increasing net of self-adjoint operators on the Hilbert space A? and Ha 6 kf for all a, then {Ha} is strongoperator convergent to a self-adjoint operator H , and H is the least upper bound of {Ha).
Proof. Since the convergence of { H a } and that of { H a , a2 ao} are equivalent, we may assume that { H a } is bounded below (by Hao)as well as above. Thus - ~ ~ H a o ~Ha~ < I a' . Thus H 2 Ha, for all a , , since ( H a l x , x ) < lima,(Harx,x)= ( H x , ~ ) If . a 3 a', then 0 6 H - Ha 6 H - H a p ;and 0 < ( ( H - H , ) x , x ) = ll(H - Ha)1/2~112 ,< ( ( H - H a , ) x , x )3 0.
Hence { ( H is strong-operator convergent to 0. The strong-operator continuity of multiplication on bounded sets of operators (see Remark 2.5.10) allows us to conclude that { H - H a } is strong-operator convergent to 0. We have noted that His an upper bound for { H a } .If K 2 Ha for all a, then ( K x , x ) > (Hax, x ) -),, ( H x , x ) . Hence ( K x , x ) > ( H x , x ) for all x in 2 ; and K 2 H . It follows that H is the least upper bound of { H a } . If { K a } is a family of positive operators acting on X , the net of finite subsums of C Ka is monotone increasing. From the preceding lemma, this net is strong-operator convergent (equivalently, the family {KO} is summable, in the sense of the discussion preceding Proposition 1.2.19, in the strong-operator topology) if the finite subsums are bounded above. 5.1.5. LEMMA.If A is a bounded operator on the Hilbert space 3P and 0 < A < I , then {A1/"} is a monotone increasing sequence of operators whose strong-operator limit is the projection on the closure of the range of A .
Proof. Passing to the function algebra representing %(A), the C*subalgebra of 2l generated by A and f (see Theorem 4.1.3), {All"} is seen to be a monotone increasing sequence bounded above by 1.From Lemma 5.1.4, {A'/"} has a strong-operator limit E, which is the least upper bound of {A1/"}.Hence {A2/"}has E 2 as strong-operator limit. But {A'/"}(= {A2I2"})is a subsequence of { A 2 / " }so , that E = E 2 , and E is a projection. Applying the Stone-Weierstrass theorem in the function algebra representing %(A), we see that A"" is the norm limit of polynomials, without constant
308
5 . ELEMENTARY VON NEUMANN ALGEBRA THEORY
term, in A . Thus A1l"x= 0 if A x = 0; and Ex = 0 when A x = 0. On the other hand, if Ex = 0, then 0 = EX,^) 2 ( A 1 ' " x , x )= llA1/2"~112 3 0, so that AliZnx= 0 and A x = 0. It follows that the (self-adjoint) operators E and A have the same null space. Hence E = R(A) (see Proposition 2.5.13). H A C*-algebra 9,acting on a Hilbert space 2,that is closed in the weakoperator topology and contains I is said to be a uon Neumann algebra. If the center of 9 consists of scalar multiples of I, we say that W is a factor.
5.1.6. EXAMPLE.In Section 4.1 we noted that the algebra d ofmultiplications by essentially bounded (measurable) functions on L,(S, X m )( = L,) is a C*-algebra. Since M,M, = M,, = M,M,, d is abelian. Let en be the characteristic function of the subset S,, of finite measure in S, where the sets S,, are so chosen that they are mutually disjoint with union S. Then Me" is a projection and E M e n= V Men= I (see Proposition 2.5.8 and. Example 2.5.12). If Tis a bounded operator on Lz commuting with d and Ten = fn,then (by a measure-theoretic argument similar to that used in proving 2.4(13)) fn is essentially bounded; for TMe.(g) = T(gen) = MgT(en) = f n g (= Mfm(g)),
for each essentially bounded g in Lz . Since TM,, and M f nare bounded and the essentially bounded functions are dense in L,, TM,, = Mfn. From Example 2.4.11, IIM,,II is the essential bound offn. As llMfnll < IlTll, the function f defined by f IS, = fn I S, is essentially bounded. Note for this that f, = T(e,,)= T(e, . en)= e,,T(e,) = en .f,,. Thus, M,Men = M,,, = M," = TM,, for each n. Since EMen= I , M , = T. It follows that d is not a proper subset of a commuting family of bounded operators on L,. We say that d is maximal abelian in this case. Since { T :((TA - A T ) x , y ) = 0 } is a weak-operator closed set and d is the intersection, over all A in d and all x and y in L , , of these sets (from the facts we established above); d is weak-operator closed. In the terminology just introduced, d is a von Neumann algebra. We now specialize to the case where S is the interval [0,1] and m is Lebesgue measure, so that the set (M,:fe C ( S ) } is a C*-subalgebra '$I of d. We assert that '$I has weak-operator closure fl (illustrating the fact that the passage from a C*-algebra ,of operators to the von Neumann algebra it generates is analogous to the transition from continuous functions to bounded measurable functions). For this, note that each f in L , is the limit almost everywhere of a sequence {fn} ofcontinuous functions such that Ilfnllm < ~ ~ f ~ ~ for each n ; and M , is the strong- (and hence weak-)operator limit of {M,,}, by the final paragraph of Remark 2.5.12.
5.2. SPECTRAL THEORY FOR BOUNDED OPERATORS
309
5.1.7. EXAMPLE.The algebra B ( 2 )of all bounded operators on a Hilbert space is an example of a factor. The projections Eo with one-dimensional range are minimal projecfions in i@(&?) (contain no other non-zero projections in g ( 2 ) ) We . shall note, at a later point (Theorem 6.6.1), that the property of having a (single) minimal projection characterizes a(%)among the factors (up to * isomorphism). On the other hand, if { En}is a family of minimal projections in B ( 2 )whose ranges correspond to an orthonormal basis { x " } for &? (that is, the range of En is spanned by x,,), then { E n }is an orthogonal family with sum I. Although we imposed the requirement that a von Neumann algebra contain I (for convenience and for simplicity of statements of theorems), this requirement entails no essential restriction. The result that follows permits us to consider the action of a weak-operator closed algebra on S,stable under the adjoint operation, on the range of its maximal projection -where it becomes a von Neumann algebra in our sense. 5.1.8. PROPOSITION. Zf2I is a weak-operator closed self-adjoint algebra of operators acting on the Hilbert space 2,the union and intersection of eachfamily of projections in 2I lie in There is a projection P in 2I, larger than all other projections in 2I, such that P A = A P = A for all A in 2I.
a.
Proof. Since R(T*) = R(T*T), from Proposition 2.5.13, once we note that the range projection of each positive self-adjoint operator in 2I lies in a, the same is true for each operator in 2I. Now Lemma 5.1.5 assures us that R(A) is the strong- (hence, weak-)operator limit of {A""}, when 0 < A < I. With A in a,Al'" E a, since 2l is norm closed. Thus R(A)E 2I. It follows that R( 7') E M for each T i n a. If E and F are projections, R(E + F ) = E v F (see Proposition 2.5.14). Thus E v F E 2I, if Eand Fare in 2I. It follows that finite unions of projections in 2I lie in 2I. If {E,} is a family of projections in 2I, the unions of finite subfamilies lie in M and form a monotone increasing net bounded above by I. From Lemma 5.1.4, this net has a least upper bound E which is its strongoperator limit. Thus E E % ; and, from Remark 2.5.9, E = V, E,,. Again, from 2.5(4) (applied in the Hilbert space P(H)), A ,E, = P - V,(P - EJ, where P is the union of all projections in M. From the foregoing, P E 2l and A ,E, E 2I. As Pcontains the range projection of each A in a,P A = A and PA* = A*. Thus PA = AP = A; and P is a multiplicative unit for M. 5.2. Spectral theory for bounded operators
We use the special lattice-theoretic properties of abelian von Neumann algebras to identify important features of their representing function systems.
310
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
These features are used, in turn, to develop the classical form of the spectral theorem for bounded self-adjoint operators. We treat the case of unbounded operators in Section 5.6. In the case of finite-dimensional Hilbert spaces, the results of this section reduce to the familiar diagonalization of self-adjoint (and normal) matrices relative to orthonormal bases. Zfd is an abelian von Neumann algebra, then d z C ( X ) , 5.2.1. THEOREM. where X is an extremely disconnected compact Hausdorff space.
Proof. From Theorem 4.4.3, d 2 C ( X ) for some compact Hausdorff space X . Since the isomorphism transports the operator order on the set of selfadjoint operators in d to the pointwise order on the set of real-valued functions in C(X), each increasing net {h}in C(X), bounded above by k , corresponds to an increasing net {A,,} of self-adjoint operators in d ,bounded above by kZ. From Lemma 5.1.4, { A , } has a least upper bound A in d .There is anf in C(X),corresponding to A , which is the least upper bound of {f.}.If { A } is an arbitrary family of real-valued functions in C ( X )bounded above by k , the least upper bounds of finite subsets of {h}form an increasing net, bounded above by k , whose least upper bound (in C ( X ) )is a least upper bound (in C ( X ) ) for {f.}.From Theorem 3.4.16, Xis extremely disconnected. H
Our next result describes the spectral resolution of a (bounded) self-adjoint operator. 5.2.2. THEOREM. ZfA is a self-adjoint operator acting on a Hilbert space X and d is an abelian von Neumann algebra containing A , there is afamily {E,} of projections, indexed by R, in d such that
(i) E, = 0 if1 < - IlAll, and En = Z ifllAll < A; (ii) El < EATif1 < 1’; (iii) E, = E,,; (iv) AE, < AE, and I(Z - E,) < A(Z - E,)for each A ; (v) A = jl!
5.2. SPECTRAL THEORY FOR BOUNDED OPERATORS
31 1
values not exceeding A, then Y c X \ f - ' ( ( A , co)) so thatf-'((A, a))E X\Y. As Y is open, X\ Y is closed; andf- '((A, a))-E X\Y .Thus Y c X, ;and X,is the largest clopen set in X on whichftakes values not exceeding A. If e, is the characteristic function of X,, e,EC(X) since X , is clopen; and E, in d corresponding to el is a projection. Conditions (i), (ii), and (iv), in the statement of this theorem, are apparent from the definition of X, and the properties of the isomorphism of d with C ( X ) .(For the second inequality in (iv), we make special use of the continuity offto conclude thatftakes values greater than or equal to A on f-'((A, 00))- .) To prove (iii), note, first, that el is the greatest lower bound in C(X) of {e,.: 1' > A} ; for if h is a (positive) lower bound in C ( X )and h ( p ) # 0, then h is not 0 at each point of the clopen set h - '((+h(p),2h(p)))- (= Y). Since h < el. if A < A'; Y c X,.. Thus Yis a clopen set on whichftakes values not exceeding L', for each A' greater than A-that is,ftakes values not exceeding A on Y. From our characterization of X, as the largest such clopen set, Y E X, and h < e l . From (ii), e, is a lower bound for {e,.: A' > A}, so that el is the greatest lower bound of this set in C ( X ) . It follows that E, is the greatest lower bound of { EA,: A' > A} in d . From Proposition 5.1.8, A , > E,, E d . From Corollary 2.5.7, A,,>, EL. is the greatest lower bound in a(*) of {Ex:A'> A}. Thus EA = AA*,AEAP. To prove (v), choose A. less than - l.411 and let {Ao, Al,. . .,A,} be a partition of [Ao, llAll] (so that A, = IlAll). If [Aj- 1 , Aj] n sp(A) # 0, let A; be a point of this intersection-otherwise, let A; be Aj-l. If this intersection is empty, f - ' ( [ A j - l , Aj]) = 0, since sp(A) is the range off. Thus f-l((Aj-17
a))= f - l ( ( A j , a))
and e A J -= , e A J It . follows that Cj"= A; (el, - e,,-,) (= h) is a linear combination of mutually "orthogonal" characteristic functions e,. - e, with coeficients in sp(A). Now each p in X lies in exactly one set Xnj\ (= Yj), j = l , . . . ,n, since X,, = 0 and X," = X. If P E Yj, then h ( p ) = A; and Aj-' < f ( p ) < A j . Hence Ilf- hll < maxj{lAj - Lj-'1}, and (v) follows.
A family {E,} of projections indexed by R, satisfying (i')
A l s l w E ,= 0 and V L E a E = , I
and (ii), (iii) of Theorem 5.2.2is said to be a resolution ofthe identity. Since (i) of Theorem 5.2.2 guarantees (i') above, the family {E,} determined in the argument of Theorem 5.2.2 is a resolution of the identity. If there is a constant a such that El = 0 when A < - a and E, = Zwhen a c A (as there is in the case of Theorem 5.2.2), we say that {E,} is a bounded resolution of the identityotherwise we say that { E,} is an unbounded resolution of the identity. At this point, we have a resolution of the identity for A in each abelian von Neumann algebra containing A . In Theorem 5.2.3, we show that a resolution of the
312
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
identity satisfying either (iv) or (v) of Theorem 5.2.2 is the resolution of the identity for A in the abelian von Neumann algebra generated by A and I; so that we may speak of the resolution of the identity for A (or the spectral resolution of A ) . 5.2.3. THEOREM. I f { F , } is a resolution of the identity and A is a (bounded) self-adjoint operator such thaf AF, < AF, and A(I - FA)< A(I - Fa)for each A, or i f A = AdF, for each a exceeding someao, then { F A }is the resolution of the identity for A in d o ,the abelian von Neumann algebra generated by A and I.
r-,,
Proof. The fact that AF, is self-adjoint is implicit in the assumption that AFa < AF,. Thus A commuteswith each F A .Since Fa < FA,when A < A’, { F A }is an abelian family. Let d be an abelian von Neumann algebra that contains A and {F,} and X be the extremely disconnected compact Hausdorff space such f in C ( X ) that d E C ( X ) . If {E,} is the resolution of the identity for A in d, correspondsto A , and el in C ( X )corresponds to E,, then enis the characteristic function of X,,the largest clopen set in Xon whichftakes values not exceeding A. Iffn in C ( X )corresponds to FA,then f,is the characteristic function of a clopen set Y , on which f takes values not exceeding A, sincef fa < A,,. Thus Ya c X,. As F, = A ,,>, FA., Y, is the largest clopen set in X contained in fl,, > a Y,. . Now A’ G A P )if p E X\Y,. , since A‘(I - FA.)< A(I - Far),so that X\YapE f-’((A, 00))- when 1’ > A. Thus Xa c Y,. when A‘ > A ; and X, is a YA,.Since Y , is the largest such clopen set, clopen set contained in n,. X, E Ya. Hence X, = Y, and En = F A .The resolution of the identity for A in d osatisfies (iv) of Theorem 5.2.2 and d oE d . From what we have just proved, that resolution coincides with {E,} (and {F,}). Suppose, now, that A = A dF, for each a exceeding some a,. If, for such an a, A E [ - a, a] and {A,, . . . ,A,} is a partition of [ - a, a ] , with A as some A k , such that ( B =) Cj”= A;(F,, - FA,-J is close (in norm) to A ; then IIAF, - BFJ is small and
p-.
k
k
C A;(F,, - FA,-^) < C &(FA, - F A , - , )= A(F, - F - J < AF,. j= 1 Thus AF, < AF,. At the same time, IIA(Z - FA)- B(I - FJl is small and BF,
=
1
j=
n
B(I - FA)=
1 j=k+ 1
n
AJ(FA,-
- 1)
2
C
Ak(F2., - FA,- 1) = &Fa
- Fa).
j=k+ 1
+
Thus A(I - FA)3 A(Fa - FA)for each a greater than ao. Letting a tend to co, Fa tends to I in the strong-operator topology, so that A(Z - FA)3 A(I - Fa). From the first part of this proof, F, = E, for each A.
In Theorem 5.2.4 we start with a bounded resolution of the identity and construct a bounded self-adjoint operator whose spectral resolution is the given resolution of the identity.
5.2. SPECTRAL THEORY FOR BOUNDED OPERATORS
313
5.2.4. THEOREM.If{ Ed}is a bounded resolution of the identity on a Hilbert l d E Aconverges to a self-adjoint operator A on Z such that space 2,then Stla ( [ A [< ( a and for which { E d }is the spectral resolution, where Ed = 0 $1 < - a and EL = I $ a < 1. Proof. If { I , , . . . ,I,} ( = 9’) and {p,, . . . ,p,,,} (= 9) are partitions of [ - a , a ] , 19’1 and 191 are the lengths of their largest subintervals, and {yo,.
. . ,y r } is their common refinement, then r
n
II
c qJ%,
1 YXEh - En-JI < 191’
- EdJ-J-
j= 1
k= 1
and r
m
II
c PJ(EPj
- EPJ-,)-
c Y;(Eh
-
E7k-l)ll
< 191,
k= 1
j= 1
so that n
II
c 1pnj-
j= 1
m
-
p p & - J%!-l)ll
< 191+ PI.
k= 1
Thus the family of approximating Riemann sums to Jta 1dEA,indexed by their corresponding partition of [ - a, a] and the set of these partitions partially ordered (and directed) by refinement, forms a Cauchy net in the norm topology on 3?(2). Since B ( Z )is complete in its norm topology, this net converges in riorm to a bounded self-adjoint operator on H.From Theorem 5.2.3, { E d }is the spectral resolution of A . Passing to C ( X ) , where d z C ( X ) and d is an abelian von Neumann algebra containing A , we see that the conditions, EA= 0 if I < - a and Ed = I if a < I , imply that the function in C(X)representing A has range in [ - a , a ] . Thus IlAll ,< a. We studied unitary operators in C*-algebras in Section 4.4, and noted, there, that exp iH is a unitary element in each C*-algebra containing the selfadjoint element H . We remarked, in the discussion preceding Proposition 4.4.10, that not each unitary element of a C*-algebra has this form. In essence, the possibility of finding “log U” in the C*-algebra generated by U (an algebra of continuous functions) may be blocked by topological (homotopy) considerations. This is not the case in the von Neumann algebra generated by U - where the topological obstructions vanish before the (essentially measuretheoretic) constructions available in von Neumann algebras. We prove this von Neumann algebra analogue to Proposition 4.4.10 in the theorem that follows. 5.2.5. THEOREM.r f U is a unitary operator acting on the Hilbert space Z and .dis the (abelian) von Neumann algebra generated by U and U*, there is a
314
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
positive operator H in d such that IlHll < 2n and U = exp iH. In addition, U is the norm limit offnite linear combinations of mutually orthogonal projections in d with coefficients in sp(U). Proof. From Theorem 5.2.1, d z C ( X ) with X an extremely disconnected compact Hausdorff space. If u in C ( X ) corresponds to U , then ii corresponds to U * ; and lu12 = 1. Let X , be the complement of the closure of the set of points at which the values of u do not lie in {expiI': I' in [0, I ] } (= C,), for I in [0,2n). Arguing as in the proof of Theorem 5.2.2, X , is the largest clopen set on which u takes values in C,. Let e, be 0 if I 0, 1 if 2n < I ; and let e, be the characteristic function of X , for I in [0, 2n). Then el is the greatest lower bound of {el. : I < A'} if I < 0 or I > 2n. As el < e,, when I < A', el is a lower bound of {el. : I < A'} for all I . To see that e, is the greatest lower bound when I E [0,2n), note that each clopen subset 0 of n,. , X,. is contained in X , (for u takes values on 0 in each C,., with I' exceeding I , so that u takes values on 0 in C,). As in Theorem 5.2.2, the projections Ed in d corresponding to e, give rise to a (bounded) resolution of the identity { E d } . From Theorem 5.2.4, I dE, converges (in norm) to a self-adjoint operator H in d . Let h be the function in C ( X ) corresponding to H . Letting X , be @ when I < 0 and X when I 2 271, X , is the largest clopen set on which h takes values not exceeding I . The range of h is contained in [0,2n] so that H is positive and IlHll < 2n. Note, too, that h cannot take the value 2n at each point of a non-null clopen set 0,; for otherwise 0, is disjoint from U, < 2n X,, . But then u(po) # 1 for somep, in 0,(otherwise 0, G X,). By continuity of u, there is a clopen subset O1 of Oo containingp, and there is a I , in ( 0 , 2 n )such that u(q)E C,,, for each q in O1. Thus O1 E X,, contrary to the choice of 0, disjoint from XA0.With this information, we can now see that X , is the largest clopen set on which exp ih takes values in C , ( I E [ 0 , 2 x ) )- whence exp ih = u, and exp iH = U. Indeed, if 0 is a clopen set such that exp ih(p) E C, for each p in 0, then either h( p ) E [0, I ] or h( p ) = 271for each p in 0. If h( p ) = 2n for some p in 0, then, by continuity of h, there is a clopen subset of 0 containingp on which h takes values near 2n -in particular, not in [0, A ] , since A 271. By choice of 0, then, h takes the value 2n on this entire clopen subset-contrary to what we have just proved. Thus h ( p ) E [0, I ] for all p in 0, and 0 c X,. If Ij(E,, - Ed,_ is close to H in norm, then
-=
,
-=
c;=
n
1 (exp iI;)(Ea, - Edj- J j= 1
is close to exp iH (= U ) in norm. From Theorem 5.2.2, we can choose I: in sp(H) if Ed, # E a j - l .With this choice, expiIJEsp(U) if Eaj # E a j - l . In Example 5.1.6 we noted that the multiplication algebra d of a o-finite measure space ( S ,9, m)is an abelian von Neumann algebra. Iff is a real-valued
5.2. SPECTRAL THEORY FOR BOUNDED OPERATORS
315
essentially bounded measurable function on S, M , is a (bounded) self-adjoint operator on L 2 .With El the projection corresponding to multiplication by the characteristic function of the set Sdon which f takes values not exceeding 1, { E d }is the spectral resolution of M,. The key observation needed for this is the fact that n,, , Sap= Sd(and, thus, A El. = El). In Theorem 5.2.6 we describe a simultaneous spectral resolution for a commuting family of operators forming an abelian C*-algebra. In this case, the (joint) spectrum of the family is the pure state space. Somewhat more precisely, we describe the spectral resolution of a representation of an abelian C*-algebra. 5.2.6. THEOREM. I f X is a compact Hausdor-space, 2 is a Hilbert space, then, to each Borel subset S of X there and (p is a representation of C ( X ) on S, corresponds a projection E(S) such that (i) E ( S )E d,the strong-operator closure of (p(C(X)); (ii) E ( S ) = A ( E ( 0 ) :S E 0, 0 open}; (iii) E(U,"= S,,) = =C : E(Sn)for each countable family {S,,} of mutually disjoint Borel subsets of X, in particular, E(Sn)E(S,,,)= 0 if n # m , and E ( 0 ) = 0; (iv) E(Xo) = I,for X o a Borel subset of X , ifthe span of the ranges of those such that fE C ( X ) and f vanishes on X\ X o is dense in X ; (v) for each x in 2,S -P ( E ( S ) x ,x ) is a regular Borel measure, px,and,for f i n C(X),
(p(n
(cp(f)x,x) =
s
f l P ) dPx(P)*
X
Proof. If 0 is an open subset of X and f in C ( X ) has range in [0,1] and vanishes on X\0, then 0 ,< q(f) < I. Thus (p(F(0))has a least upper bound E ( 0 )in the abelian von Neumann algebra d,where F(0) is the set (directed by its natural order) of such functionsf. As { f ' : f E F ( 0 ) ) = F(0), E(0) is a projection. With S a Borel subset of X , let E ( S )be A{ E(0):S G 0 , O open}. I f x is a unit vector in S, f -,(tp(f)x, x) is a state of C(X). From the Riesz representation of such functionals (see the discussion preceding Lemma 1.7.7), there is a regular Borel measure px on Xsuch that (cp(f)x, x ) = J f ( p )dpx(p). By (inner) regularity of p x , given an open set 0, there is a compact subset X of 0 such that px(-X) is close to px(0). Since X is a normal space, there is a continuous function f on X with range in [0,1], vanishing outside 0, and 1 on X. Then Px(.x) G
s
f(P)dPx(P)= ( P ( f ) X , X >G ( E ( 0 k X ) -
316
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
It follows that p x ( 0 ) 6 ( E ( O ) x , x ) . From the definition of E(0), ( E ( O ) x , x ) 6 p x ( 0 ) ,so that (E(O)x,x) = px(0). From (outer) regularity of p x , we have that px(S)= inf{px(0):S c 0, 0 open} = inf{(E(O)x,x):
S G 0, 0 open} = ( E ( S ) x , x )
for each Borel set S. If, now, {S,} is a family of disjoint Borel subsets of X,then m
m
1 (E(Sn)x,x)*
px(Sn) =
n= 1
In particular, if x is a unit vector in the range of one of the E(S,,),say, E(S,), then m
12 ( E ( \n=]
Sn)x,x) = /
1 (E(S,,)x,x)2 ( E ( S l ) x , x ) = 1. n= 1
Since 0 6 (E(Sn)x,x) for all n, (E(S,)x, x) = 0 unless n = 1. It follows that E(S,)E(S,,,) = 0 if n # m and that E(U,"= S,,)= E(S,,). With X, as in (iv), if 0 is an open set containing X, and f (real-valued) in C ( X )vanishes on X\0, then the range projection of cp( f o )is a subprojection of E(0),wheref, = )If ll-llfl. But cp( f o ) and ~ ( fhave ) the same range projection (for dlfl)= c p ( f + ) + df-), df) = 4 u + ) - cpu-1, and f + f - = O-see Remark 3.4.9). Thus E ( 0 ) contains the range projection of the image of each function in C ( X ) vanishing on X \ X o ; and, by assumption, E(0)= I. Hence E(x,) = I. We apply this theorem to the important special case of R-essential representations of C({R, co}) (that is, representations essential on the ideal of functions in C({R, a})vanishing at co, where, as usual, {R, co} denotes the one-point compactification of R). 5.2.1. COROLLARY. Each R-essential representation cpo of C({R, co}) corresponds to a (possibly unbounded) resolution of the identity { E L }such that, for each f in C({R, co}) with Am) = 0, (cpo(f)x,x> =
1
A4W
A X , x>.
R
Proof. From Theorem 5.2.6, there is a projection-valued measure S + E(S) on { R, co} such that (q0(f ) x , x) = JR A I ) dp,.(I), where feC({R, co}),f(co) = 0, and px(S) = ( E ( S ) x , x ) . If EA= I - E((1, a)),then { E l } is a (possibly unbounded) spectral resolution of the identity. To see this, note that E((I, a))< E((A', co)), when I' < I , so that EA,< E A ,in this case. Since cp, is R-essential, from (iv) of Theorem 5.2.6, we have that E(R) = I. As
317
5.2. SPECTRAL THEORY FOR BOUNDED OPERATORS
(m, a)= U n B m ( n , n = 0. It follows that
+ 11, E((m,a))= I,"=,E((n,n + 11). and
I = I - A E((A,C O ) ) = V(I- E((1, 00)))
= YEA. I
1
1
A m E((m,00))
At the same time, m
m
C
I = E(R)=
E((n,n
+ 11) = V
E((m,CO)),
m=-w
n=-m
so that oc
02
0=I-
V m= - w
OD
A
E((m,00))=
A
( I - E ( ( ~ , c o )= ))
m=
m = -a:
Em
-a,
and m
O,
A
1
m= - m
Em=O.
Let A(n) be I + n-'. Then
c E((4n + m
+ 1, a))+
E ( ( 4 00)) =
111 W l ) ,
n= 1
so that m
A
n= 1
L1 m
d EA+1
A
A ( I - E ( ( 4 n + 1),4n)]))
1
= I - E((A, ~ 0 ) )= EI
Since Q
EL Q
A E.I~Q A I
d El,
n= 1
it follows that EL = A I < I , E1, and { E L }is a resolution of the identity. We have
E((A,A'])
= E((A,GO)) - E((A',C O ) ) = E,I.*-
El,
so that
PA(AA'])
=
(EA*x, X) - (EGGx).
Combining this with (v) of Theorem 5.2.6, we have (1)
(cpo(f)x,x) =
s.
f(4dcLx(4 =
s.
A4d(E,x,x)
when fe C({R, co}) and f vanishes outside a finite interval. Since such f are norm dense in the set of those functions in C({R, 00)) vanishing at CO, (1) is valid for allfin this set. W
318
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
Using Theorem 5.2.6, we can produce the spectral resolution by measuretheoretic means. If A is a (bounded) self-adjoint or normal operator on X and %is the (abelian) C*-algebra generated by A (and A*) and I, then the mapping f - + f ( A ) provides us with a representation of C(x>on X, where X = { (w, 00} if A is self-adjoint and X = {C, a}, the one-point compactification of C, if A is normal. From Theorem 5.2.6, there is a projection-valued measure assigning a projection E ( S ) on X to a Borel subset Sof Xand such that, for eachfin C(X),
j
( f ( A ) x , x )=
f(P)&x(P),
X
where pX(S)= ( E ( S ) x ,x). If 0 is an open set disjoint from sp(A) andfin C ( X ) is in 9 ( 0 )(see the notation of Theorem 5.2.6), thenAA) = 0, so that E ( 0 ) = 0 and p x ( 0 ) = 0 for each x in X. We write ( A A ) ~x> , =
J
~
pdcLx(p), )
sp(A)
and speak of S + E(S) as the spectral measure for A. In case A is self-adjoint, Corollary 5.2.7 shows us how to pass from this spectral measure for A to a spectral resolution {E,}. From the proof of that corollary, we have that E, = Z - &(A, a)).As just noted, E((A, a))= 0 if (A, 00) is disjoint from sp(A). Thus E, = I if A > IlAll. At the same time, I - E((1,co))= E(X\(A, 00)) = 0 if A .c - IlAll, so that E, = 0 when A - llAll. Thus { E n }is a bounded resolution of the identity in this case, and
-=
(2)
(f(& x) =
J
IlAll
f(4 d(E,x, x> -llAll
(at first, for eachfin C ( X ) vanishing at a,but then for eachfcontinuous on sp(A) since each such agrees on sp(A) with some function vanishing at 00). In particular
s
IlAll
( A X ,X)
=
A d(E,x, x)
- IlAll
for each x in X, so that AE, < AE, and A(Z - EL)< A(I - El). It follows (from Theorem 5.2.3) that {E,} is the spectral resolution of A. Since px(S) = ( E ( S ) x ,x) for each Borel subset S of X and x in X and (f(A)x,x)=
1
AP)dPx(P),
X
polarization of ( E ( S ) x , y ) allows us to define a complex (Radon) measure on X as a linear combination of positive measures pz and (3)
( f ( A ) x , y )=
1
~(P)~P,,(P)
X
5.2. SPECTRAL THEORY FOR BOUNDED OPERATORS
319
for eachfin C(X) and all x, y in X. If A is self-adjoint, (3) amounts to the formula,
j
IlAll
( f ( A ) x , y )=
f(4~(J%x,JJ),
-llAll
which “polarizes” to (2). If A is normal, (3) provides us with the possibility of defining g(A) when g is a bounded Borel function on @. Note that ISg(P)dPx,,(P)I G llxl1 IIYII suPlg(P)l (this is apparent from (3) when g E C ( X ) , and extends by a measure-theoretic argument to the case where g is a bounded Borel function). Thus ( X J )
-+
jxg(P)dPx,y(P)
is a bounded conjugate-bilinear functional on & with bound not exceeding sup)g(p)Jand, so, corresponds to an operator g ( A ) satisfying
= (g(Aly9 x> = ( g ( A ) * x , y )
for all x and y in %. Thus (5)
&A)
=g
W*
for each bounded Borel function g on C and each normal A . Since A and A* commute with a,g(A) and g(A) commute with and, hence, with each other. Thus g ( A ) is normal. We proceed, now, to establish the other properties of a (bounded) Borel function calculus for (bounded) normal operators. If g and h are bounded Borel functions on C (or R if A is self-adjoint), then, for each x in a?, ( ( a s + h)(A)x,x> =
Ix
( W ( P ) + h(PN dPx(P) = ((ag(A) + h(A))x,x),
so that (6)
(ag
+ h)(A) = ug(A) + h(A).
320
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
If g is the characteristic function of a Borel subset S of X,then, for each x in X, (g(A)x,x > =
J
g(p)dlcx(P)= PX(s) = (E(s)x, x > . X
Thus g(A) = E ( S ) ; and, in particular g2(A)= g ( A ) = g(A)2. If h is the characteristic function of a Borel set disjoint from S, then, from Theorem 5.2.6(iii), 0 = g(A)h(A) = ( g h)(A). It follows, now, that g2(A) = g(A)2, so that ( g + h)'(A) = [g(A) + h(A)I2 and (9 . h)(A) = g ( 4 . h ( 4 ,
(7)
when g and h are finite linear combinations of characteristic functions of disjoint Borel subsets of X ("step functions"). Since each bounded Borel function is a uniform (norm) limit of such step functions (and Ilg(A)II < llgll for each bounded Borel function g), we have (7) for arbitrary bounded Borel functions g and h. The identities (5), (6), and (7) that we have established thus for assure us that the rule (9 h)(A) = g(h(4)
(8)
O
holds when g is a polynomial (in z and 5) and h is an arbitrary bounded Borel function. Using the Stone-Weierstrass theorem (3.4.14) to approximate a continuous function uniformly on a closed disk in C containing the range of h by a polynomial (in z and 5), it follows that (8) is valid for each continuous function g and each bounded Borel function h. Since g(h(A)) = g(h(A)) = g(h(A))* if g is real-valued and g(h(A)) 2 0 if g 2 0, we have that {g,,(h(A))}is an increasing sequence of self-adjoint operators when {g,,} is an increasing sequence of bounded Borel functions. If each g,, is continuous and tends pointwise to a bounded Borel function g, then g,,(h(A))= (g,, h)(A) and {gn h } is an increasing sequence tending pointwise to g h. It will be useful for us to note that if {f,} is an increasing sequence of bounded Borel functions tending pointwise to the bounded Borel functionf, then { f , ( A ) }is an increasing sequence of self-adjoint operators with least upper bound f ( A ) (and a similar conclusion holds for decreasing sequences). We say that the mappingf-Ad) with this monotone sequential convergence property is m-normal. To prove this, choose x in X and note that 0
0
0
( f , ( A ) x , x )=
1
X
f"(P)dPX(P)
+
j
f(P)dPx(P) = (f(A)x,x), X
from the monotone convergence theorem. Thus, in the case of the continuous the characteristic function of an open set 0 in C (and, so, for a closed set as well). To construct the
g,,, we have (8) for their limit g . In particular, we have ( 8 ) for g
5.2. SPECTRAL THEORY FOR BOUNDED OPERATORS
321
sequence {g,,} for g , express 0 as a countable union of open disks Oj(with radius r,). Letfj" be a continuous function on a3 with range in [0,13, vanishing outside Lo,, and 1 on the closed disk with the same center as 0,and radius ( n - l)rj/n. Then f i n v fin v * . v f,,,, will serve as g,,. Let S be the family of Borel sets S whose characteristic function g satisfies (8) for all bounded Borel functions h. We have just seen that 9 contains all open and all closed sets. From the properties we have established for the mapping f + A A ) (in particular, a-normality) we see that 9 is a a-algebra. Hence S is the family of all Borel sets; and (8) holds for all bounded Borel functions h, when g is the characteristic function of a Borel set. As a consequence, (8) is valid for each step function g and then, by passing to (norm) limits, (8) follows for each pair of bounded Borel functions g and h. We summarize this discussion in the theorem that follows.
5.2.8. THEOREM. If A is a (bounded) normal operator on the complex Hilbert space X the * homomorphism f +f(A) of C ( X )into the C*-algebra % generated by A, A*, and I, where X is the one-point compactijkation of @, extends to a a-normal * homomorphism g -+ g(A) of the algebra W of bounded Borel functions g on C into the abelian von Neumann algebra d consisting of operators commuting with each operator commuting with %. Ifg in W vanishes on sp(A), then g(A) = 0. With g and h in W,g ( A ) = g(A)* and ( g 0 h)(A) = g(h(A)). Letting S be a Borelsubset of X , g be its characteristicfunction, and E(S)be g(A), the mapping S + E(S) is a projection-valued measure on X . Moreover Ilf(A)II G sup{If(a)l: and
=
j X
f ( P ) dPx(P) =
SP(41
1
f(P)dPx(P)
sp(A)
for each f in 93,where px(S)= ( E ( S ) x , x ) . If A is self-adjoint its spectral resolution is { E A } ,where EA= I - E((I, 00)).
I f f is the ideal of functions in 9 vanishing on sp(A), then 9 is the kernel of the homomorphism of W onto W(sp(A)) obtained by restricting a function in W to sp(A). Thus &?/Yz W(sp(A)). As noted in Theorem 5.2.8, the kernel of the (a-normal) homomorphism, g -,g(A), of W into d, contains 3 Thus the mapping, g + 9 -,g ( A ) , gives rise to a homomorphism of W(sp(A)) into d.If g E W(sp(A))and we define $jto be g on sp(A) and 0 on the complement of sp(A), then 8 E 9and g ( A ) is the image of g under the homomorphism described. We write g ( A ) for this image. From this same observation, we see that g + g(A)is a a-normal homomorphism of W(sp(A)) into d. In the theorem that follows, we prove that our bounded Borel function calculus is unique. (Compare Theorem 4.4.5 and Remark 4.4.6.) The
322
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
uniqueness is stated in terms of B(sp(A)), although the preceding paragraph applies to any Borel subset of C containing sp(A) and the following result (and argument) apply to each bounded Borel subset of C containing sp(A).
5.2.9.THEOREM.I f A is a (bounded)normal operator on a complex Hilbert space 2, B(sp(A)) is the algebra of (complex-valued)bounded Borelfunctions on sp(A), cp is a a-normal homomorphism of B(sp(A)) into an abelian von Neumann algebra d, cp( I) = Z, and cp( z) = A , where z(a) = a for each a in sp(A), then cp maps W(sp(A)) into d o the , abelian von Neumann algebra generated by A, A*, and Z, and cp(g) = g(A) for each g in W(sp(A)). Proof. With complex conjugation as involution (* operation) and sup{lg(a)l : a E sp(A)} as Ilgll, g(sp(A)) is a C*-algebra; for 11s .g1( = 11g112.If g is real-valued and aEC\R, then g - a1 has an inverse h in B(sp(A)). Since Z = [cp(g) - aZl . cp(h), a#sp(cp(g)). The elements of d are normal, so that cp(g) is self-adjoint. Thus cp is a * homomorphism of the C*-algebra B(sp(A)) into d. From Theorem 4.1.8(i), cp is order preserving and does not increase norm. Applying Theorem 4.4.5 to cp restricted to C(sp(A)), we have that q ( f )= A A ) for each f in C(sp(A)) a n d A A ) E d o . Since cp and the mapping g + g ( A ) are a-normal, we have, as in the argument proving (8), that cp(g) = g ( A ) ( € d o when ) g is the characteristic function of an open set. If 4 is the family of Borel sets whose characteristic function g satisfy cp(g) = g(A), then, again, by a-normality, 4 contains the union of each countable subfamily. Moreover, since cp( 1) = Z,9contains the complement of each set in 3F Thus 4 coincides with the family of all Borel subsets of sp(A); and cp(g) = g(A) (E d o for ) the characteristic function g of such a set. Since cp is linear and norm continuous, Ilg(A)II 6 Ilgll, and the step functions are norm dense in ) each g in &?(sp(A)). 4 B(sp(A)), it follows, now, that cp(g) = g(A) (E d o for Lemma 5.2.10 provides the foundation for developing the Borel function calculus in purely topological-function-theoretic terms. The approach of Theorem 5.2.8, stemming from Theorem 5.2.6, is essentially measuretheoretic, while the construction of the spectral resolution { E A }in Theorem 5.2.2is topological and function-theoretic. We shall take the latter path, and refer to the following results, when we treat the function calculus for unbounded normal operators (Section 5.6). Recall that a subset of a topological space Xis nowhere dense (in X ) if its closure has empty interior and that a subset is meager (or of thefirst category) in X if it is a countable union of sets nowhere dense in X . (See [K: p. 2011.)A subset of a nowhere-dense set is nowhere dense, so that a subset of a meager set is meager. A countable union of meager sets is meager. 5.2.10.LEMMA.If X is an extremely disconnected compact Hausdorff space, each Borel subset of X differsfrom a (unique) clopen set by a meager set.
5.2.
SPECTRAL THEORY FOR BOUNDED OPERATORS
323
Each bounded Borelfunction g on X differsfrom a (unique)continuousfunction f on a meager set. The mapping that assignsf to g is a (conjugation-preserving,0normal> homomorphism of B(X),the algebra of bounded Borel functions, onto C ( X ) with kernel consisting of those functions vanishing outside a meager set. Proof. Let 9be the family of subsets of Xthat differ from a clopen set by a meager set. If S Eand~ X o is a clopen set such that (S\Xo)u(Xo\S)is meager, then X\S and X\X, differ by this same set. As X\X, is clopen, X\Se$? Each open set 0 lies in % since 0- is clopen and 0-\0is nowhere dense. If S j E 9for j = 1,2, . . . and X j is aclopen set such that (Sj\Xj) u (Xj\Sj) (= M j ) is meager, then
,
; S j E $?Hence 4” contains the 0As UT= M j is meager and UT= X j is open,=U algebra generated by the open subsets of X ; that is, 9contains the Borel subsets of X . The Baire category theorem [K: p. 200, Theorem 341 assures us that the complement of a meager set is dense in X , so that two continuous functions agree on the complement of a meager set only if they are equal. Thus there is at most one continuous function agreeing with a given bounded Borel function on the complement of a meager set. If S is a Borel subset of X , g is its characteristic function, X, is a clopen subset of X such that (X,\S)v (S\Xo) (= M ) is meager and f is the characteristic function of X o , then f is continuous and g - f is 0 on X\M. We see, from this and the preceding “uniqueness” remark, that there is at most one clopen set differing from S by a meager set. At the same time, we see that a finite linear combination of characteristic functions of (disjoint) Borel subsets of X(step functions) agrees with a (unique) continuous function on the complement of a meager set. Since the step functions are (supremum-)norm dense in B ( X ) ;if g is in B(X),there is a sequence {g,,} of step functions such that 119 - grill + 0. Let {fn} be a sequence of continuous functions such that fn and gn agree on the complement of a meager set M,,. Then [If,-f,ll < 119. - gml(,sincefn - f , and g,, - gm agree on the complement of M,, u M,, a dense set (so that I( f n -f,)(p)l < 119. - gml(for eachp in this dense set). Thus {f,}is a Cauchy sequence and converges in norm to somef in C(X).As { g n }tends to g and {fn} tends tof pointwise, f and g agree on the complement of UTZl M j , a meager set. If g1 and g2 in B(X)differ fromf l andf 2 in C ( X )on the meager sets M1 and M 2 , then g l ,ag, g2 and g l g 2 differ fromyl, afl +f 2 and f i f i on a subset of M I v M 2 . Thus the assignment to g in B(X)of the unique f in C ( X )differing from g on a meager set is a (conjugate-preserving) homomorphism of B(X) onto C ( X ) .Of course g corresponds to 0 if and only if g vanishes outside a meager set. If {g,,} is a monotone increasing sequence of bounded Borel
+
324
5 . ELEMENTARY VON NEUMANN ALGEBRA THEORY
functions on X tending pointwise to the bounded Borel function g and fn in C ( X ) differs from g,, on the meager set M,,, then fn(p)
5.2.12. REMARK.Lemma 5.2.10 provides us with another means of proving Theorem 5.2.5. In the notation of the proof of Theorem 5.2.5, define ho(p)to be that I in [0,2n) such that exp iA = u ( p ) . Then ho is continuous on X\u-’(l) and ho is 0 on u-l(l). Hence h o e B ( X )and there is a function h in C ( X )agreeing with ho on the complement of a meager subset M of X. Thus exp ih and exp iho ( = u) agree on the dense subset X\M. Since exp ih and u are continuous, exp ih = u and exp iH = U , where H in d corresponds to h. 5.2.13. REMARK.If d is an abelian von Neumann algebra, d z C(X), and A in d is represented by f in C ( X ) ;the mapping g + g 0f is a 0-normal homomorphism of B(sp(A)) into B(X).Composing this mapping with the 0normal homomorphism of g ( X ) onto C ( X ) (described in Lemma 5.2.10) and then with the (cr-normal) isomorphism of C ( X ) onto d yields a a-normal homomorphism cp of W(sp(A)) into d carrying 1 onto I and the identity transform I on sp(A) onto A. Theorem 5.2.9 applies and cp(g) = g ( A ) for each g in B(sp(A)). The process of forming cp recaptures the bounded Borel function calculus by topological-function-theoretic means. At the same time, we see that g(A), obtained from g 0f with f representing A in C ( X ) ,is independent of the abelian von Neumann algebra d containing A (and its isomorphic C ( X ) ) we use. 4 We reinforce the preceding “independence” comment in the proposition that follows. We say that a * homomorphism $ of one von Neumann algebra into another is a-normal when $ maps the least upper bound of each increasing sequence of self-adjoint operators bounded above in the first algebra onto the least upper bound of the image sequence. (Compare Proposition 4.4.7.)
5.3. TWO FUNDAMENTAL APPROXIMATION THEOREMS
325
5.2.14. PROPOSITION. If cp is a a-normal homomorphism of one von Neumann algebra W ,into another W 2mapping I onto Z, then (p(j(A)) =f(cp(A)) for each normal A in 9,and each bounded Borel function f on sp(A). Proof. From Proposition 4.4.7, sp(cp(A)) c sp(A). If d ois the abelian von Neumann algebra generated by A , A*, and Z, then d oE W 1and cp(do) is contained in an abelian von Neumann subalgebra d of W 2containing cp(A). The mapping that assigns cp(h(A))to h in B(sp(A)) is a a-normal homomorphism. Composing this mapping with the a-normal homomorphism of g(sp(cp(A))) into g(sp(A)) that assigns to each h the function equal to it on sp(cp(A)) and 0 on sp(A)\sp(cp(A)) (= Y ) yields a a-normal homomorphism $ of B(sp(cp(A))) into d .Let g be the characteristic function of Y (an open subset of sp(A)). Let 0 be an open subset of C such that Y = 0 n sp(A). As noted in the proof of Theorem 5.2.8, the characteristic function of 8 is the pointwise limit of an increasing sequence of positive continuous functions on 8. The sequence {f,}of restrictions of these functions to sp(A) is increasing and tends pointwise to g. Moreover, eachf, vanishes on sp(cp(A)) so thatf,(cp(A)) = 0. From Proposition 4.4.7, 0 =f,(cp(A)) = cp(f,(A)).Thus, by a-normality of cp and the mapping h + h(A) of B(sp(A)) into d o ,&(A)) = 0. For any h in g(sp(A)) vanishing on sp(cp(A)), we have g . h = h, so that g(A)h(A) = h(A) and q(h(A)) = 0. It follows that, with I and the identity transforms on sp(A) and SP(cp(A)), c p ( 4 = c p ( l ( 4 ) = c p ( [ l * (1 - g)l(A)) = $ ( l o ) and $ maps theconstant function 1 on sp(cp(A)) to cp((1 - g)(A)),which is cp(Z). From Theorem 5.2.9, $(h) = h(cp(A)) for each h in W(sp(cp(A))). With f in W(sp(A)) and f o its restriction to sp(cp(A)),f(cp(A)) =fo(cp(A))(in essence, by definition), so that cp(j(A)) = c p ( C f . (1 - g ) l ( 4 1 = $(fa) =fo(cp(A)) = f ( c p ( 4
5.2.15. REMARK.Again, we recapture the composite function rule,
(f g ) ( A ) =f ( g ( A ) ) , for f and g bounded Borel functions on C. The mapping 0
f + (f.g ) ( A ) of B ( D ) is a a-normal homomorphism mapping the identity transform onto g ( A ) and 1 onto Z, where D is a disk containing sp(g(A)) u range(g). It follows from Theorem 5.2.9 that (f g ) ( A ) = f ( g ( A ) ) . 0
Bibliography: [7, 17, 18, 231
5.3. Two fundamental approximation theorems If 8 is a family of bounded operators on the Hilbert space 2,8' will denote those bounded operators on 2 commuting with all operators in Z T h e set 9' is called the commutant of Z Note that 9' is weak-operator closed.
326
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
5.3.1. THEOREM (Double commutant). Zf2I is a self-adjoint algebra OJ operators containing the identity operator and acting on the Hilbert space M, then the weak- and strong-operator closures of 2I coincide with (a‘)‘. Proof. Since 2I is convex, Theorem 5.1.2 assures us that the weak- and strong-operator closures of 2I coincide. We shall conclude this, once again, in the present case, by showing that 3”is the strong-operator closure of 2I. Since % c 3”and (a’)’is weak- (and strong-)operator closed, both the weak- and strong-operator closures of 2I are contained in a’’. Suppose T Ea’‘and xl ,. . .,x, are given in %. We want to find Toin 2I such that ll(T - To)xjll < 1 for eachj. Let 2 be the n-fold direct sum of X with itself. With A in a(%),let A be A @ . * * @ A (so that d ( y l , .. . ,y,) = ( A y , , . . . ,Ay,)). If 1 = (xl ,. . . ,x,) and = {A:A E%}, then is a selfadjoint algebra of operators on and [&] is invariant under From the final paragraph of Section 2.6, Subspaces, the orthogonal projection E with range “&I commutes with (that is, E E @). If we show that TE then T commutes with E, and the range of E is stable under T. In this case, T1 = ( T x , , . . . ,Tx,,) is in this range, since 1 = (Ix, ,... ,ZX,)E 42). As Bi?is dense in “&], there is a To in 3 such that ll(T - T0)xjll .c 1 for allj. It remains to prove that T E ~ ‘ In ’ . Section 2.6, Matrix representations, we observed that operators in a(*) can be represented as n x n matrices with entries from a(%). In this representation, T has each diagonal entry T and all others 0. At the same time, simple matrix calculations show that consists of those matrices with all entries in a’, and consists of those matrices with a single operator from 2I“ at all diagonal entries and 0 at all other entries. Since T is assumed to be in 2I”, T E ~ ” .4
*,
a
a
a a. a”,
a‘
‘@I
There are several easy consequences of the double commutant theorem. If
9 E a(%)and 9* = { A * :A E F } then , {9 u F*}” is the von Neumann algebra generated by R If d is a factor, {du A’}’= d n A‘,which is the set of scalars. Thus {dud‘}” = B(X). More generally, { W U ~ } ’ is ’ v‘, where W is the center of the von Neumann algebra W ; for {W u w’}’= W n w’ = W. At the same time, we see that W is the center of w’.It follows that 9‘is a factor if W is a factor. As another simple consequence of the double commutant theorem, we may if each projection in 9’ conclude that a von Neumann algebra W on % is a(%) is either 0 or I. For this, we note that, from Theorem 5.2.2(v), each von Neumann algebra is the norm closure of the linear span of its projections.Thus w’ is { U } ,and, under the present assumption, W (= @’) = a(%). Our second approximation result (the Kaplansky density theorem) requires for its proof certain facts about strong-operator continuity of some classes of functions. These continuity results are interesting and useful in their own right. The most basic of them, strong-operator continuity of multiplication on bounded subsets of B ( X ) ,was established in Remark 2.5.10.
327
5.3. TWO FUNDAMENTAL APPROXIMATION THEOREMS
5.3.2. PROPOSITION. Each continuousreal- or complex-valuedfunctionf(on R or C)is strong-operator continuouson bounded sets of (self-adjoint or normal)
operators on the Hilbert space S. Proof. We may suppose that the bounded set of normal operators under consideration is contained in the ball of radius r. If A . is a normal operator in this ball, E > 0, and x l , . . . ,x,, is a set of vectors in 2,we want to find vectors y , , . . . ,y, and a positive 6 such that IIcf(A) -f(Ao))xkll < E if ll(A - Ao)yjll < 6, A is normal, and IlAll 6 r. If we can accomplish this with x l , . ..,x,, replaced by a single vector xo, then we can do it for x l , . .. ,x,,-increasing the set of y’s successively. Replace E by E ( I x ~ I I If
’.
then IIcf(A) -f(Ao))xoll < E . Thus we may assume that llxoll = 1. From the Weierstrass approximation theorem (see Remark 3.4.19,there is a polynomialp (in z and Z) such that [Ifpllc, < 4 3 , where @,isthe closed disk in C with center 0 and radius r. Using the strong-operator continuity of multiplication on bounded sets of operators and of the adjoint operation on the set of normal operators (see Remark 2.5.10), we can find vectorsyl, . . . , y , and a positive 6 such that Il(p(A) - p(Ao))xoll < 4 3 if ll(A - Ao)yjlI < 6, A is normal, and IlAll Q r (where Z is replaced by A* in determiningp(A)). In this case,
Il(f(4 -f(Ao))xoll
Q Il(f(A) - P(A))XOll
+ Il(P(A) - P(A0))Xoll
+ Il(P(A0) -f(Ao))xoll ,< I l f ( 4 - P(A)II + IlP(A0) -f(Ao)ll + 4 3 Q 2 l l f - Pllc, + 4 3 < E . To conclude that IIflA) - p(A)II and Ilf(Ao) - p(Ao)II are majorized by Ilf - p(lc,, in the preceding inequality,we pass to the function representation of the (abelian) C*-algebra generated by A and A*. This representation can be viewed as taking place on sp(A) ( G C,) with A represented by z, A* by Z, f ( A ) by fl sp(A), and p(A) by p I sp(A). Since the representation is an isometry, IIA4 - P(4II = Ilf- Pllsp(A)(and, similarly, IIAAo) - P(A0)II = I l f - Pllsp,Ao)). Presently we shall prove a stong-operator continuity result applicable to all (rather than bounded sets of) self-adjoint operators. An important transform from self-adjoint operators to unitary operators is needed for this - the Cayley transform. It assigns to a self-adjoint operator H the (unitary) operator
328
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
( H - iZ)(H + iI)-' (= U(H)). Since
+ iI)(H - il)(H + iI)-' = (HZ + I)(H2 + 1)- ' = I ,
U(H)U(H)* = U(H)*U(H)= ( H - i I ) - ' ( H
U ( H )is unitary. Now -i(z + l)(z - 1)-' is a real number t, provided IzI = I and z # 1. In this case, z = (t - i)(t + i)-'. Similarly, -i(U + I)(U - I ) - ' ( = H) is defined if 1 4 sp( U ); and H i s self-adjoint if U is unitary. Note for this that (U*
+ I)(U - I ) = U - u* = - (U* - I)(U + I ) ,
so that H* = i(U* + I)(U* - I)-'
+ I)(U - I)-' = H . U ( H ) = U and -i(U(H) + I ) ( U ( H )- I ) - ] =
-i(U
Analogous computations yield = H. (These identities result, as well, from the corresponding facts for real and complex numbers through the use of the function representation for %(H).) 5.3.3. LEMMA.The Cayley transform is strong-operator continuous. Proof. We have
( H + iI)(U(H)- U(Ho))(Ho+ iZ) = 2i(H - Ho),
+
Il(U(H) - U(H0))xOll = 211(H + iI)-'(H - Ho)(Ho iI)-'xoll
< 211(H - Ho)(Ho+ iI)-'xoll
+
(noting that ll(H iZ)-'ll < 1, from the function representation of%(H)). The strong-operator continuity of H -+ U ( H ) follows from this inequality. 5.3.4. THEOREM.I f h is a continuous real-valuedfunction vanishing at co on
R,then h is strong-operator continuous on the set of self-adjoint operators. Proof. Letflz) be h(- i(z + l)(z - l)-') when z # 1 and IzI = 1. If we definefll) to be 0,f is a continuous function on the unit circle in @ with values in 08, since h vanishes at 00 on R.From the discussion preceding Lemma 5.3.3, f l U ( H ) ) = h(H)for each self-adjoint H. Sincef is continuous on the unit circle in @, f gives rise to a strong-operator continuous function on the (bounded) set of unitary operators, from Proposition 5.3.2. As h is the composition of the Cayley transform andf, applying Lemma 5.3.3, we see that h is strong-operator continuous. W
We write 2I- for the strong- (equivalently, weak-)operator closure of an algebra 2I and (much less frequently used) %= for its norm closure.
5.3. TWO FUNDAMENTAL APPROXIMATION THEOREMS
329
isIa self-adjoint 5.3.5. THEOREM(Kaplansky density theorem). If '$ algebra of operators, then each A in the unit ball of %-, lies in (a);, the strong-operator closure of the unit ball(21)1of 2t. I f H is a self-adjoint operator in then H i s in the strong-operator closure of the set of self-adjoint operators in Proof. If H is a self-adjoint operator in 2l- and { T,} is a net of operators in 2t weak-operator convergent to H , then {+(T, + T:)} is a net of self-adjoint operators in 2t weak-operator convergent to H. Thus His in the weak-operator closure of the set of self-adjoint operators in 2t. Since this set is convex; from Theorem 5. I .2, H is in its strong-operator closure. Suppose, now, that His a self-adjoint operator in and { H a }is a net of self-adjoint operators in 2t strong-operator convergent to H . Ifflt) is t , for t in [ - 1,1], and t for t not in [ - I , 11, then f is real-valued, continuous, and vanishes at 00, on R. From Theorem 5.3.4,fgives rise to a strong-operator continuous function ;and { A H , ) } is strong-operator convergent toAH).Since f is the identity mapping on s p ( H ) , A H ) = H . Sincef has bound I , IIAH,)II < I. Thus His in the strong-operator closure of the set of self-adjoint elements in the unit ball of the norm closure %= of 2t. But each self-adjoint element in is a norm limit (hence, strong-operator limit) of self-adjoint elements in Thus H is the strong-operator limit of such elements. ,then T',the 2 x 2 matrix with entries 0 on the diagonal and T If T E(W), and T* at the other positions, acting on X @ X ,is self-adjoint and in the unit ball of a;, where (u2 is the algebra of 2 x 2 matrices with entries from %. From what we have proved to this point, T'lies in the strong-operator closure of the unit ball of 212.Thus each entry in T',in particular T, is in the strongoperator closure of the set of corresponding entries of elements in the unit ball of 212 -and these entries are all in the unit ball of 2t.
Note that we d o not assume that I E % in Theorem 5.3.5.
5.3.6. COROLLARY. If% is a self-adjoint algebra of operators acting on the then H is in the strongHilbert space A? and H is a positive operator in ('K),, operator closure of From Proof. Since H 2 0, H = K2, with K a positive operator in Theorem 5.3.5, K is the strong-operator limit of a net {K,} of self-adjoint operators in By strong-operator continuity of multiplication on the unit ball, { K:} is strong-operator convergent to K2 (= H). Since K, is self-adjoint, K:E(YI+)~. 5.3.1. COROLLARY. I f 9I isa C*-algebraacting on the Hilbert space X and U is a unitary operator in 2t- , then U is in the strong-operator closure of the set of unitary operators in a.
330
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
Proof. From Theorem 5.2.5, U = exp iH, with H a self-adjoint operator in %-. Using Theorem 5.3.5 (with the easy extension of its assertion to a ball of arbitrary radius), we have that His the strong-operator limit of {H,,},with Ha a self-adjoint operator in the ball of radius IlHll in %. Since t expit is continuous, {exp iH,} is strong-operator convergent to exp iH (= U). In each of the statements of the results related to the Kaplansky density theorem, except Corollary 5.3.7, we did not have to assume that 2l is norm closed. This assumption is needed in Corollary 5.3.7 (see Exercise 5.7.34). The Kaplansky density theorem, as well as the double commutant theorem, are used so often in what follows that we shall generally make use of them without mention or citation. Bibliography: [9, 11)
5.4. Irreducible algebras - an application A family 9 of bounded operators on a Hilbert space X is said to act topologically irreducibly when (0)and i?fare the only (closed) stable subspaces under S If (0) and 2 are the only linear manifolds (not necessarily closed) in 2 stable under g we say that 9 acts algebraically irreducibly.
5.4.1. THEOREM.If9is a self-adjoint family of bounded operators acting on the Hilbert space X, then 9acts topologically irreducibly on X if and only if' 9' consists of scalars or, equivalently, 9"= B ( X ) . Proof. Note, first, that if 9'consists of scalars, 9" = B ( 2 ) . If 9"= a(%),then 9"' consists of scalars. However, 9 c F", so that 9'E "9'; and 9' commutes with (F')', so that 9' G 9'". Thus 9' = F'", and 9' consists of scalars. With 9a self-adjoint family of bounded operators, 9' is a von Neumann algebra. From Theorem 5.2.2(v), then, 9' consists of scalars if and only if each projection in F' is either 0 or I. Since 9is self-adjoint, a projection lies in 9' if and only if its range is stable under 9(see Section 2.6, Subspaces). Thus 9 acts topologically irreducibly if and only if 9' consists of scalars. Making essential use of the Kaplansky density theorem, we shall prove (Corollary 5.4.4) that topological and algebraic irreducibility are the same for a C*-algebra. Toward this end, we need the following result. 5.4.2. LEMMA.I f { x l , ...,x n }is an orthonormalset in the Hilbert space 2 and zl, . . . ,z, are vectors in the ball of radius r and center 0 in X, there is a B in a(%)such that IlBll 6 (2n)"'r and Bxj = z j for allj. I f A x j = zj for some seuadjoint operator A , then B may be chosen self-adjoint. Proof. Let E be the orthogonal projection of X onto the (finitedimensional) subspace spanned by { x l , .. . , x n } .If T x is & ,(Ex, x j ) z j , then
5.4. IRREDUCIBLE ALGEBRAS-AN
33 1
APPLICATION
TE = T and IITxll< r
I(Ex,xj)I
< r(
j= 1
i
/(Ex,xj>12)1'2(
i
< n'/2rllExll
j= 1
j= 1
= 211TIIZ
< 2nr2.
5.4.3. THEOREM.rfthe C*-algebra 2l acts topologically irreducibly on the Hilbert space X, { y l ,. . . ,y , } is a set of vectors and { x , , . . . ,x,} is a linearly independent set of vectors in 3?,there is an A in 2l such that A x j = y j . IfBxj = y j for some self-adjoint operator B, then A can be chosen selfradjoint. Proof. Replacing {xl,. . . ,x,} by an orthonormal basis for the space they span and { y , , . . . ,y n }by the transform of this basis under the mapping taking x j to y j , we may assume that { x , , . . . ,x,} is an orthonormal set. With Bo in a(#) such that Boxj = y j , choose A . in 2l such that llAoxj - Boxill = llAoxj - yjll 6 [2(2n)"2]-1. For the choice of A o , we use the fact that 2l- = a'' = a(#)(Theorem 5.4.1). Choose B1 such that B l x j = y j - A o x j , with I(B1)I < 5 (using Lemma 5.4.2).Note that A . can be chosen self-adjoint if Bo can, since the self-adjoint operators in CU are strong-operator dense in those of its strong-operator closure, a(%). In this case, B1 can be chosen selfadjoint, from Lemma 5.4.2. Theorem 5.3.5 (Kaplansky density) provides us with an operator A l in 2l (self-adjoint if Bo is self-adjoint) such that llAlll d and (IAlxj- Blxjll < [4(2n)1'2]-1. Suppose, now, that Bk has been constructed so that llBkll < 2 - k , Bkxj = y j - Aoxj - A l x j - . . . - A k - l x j , and Bk is self-adjoint if Bo is. Choose Ak in 2l (self-adjoint if Bk is) such that llAkll < 2 - k and IIAkxj- Bkxjll < [ 2 k +1(2n)112]-1(employing Kaplansky density for this choice). From Lemma 5.4.2, there is a B k + , with llBk+,ll < 2 - ( k + 1 )and Bk+ 1 X j = J'j - Aoxj - ' ' ' - AkXj (self-adjoint if Ak is). The sum Ak converges in norm to an operator A in 2l (self-adjoint if Bo is); and
x:=o
m
yj - AXj
= yj
-
2 AkXj = h ( y j - AoXj - . k=O
'
. - AkXj)
k
=limBk+ixj=O. k
In the preceding proof, we could have chosen A . such that llAoxj - yjll < [ 2 ( 2 n ) " 2 ] - ' ~and )lAo))< IIBoll, B1 such that llBlll < ~ / 2 A , l such that
332
5 . ELEMENTARY VON NEUMANN ALGEBRA THEORY
llAlll d 4 2 , and, finally, Ak such that IlAkll d 2 - k ~ With . these choices, llAll < IlAkll d llBoll E for each preassigned positive E , though, in this process, A depends on the given E . A function calculus argument (see Exercise 5.7.41) proves that A may be chosen so that llAll d IIBoll. The property of 2l described in the first sentence of Theorem 5.4.3 is referred to as transitioity. It is an immediate consequence of the transitivity of 2l that it acts algebraically irreducibly.
+
5.4.4. COROLLARY. If the C*-algebra 2l acts topologically irreducibly on the Hilbert space 2,then it acts algebraically irreducibly. Theorem 5.4.3asserts a form of “self-adjoint transitivity” for 2l as well as “general transitivity.” In Theorem 5.4.5 we establish “unitary transitivity” for 2l -a result we shall need in the deeper study of states (see Section 10.2). We need no longer distinguish topological and algebraic irreducibility for C*algebras. We shall speak of C*-algebras “acting irreducibly.”
I f 2 l is a C*-algebra acting irreducibly on 2 and V is a 5.4.5. THEOREM. unitary operator such that V x l = yl ,. . . , Vx, = y,, then there is a self-adjoint operator H in 2l such that U x , = y l , . . . ,Ux, = y,, where U = exp iH. Proof. Since two operators that agree on a basis for the space generated by x1,..., x, agree on the space, we can assume that { x , ,...,x,} is an orthonormal set. The same is then the case for { y l ,. . . ,y,}. Let { x l , .. . ,x m } and { y , , . . . ,y,} be extensions of { X I , . . .,x,} and { y l , . . .,y,} to orthonormal bases of the space generated by x I ,. . . ,x , , y l , . . . ,y,. Define Vo on this space by Voxj = y j for allj . We may replace V by Vofor the purposes of finding U of the statement of this theorem. We may replace { x , ,. . . ,x,} by an orthonormal basis that diagonalizes V o . Hence, we may assume that Voxj = cjxj for all j , with lcjl = 1. Let a , ,. . . ,a, be real numbers such that cj = exp iaj. From Theorem 5.4.3, there is a self-adjoint H in 2l such that H x j = a j x j . If U = exp iH, then U is a unitary operator in 53,U is a norm limit of polynomials in H, and U x j = c j x j .
Bibliography: [ 5 , 81 5.5. Projection techniques and constructs In this section we study some topics involving projections and constructs with von Neumann algebras related to them.
Central carriers. If is in the von Neumann algebra 9 and { P a }is a family of central projections in W (that is, each Pa is in the center $7 of 9)such that P,A = 0 for all a, then PA = 0, where P = Va Pa. Note, for this, that A x is orthogonal to the range of Pa for all a, and, hence, to the union of these spaces. Thus PAX = 0 for all x and PA = 0.
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5.5. PROJECTION TECHNIQUES AND CONSTRUCTS
5.5.1. DEFINITION. The central carrier CA of an operator A in a von Neumann algebra 92 is the projection I - P,where P i s the union of all central projections Pa in W such that P,A = 0. H Since the center ‘37 of W is 9n W‘, ‘37 is a von Neumann algebra. From Proposition 5.1.8, P and hence C , are in W. As we noted, P A = 0, so that CAA = A . We could equally well have defined CAas the intersection of all central projections Q such that Q A = A . The context will usually make clear the von Neumann algebra relative to which a central carrier is formed. It may, however, require clarification, in which case some phrase such as “relative to 9’’ will appear. That the von Neumann algebra plays a role in determining central carriers can be illustrated by identifying the central carrier of a projection E (different from 0 and I ) relative to the algebra of all bounded operators and relative to the von Neumann algebra generated by Eand I. In the first case the central carrier is I and in the second it is E. 5.5.2. PROPOSITION. The central carrier of an operator A in a von Neumann algebra 92 acting on a Hilbert space % has range [%‘A(%)]. Proof. Since C,A = A , A(%) is contained in the range of C A .Since CA commutes with W,and W is a self-adjoint family of bounded operators on %, the range of CAis stable under W.Thus [.@A(%)] is contained in the range of CA . The projection Q with range [%A(%)] commutes with W and since [%‘A(X)] is stable under W and 9‘. Thus Q is in the center of Ye (see the discussion following Theorem 5.3.1). As [WA(.@)] contains the range of A , Q A = A . Hence C, < Q (for ( I - Q ) A = 0, so that I - Q < I - CA).From the earlier discussion, Q < C,, SO that Q = C A . %‘I,
5.5.3. PROPOSITION. I f {E,} is a family of projections in a von Neumann If Q is a central projection in 9, algebra W and E = V, E,, then CE= V, CEO. then QC, = CQAfor each A in Ye.
Proof. Since E, Q E Q CE, CEa 6 C E ; and V, CEa< CE. If P = CE- V, CEa,then PE, = 0 for each a, so that PE = 0, and 0 = PC, = P. Since BQA(X) = Q W A ( X ) ,it follows from Proposition 5.5.2 that QC, = CQA.
The analogous identity for intersections of projections is not valid. If {x,,}is an orthonormal basis for a (separable) Hilbert space % and E,,is the projection with range spanned by { x j : j 2n } , then A,,E, = 0. If central carriers are formed relative to @(%), then CEn= I and Co = 0. Thus A,, CEn= I # C,. 5.5.4. THEOREM.rfW isa von Neumann algebra with center % acting on the ifand only if Hilbert space X, then C;= AjAS = 0, with A, in W and AS in W’, there are operators Cj,,j , k in { 1, . . . ,n } , in ‘37 such that ‘j“= AjCik = 0f o r k in
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5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
{l,. . . , n ) and C;,l CjkA;= AJ for j in (1,. . .,n ) . In particular, AA' = 0 , with A in W and A' in w',i f and only i f CACA. = 0. Proof. Given operators n
n
AjAj = j= 1
cjk
with the stated properties, we have
c AjCjkA; cY 1"Ajcjk1A; n
=
j=lk=l
k=l
= 0.
j=1
Suppose, now, that I;=AjAS = 0. Let 9:be the algebra of n x n matrices with entries from 9' acting on the n-fold direct sum of 3E" with itself. Then 9: has commutant consisting of those n x n matrices with some element of W repeated at each diagonal position and 0 at each off-diagonal position. Let ;? be the n x n matrix whose first row has the entries A l ,. ..,A , and all of whose other entries are 0. Let [ c j k ] be the union of all projections E' in such that dE' = 0.Then dP = 0, where P = [cjk]. Let F be the projection with entry the projection F' in 9' at each diagonal position and 0 at all other positions of an n x n matrix. Since AjF' = F A j for allj, A F P = P'aP = 0. It follows from this and the construction of P that the range of P contains that of PIP. Hence I-
-
FP = PFP = (PFP)*= PF; and cjk commutes with each projection F' in 9'. From Theorem 5.2.2, cjk E V . Since dP = 0, matrix multiplication yields the equations AjCjk = 0 fork in { l , . . . , n } . If a ' i s the n x nmatrix with entries A ; , . . . ,A: in the first column and 0 at all other entries, ad' = 0. Thus d annihilates the range projection of d' and PA' = 2'. Again, matrix multiplication yields C t Z lCjkA; = A; f o r j in { l , . . . , n } . If AA' = 0 with A in W and A' in w',applying the result just proved, there is a central operator Q such that 0 = A Q = Q A and QA' = A'. Moreover, the 1 x 1 matrix with Q as entry is the projection P. Thus Q may be chosen as a projection in V. Since Q A = 0, Q ,< I - C A . AS QA' = A', CA.,< Q . Thus CACA, = 0. Conversely if C A C A = , 0, then AA' = CAACA.A'= ACACASA' = 0. Theorem 5.5.4says, in effect, that the algebra generated by W and W'is isomorphic to the (algebraic)tensor product of W and w'as modules over W. In particular, when W is a factor, Vis the scalars; and the algebra generated by W and w'is the more standard tensor product of algebras over the field of scalars.
Some constructions. If W is a von Neumann algebra acting on a Hilbert space if and E' is a projection in W', the range of E' is stable under 9.We noted in Section 4.5that the restriction of W to this range is a representation of 9.The image of t h s representation, a self-adjoint algebra of operators on E'(&'), is * isomorphic to WE' (and will often be denoted by BE'). In the discussion that follows, we shall prove that WE', acting on E ' ( i f ) , is a von Neumann algebra; and we shall identify its center and commutant.
5.5. PROJECTION TECHNIQUES AND CONSTRUCTS
335
If9 is a von Neumann algebra acting on the Hilbert 5.5.5. PROPOSITION. space 2 and E' is a projection in W',then the mapping AE' -+ AC,. is a * isomorphism of BE' onto WC,..
Proof. Since (aA + B)E' = aAE' + BE', ABE' = AE'BE', (aA + B)CE, + BCE,, and ABCE, = ACEeBCE. ; the mapping AE' -+ ACE, is a * homomorphism of BE' onto WCE.once we note that it is single-valued ("well defined"). The same operator in WE' may appear as AE' and BE' for some A and Bin 9. In this case, we wish to conclude that ACE, = BC,. .Equivalently, if TE' = 0 for Tin 9, we wish to conclude that TCE, = 0. But Theorem 5.5.4 tells us that CTC,. = 0 when TE' = 0, SO that 0 = CTCE'T = C,TCE* = TCE.. At the same time, we see that TE' is 0 if TCE, = 0 (for, then, 0 = TC,.E' = TE'). Thus the mapping AE' -+ACE* is a * isomorphism (clearly onto WC,.). = aACE.
w
5.5.6. PROPOSITION. If9 isa von Neumann algebra with center W, acting on then WE', acting on El(&), is a the Hilbert space 2,and E' is a projection in W', von Neumann algebra with center WE' and commutant E'B'E'.
Proof. The mapping A -+ AE' of W onto WE' is weak-operator continuous. From Theorem 5.1.3, (3?(~%))~ is weak-operator compact so that its intersection (W)l with the weak-operator closed W is weak-operator compact. Thus (a), E' is weak-operator compact (hence closed). From the Kaplansky density theorem, ( B E ' ) ,is dense in the unit ball of the weak-operator closure of BE'. We show that (WE'), = (a), E', from which (WE'), is the unit ball of the weak-operator closure of WE';and WE' is weak-operator closed. Choose AE' in ( B E ' ) ,, and observe that AE' = ACEVE'. With T, in 9,if IIT,,C,. - TJI-+ 0, then IIT,CZ, - TCECll = IITnCr - TCE.11 -+ 0. Thus T E W and T = TC,. E W C ~ , . Thus B C E ,is a C*-algebra (norm-closed with unit C,.). From Proposition 5.5.5, TCE, -+ TE'is a * isomorphism of WCE,onto WE'. Since a * isomorphism of a C*-algebra is an isometry (see Theorem 4.1.8(iii)), IIAC,.ll= IlAE'll < 1. (a),E', from which (WE'), c (a),E'. The reverse Thus AE' = ACE~E'E inclusion is immediate; and (W)l E' = (WE'),. If T is in the center of WC,,, then, in particular, T = TC,.. Thus 0 = TA(Z - C E f = ) A(Z - CE,)T, for A in 9%'.AS A = ACE, + A(Z - C E pand ) ACE, E B C E , ,TA = AT. It follow^ that T EW, SO that T EWCE..Of course WC,. commutes with BC,.. Combining these conclusions, WCE,is the center of BC,.. Hence WE' is the center of WE'. If T' in B(E'(%)) commutes with WE', then, denoting by T' again the operator on 2 that is 0 on ( I - E ' ) ( H ) and T' on E ' ( X ) , with T in 9, T' = E'T'E' and T ' T = T E ' T = T E ' T = T T . Thus T e a l , so that T' E E'W'E'. Ofcourse E'W'E'commutes with BE'. It follows that E'W'E' is the commutant of WE' on E ' ( X ) .
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5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
5.5.1. COROLLARY. r f W is a von Neumann algebra acting on the Hilbert then E B E acting on E ( H ) is a von Neumann space 2 and E is a projection in 9, algebra with commutant B'E.
Proof. From Proposition 5.5.6, EWE acting on E(H)is the commutant of W'E, and W'E is a von Neumann algebra. Thus EWE is a von Neumann algebra and its commutant in W(E(A?)) is W'E.
If {2,} is a family of Hilbert spaces and (9,) is a family of von Neumann algebras such that 9, acts on X,, we can form 1@A?, (= &')and the family W, of all bounded operators on H of the form 1@ A,, where A , E 9, and { llA,ll} is bounded (see Section 2.6, Direct sums, for a discussion of direct sums of Hilbert spaces and operators). It is clear that W is a self-adjoint algebra of operators on A?.Suppose that A is in the strong-operator closure of 9.We denote by Xb again the subspace of A? consisting of those vectors all of whose components, except possibly the component in A?,,are 0. Each operator in W commutes with P a ,the orthogonal projection of X onto X,, so that the same is true of A . Now AP, is in the strong-operator closure of WP, since right multiplication by P, is strong-operator continuous. Hence A,, the restriction of AP,, to X,, is in a,; and A = C @ A , E 9. It follows that W is a von Neumann algebra. We call W the direct sum of {a,}and denote it by 1@ 9,. Since P, E 9, each A' in w'commutes with P, . Thus A' = 1 @ A ; , where A ; is the restriction of A'P, to A?,.Hence 9'E 1 @ 9:.As the reverse inclusion is apparent, we see that w' = I@ B:. Cyclicity, separation, and countable decomposability. We have noted and used the fact that the range of a projection in a von Neumann algebra is stable under the action of the commutant. For certain of these projections, the image of a single vector acted on by the commutant is dense in the range. Such projections play an important role in the theory. In the discussion that follows, we study their properties. We begin by noting that the projection E with range [ W ' q is in 9 (= W"), where W is a von Neumann algebra and Y is a set of vectors, since [W'U is stable under the self-adjoint family 9'. 5.5.8. DEFINITION. A projection E in a von Neumann algebra W acting on when its range is [W'x] the Hilbert space H is said to be cyclic in W (or under 9') for some vector x. In this case, x is said to be a generating vector for E (under 9') If. [W'x] = H,we say that x is a generating vector under 9'. More generally, we say that a family Y of vectors is generatingfor W' when [B'yl = &', and that Y is generatingfor E when [# Y l is the range of E. If A , in W,is 0 when Ay = 0 for all y in Y , we say that Y is separating for 9. In particular, we speak of a as a separating vector for 9. vector y , such that A = 0 if Ay = 0 and A E 9, 5.5.9. PROPOSITION. I f E is a cyclic projection in the von Neumann algebra
W with generating vector x and F is a projection in W such that F < E , then F is
5.5. PROJECTION TECHNIQUES AND CONSTRUCTS
331
cyclic in W with generating vector Fx. Each projection in W is the union of an orthogonal family of cyclic projections in 9.
. Fiscontinuous Proof. Note that [A’Fx: A‘€.%’] = [FA’x: A ’ E ~ ’ ]Since and { A ’ X : A ’ E . % ’ >is dense in E ( H ) , {FA’~:A’EW’}is dense in F E ( 2 ) (= F ( X ) ) .Thus F is cyclic in B and Fx is a generating vector for F. Suppose E is an arbitrary projection in 9. If Eis 0, then E is cyclic in W with generating vector 0. If E # 0 and x is some non-zero vector in its range, then [W’x] is the range of a cyclic projection Eo. Since the ranges of Eo and E are Eo < E and Eo E 9‘‘ = W. Thus E has a stable under the self-adjoint family 9‘; non-zero cyclic subprojection if E # 0. The set of orthogonal families of nonzero cyclic subprojections of E is non-empty, and the union of each totally ordered subset is an upper bound for that subset under inclusion ordering. Zorn’s lemma guarantees the existence of a maximal orthogonal family { E,} of non-zero cyclic subprojections of E. If E - V, E, is not 0, it contains a non-zero cyclic subprojection Eo. Adjoining Eo to {E,} contradicts the maximality of { E , } . Thus E is the union of the orthogonal family {E,} of non-zero cyclic projections. The simple Zorn’s-lemma argument employed at the end of the preceding proof will be needed (with minor modifications) frequently. For the most part, all that will appear will be the imperative, “Let { E,} be a maximal orthogonal family of cyclic subprojections.”
5.5.10. PROPOSITION. If {Q,,} is a countable, orthogonal family of central projections in a von Neumann algebra W and {En}is a family of cyclic projections in W such that E,, < Q,,, then 1En is cyclic in W. Proof. If x,, is a generating unit vector for En under W‘, then {x,} is an orthonormal set, so that C n - l x , , converges to a vector x. As [w’x,] = [B’Q,,x] E [w’x], the range of 1Enis contained in [W’x]. Since the range of C E,, is stable under w’ and x is in that range, it coincides with [W’x]. That is, x is a generating vector for 1En, and C En is cyclic in 9.
5.5.1 1 . PROPOSITION. If9 is a von Neumann algebra acting on the Hilbert space 2,a subset Y of # is generating for W ifand only ifit is separatingfor 9’. Proof. Suppose Y is generating for W,A ‘ E W ’ ,and A‘y = 0 for ally in Y. Then 0 = AA‘y = A’Ay for all A in W and y in Y. As { A y :Y E Y, A E W}spans 2,A’ = 0. Thus Y is separating for 9’. If Y is not generating for 9, then [WY] is the range of a projection E‘ in w’, different from I . Thus I - E‘ # 0, and ( I - E’)y = 0 for all y in Y (since y is in the range of El). Hence Y is not separating for w’ in this case.
The special case of the preceding result in which Yconsists of a single vector is the one used most frequently.
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5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
A vector is generating for a von Neumann algebra i f 5.5.12. COROLLARY. and only i f it is separating for the commutant. I f 9 is a von Neumann algebra and E and E‘ are 5.5.13. PROPOSITION. projections in Wand 9‘with ranges [W‘x] and [Wx],respectively, then CE= CE.. Proof. From Proposition 5.5.2, CE and CE. have ranges [Ww’x] and [W’Wx], respectively. Since 9and w’commute, these subspaces coincide; and
cE=cE‘.
5.5.14. DEFINITION. A projection E in a von Neumann algebra W is said to be countably decomposable relative to 9 when each orthogonal family of non-zero subprojections of E in W is countable. When I is countably we say that W is countably decomposable. H decomposable relative to 9, The term “c-finite” is often used in place of “countably decomposable.” The von Neumann algebra W relative to which countable decomposability is asserted for a projection Eis important, as can be seen by taking E to be Iand W to be, first, 9(Z)with X non-separable, then to be { a ] } .The (minimal) projections corresponding to an orthonormal basis for X‘ form an uncountable orthogonal family of subprojections of I, so that I is not countably decomposable relative to B ( 2 ); but I is countably decomposable relative to { a I } . Despite the necessity for caution when a projection is claimed to be countably decomposable, reference to W will be omitted when no confusion can arise. If &? is a separable Hilbert space, each orthogonal family of projections is countable, so that a(#)and each von Neumann algebra on A? are countably decomposable. 5.5.15. PROPOSITION. If E is a cyclic projection in a von Neumann algebra 9,then E is countably decomposable. Proof. If x is a unit generating vector for E under 9‘and {E,} is an orthogonal family of non-zero subprojections of E in 9, then, from Proposition 5.5.9, E,x is a generating vector for E,. Since E, # 0, E,x # 0. From Bessel’s inequality (Remark 2.5.17), CaIIEaxllz< llxllz = 1. If the set of indices a is uncountable, there is a positive integer n such that l / n < IIE,xll for is not finite. Thus {Ea} is an infinite set of indices a. In this case, C,llE,~11~ countable. It follows that E is countably decomposable. H
5.5.16. PROPOSITION. A centralprojection P in a von Neumann algebra W is the central carrier of a cyclic projection in 9 if and only i f P is countably decomposable relative to the center W of 9.
5.5. PROJECTION TECHNIQUES AND CONSTRUCTS
339
Proof. Suppose P = C, with E a cyclic projection in 9, and x is a unit generating vector for E. If { P a } is an orthogonal family of central subprojections of P , then CallPaxll* 6 llxll* = 1. Thus (as in the proof of Proposition 5.5.15) Pax = 0 for all but a countable set of indices. If Pax = 0, then 0 = A'P,x = P,A'x for each A' in 9'. As [W'x] is the range of E, P,E = 0. From Theorem 5.5.4, 0 = PaC, = P a p = Pa. Thus P is countably decomposable relative to W. Suppose now that Pis countably decomposable relative to $9. Let { E,} be a family of non-zero projections cyclic in Wmaximal with respect to the property that their central carriers form an orthogonal family of subprojections of P . By hypothesis { CEa}and consequently { E,} are countable. From Proposition 5.5.10, C E, is cyclic in 9; and, from Proposition 5.5.3, its central carrier is V, CEa.If P - V, C,. # 0, it contains a non-zero projection Eo cyclic in 9, Since C,, is orthogonal to each CEO,adjoining Eo to {E,} contradicts the maximality of { E , } . Thus P = V, C,. ;and 1E, is a cyclic projection in W with central carrier P. H
An important consequence of the preceding result deals with the case where
W is abelian. 5.5.17. COROLLARY. If d is a countably decomposable abelian von Neumann algebra acting on the Hilbert space X, d has a separating vector. Ifd is maximal abelian, in addition, the separating vector is generating for d . Proof. Since d is its own center and Zis countably decomposable relative to d ,Z is (the central carrier of) a cyclic projection in d .With x a generating vector for d ' ,x is separating for d (= d"),from Corollary 5.5.12. If d is as well. W maximal abelian, x is generating for d (= d')
A partial converse to the preceding result states that if an abelian von Neumann algebra has a generating vector then it is maximal abelian. While an ad-hoc argument could be given to establish that converse, at this point it is best to defer further discussion to Section 7.2 (see Corollary 7.2.16), where suitable techniques are developed. An illustration of the situation discussed in Corollary 5.5.17 is provided by the example (see Example 5.1.6) of the multiplication algebra of the unit interval under Lebesgue measure. In this case the algebra is maximal abelian; and the constant function 1 is a separating (and generating) vector for it. 5.5.18. PROPOSITION. If W is a countably decomposable von Neumann algebra acting on the Hilbert space 2,there is a centralprojection cyclic in W whose orthogonal complement is cyclic in 9'. Proof. Let {x,,} be a set of unit vectors in X maximal with respect to the property that {En}and { E i } are orthogonal families of projections, where E,,
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5 . ELEMENTARY VON NEUMANN ALGEBRA THEORY
has range [W’x,,] and Ei has range [Wx,,]. Since { E n is } an orthogonal family of projections in 9,it is countable. We may assume that the index n is a positive integer. As {x,} is an orthonormal set, E n - l x , converges to a vector x in S. If E is 1En and E‘ is 1E ; , then E and E‘ are cyclic projections in 9 and W’, respectively;and xis a generating vector for each. Of course, x is in the range of both Eand E’. In addition [wlx,] = [a’E:x] G [W‘x].Thus [ a ’ x ] is the range of E. Symmetrically, [ a x ] is the range of E‘. If (I- E)(I - E’) # 0, a unit vector x o in the range of (I - E ) ( I - E‘) will generate cyclic projections Eo in W and Eb in W‘orthogonal to { E n }and { EA}, respectively. Adjoining xo to {x,,} contradicts the maximality of {x,,}.Thus (I - E)(I - E’) = 0; and, from Theorem 5.5.4, C,-EC,-E,= 0. Since I - C,-E < E, I - C,-E is cyclic in W (from Proposition 5.5.9). Similarly I - C,-E, is cyclic in 9’.As Cj-E < I - Cj-E’,C j - Eis cyclic in 9’. Thus I - C , - E is a central projection cyclic in 9 whose orthogonal complement, Cj-E, is cyclic in 9’. H 5.5.19. PROPOSITION. I f E is the union of a countable family { E n }of cyclic then E is countably decomposable in W. projections in a von Neumann algebra 9, Proof. Let x,, be a unit generating vector for En under 9‘. Let {Fa: a E A} be an orthogonal family of non-zero subprojections of E in 9; and let % be { a :Fax, # O } . If a is not in %, Fax, = 0; and (0) = [&?’Fax,,]= [FaW’x,]. Since Fa < V, En,Fa does not annihilate the range [W’x,,] of each En.Thus U % = A. On the other hand, CacAllFaxnllZ < Ilx,,llz = 1, so that Fax, # 0 for at most a countable number of elements a of A -that is, % is countable. If K is the cardinal number of A, K < KO * KO = KO(see the final paragraph of the proof of Theorem 2.2.10), since A = U %. H
5.6. Unbounded operators and abelian von Neumann algebras In this section we study the spectral theory of unbounded self-adjoint and normal operators. We associate an (unbounded) spectral resolution with each (unbounded) self-adjoint operator (compare Theorem 5.2.2 and the discussion following it). We extend the function calculus (both continuous and bounded Borel) to (unbounded) normal operators (compare Theorem 5.2.8). We begin with a discussion (really, a continuation of Example 5.1.6) that details the relation between unbounded self-adjoint (and normal) operators and the multiplication algebra of a measure space. If we are prepared to confine attention to, say, the case of separable Hilbert space, this discussion combined with some of the general theory of abelian von Neumann algebras to be developed in Chapter 6 contains all that we need in dealing with unbounded self-adjoint (and normal) operators.
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341
If g is a (complex) measurable function (finite almost everywhere) on S now, without the restriction that it be essentially bounded -multiplication byg will not yield an everywhere-defined operator on L,(S), for many of the products will not lie in L2(S).Enough functionsfwill have productfg in L2(S), however, to form a dense linear submanifold 9 of L 2 ( S ) and constitute a (dense) domain for an (unbounded) multiplication operator M,. To see this, let En be the (bounded) multiplication operator corresponding to the characteristic function of the (measurable) set on which 191 < n. Since g is finite almost everywhere, {En}is an increasing sequence of projections with union I. The union goof the ranges of the En is a dense linear submanifold of L,(S) contained in 9.A measure-theoretic argument shows that M , is closed with go as a core. In fact, if { f , } is a sequence in 9 converging in L,(S) tofand {gf,} converges in L 2 ( S )to h, then, passing to subsequences, we may assume that { f , } and {d,}converge almost everywhere tofand h, respectively. But, then, { a f , } converges almost everywhere to gf, so that gfand h are equal almost 9 ,= M,(f), and M , is closed. Withf, in 9, everywhere. Thus gj"~L 2 ( S ) , f ~ h { E n f o }converges tofo and {M,Enfo} = {E,,M,fo} converges to M,fo. Now E,f0 e g o ,so that gois a core for M,. Note that M,E,, is bounded and that its bound does not exceed n. Using the lemma that follows, we see that M Bis an (unbounded) self-adjoint operator when g is real-valued, since M,E,, is a bounded self-adjoint operator in that case.
5.6.1. LEMMA.Ij" {En} is an increasing sequence of projections on the ;l En(#) Hilbert space 2 and A . is a linear operator with dense domain U= (= go)such that AoEn is a bounded self-adjoint operator on Z,then A . is preclosed and its closure is self-adjoint. If A is closed with core goand AE,, is a bounded self-adjoint operator, A is self-adjoint. Proof. With x and y in go,there is an m such that
( A o x , y ) = (AOEnlX,Y) = (X,AoEmY) = (X,AoY). Thus y ~ g ( A ; and ) A,* is densely defined. From Theorem 2.7.8(ii), A . is preclosed. With A the closure of Ao, it remains to prove the last assertion of this lemma. With x and y in 9 ( A ) , we can choose sequences {x,,} and {y,,} in go converging to x and y , respectively, such that {Ax,,} and { Ay,} converge to A x and A y . Then (AXfl,Yn) = (AEmxfl,yfl)= (Xfl,AEmYn) = (xn,Ayn). Now ( A x , , , y n )tends to ( A x , y ) and (x,,, Ay,,) tends to ( x , A y ) as n tends to infinity. Thus ( A x , y ) = ( x , A y ) ; and A is symmetric. Note that ( A iI)E,, has range En(&') since AE, is bounded and self-adjoint (so that AE,, f iE,,has a bounded inverse on E , , ( 2 ) ) , Thus A _+ iI has a dense range. From Lemma 2.7.9, this range is 2 ;and, from Proposition 2.7.10, A is self-adjoint. H
*
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If Ma is unbounded, we cannot expect it to belong to the multiplication algebra d of the measure space (S,Y: m).Nonetheless, there are various ways in which M gbehaves as if it were in d -for example, M gis unchanged when it is “transformed” by a unitary operator U commuting with R In this case (see Example 5.1.6), U E ~so, that U = M u where u is a bounded measurable function on S with modulus 1 almost everywhere (see Example 2.4.1 1). Withf in 9 ( M g ) ,gujc L 2 ( S ) ;while, if guh E L2(S), then gh E L,(S) and h E 9 ( M a ) . Thus U transforms 9 ( M g )onto itself. Moreover
( U * M g U ) ( f )= Uguf= lu12gj-=
trf:
Thus U*MaU = M g . The fact that Ma “commutes” with all unitary operators commuting with d in conjunction with Theorem 4.1.7 and the double commutant theorem (5.3.1) (from which it follows that a bounded operator having this property lies in d)provides us with an indication of the extent to which Ma “belongs” to R We formalize this property in the definition that follows. 5.6.2. DEFINITION. We say that a closed densely defined operator T is affiliated with a von Neumann algebra W (and write T q B ) when U*TU = T for each unitary operator U commuting with 9.
Note that the equality, U*TU = T, of the preceding definition is to be understood in the strict sense that U*TU and T have the same domain and (formal) equality holds for the transforms of vectors in that domain. As far as the domains are concerned, the effect is that U transforms 9 ( T ) onto itself. 5.6.3. REMARK.If Tis a closed densely defined operator with core goand U* TUX = Tx for each x in goand each unitary operator U commuting with a von Neumann algebra 9, then T q 9. To see this, note that, with y in 9(T), there is a sequence { y,,} in gosuch that y,, + y and Ty, + Ty (since g ois a core for T).Now Uy, -+ U y and TUy, = UTy,, + UTy. Since Tis closed, U y E 9(T) and TUy = UTy. Thus g(T)G V*(g(T)).Applied to U*, we have 9 ( T )s U(g(T)),so that U ( 9 ( T ) )= a ( T ) . Hence 9(U*TU) = 9(T)and U* TUy = Ty for each y in 9(T). Our discussion to this point establishes that M gis a closed operator (selfadjoint, when g is real-valued) affiliated with the multiplication algebra d. Conversely, if A is a closed (unbounded) operator affiliated with d,then A has the form Ma (and g is real-valued when A is self-adjoint). We prove this in the discussion that follows. Since each Bo in d is a linear combination of four unitary operators in d (see Theorem 4.1.7) and UA E A U for each such unitary operator (as d c_ d‘ and A q d), we have BOAc ABo. In particular, if E is the projection
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343
corresponding to multiplication by the characteristic function of a measurable subset So of S , EA c AE, so that E f e g ( A ) iffeQ(A). Let Q 1 be the set of essentially bounded functions in Q(A). Iff€ Q(A) and E, is multiplication by the characteristic function of { x :1fix)l < n } , { E n }is an ascending sequence of projections in .d tending to I in the strong-operator topology (sincefis finite almost everywhere). From the foregoing Enf e g1, {E,f)tends to f and AE, f = E,Af+ Afas n + co. Thus g1is a core for A . Iffand g are in g1,then f A g = M f A g = AMJg = A(f g ) = A M , f = MgAf = gAf. (1) Let { S , } be a family of mutually disjoint sets of finite positive measure with union S. If x, is the characteristic function of S,, there is a sequence {f,,}of elements of g1tending to x,. If
S,O = {s :S E S,, f,,(s) = 0 for all j } and x is the characteristic function of S,O, then 0 = M X f n+ j Mxxn= x , so that S,O has measure 0. Let g(s) be [ ( A f , , ) ( ~ ) ] [ f , , ( s ) ]where - ~ , seS,\S,O and j is the least integer such thatf,,(s) # 0. Then g is a measurable function on S defined almost everywhere. For eachfin g1,f,,(s)(Af)(s)=fis)(Af,,)(s) for all n a n d j except on a set of measure 0, from (1). Thus ( A f ) ( s )= g(s)fis) almost everywhere. We have noted that M gis closed and affiliated with d.Since M gis an extension of the restriction of the closed operator A to the core g1for A , we have that A c Mg.As we have noted earlier, the set of functions h in L2 vanishing on the complement of sets {s:s E S, 1g(s)l < m } forms a core for M g. With h such a function, let {f,}be a sequence of functions in B1tending to h. If x is the characteristic function of the set of points at which h does not vanish, we may replace f, by x f , . Changing notation, we may assume that each f, vanishes when h does. In this case, since M g M , is bounded, Af, = M g M x f ,+ MgMxh= Mgh.
As A is closed, h E 9 ( A ) and Ah = M,h. It follows that A = M g . If A is self-adjoint, Mgxis a bounded self-adjoint operator, so that gx is realvalued almost everywhere. Hence g is real-valued almost everywhere. Again, as with bounded multiplication operators, the projections E d , corresponding to multiplication by the characteristic function of the set where g does not exceed 2, form a spectral resolution {Ed}of A (see the discussion following Theorem 5.2.5). In this case, if A is, in fact, unbounded, there will be no non-negative real number a (replacing llAll when A is bounded) such that EA= 0 if A, c - a and Ed = I if a < A, ;and we speak of {Ed}as an unbounded resolution of the identity (see the discussion preceding Theorem 5.2.3). We summarize the foregoing conclusions in the theorem that follows. 5.6.4. THEOREM. r f ( S , x m ) is a o-finite measure space and d is its multiplication algebra acting on L2(S), then A is a closed densely defined
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5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
operator affiliated with d ifand only i f A = M Bfor some measurable function g finite almost everywhere on S. In this case, A is self-adjoint ifand only i f g is realvalued almost everywhere.
An unbounded self-adjoint operator A can be associated (“affiliated”) with an abelian von Neumann algebra. As in the case of a bounded self-adjoint operator (see Theorem 5.2.2),we can use Theorem 5.2.1 to locate a function on an extremely disconnected compact Hausdorff space that “represents” A. As might be expected, this function is neither everywhere defined nor bounded. We will be able to use this representing function (as in Theorem 5.2.2)to find a resolution of the identity (unbounded) for A. (See the discussion of resolutions following Theorem 5.2.2.) In preparation for this analysis, in the definition that follows we describe the functions that appear. 5.6.5. DEFINITION. If X is an extremely disconnected compact Hausdorff space a normalfunction on Xis a continuous complex-valued functionfdefined on an open dense subset X\Z of Xsuch that limq+plf(q)l= 00 for eachp in Z (where q E X\ Z ) . A self-adjoint function on Xis a real-valued normal function on X. Iff is self-adjoint and defined on X\ Z , we denote by Z, those pointsp of Z such that lim,,,f(q) = +00 and by Z - those p in Z such that lim,,,f(q) = - 00. We denote by N ( X )and Y ( X )the sets of normal and selfadjoint functions on X. H It is one of the many surprising properties of extremely disconnected compact Hausdorff spaces (and their associated constructs) that Z = Z, u Z - (that is, there are no points near which an f i n Y ( X ) takes arbitrarily large positive and negative values). This follows from the fundamental property of such spaces that disjoint open sets O1 and have disjoint closures (X\Ol is closed and, thus, contains 0; ,a clopen set, so that X\0; is closed and contains 0;).To see this, note that the sets
are disjoint open sets (sincefis continuous on X\ Z and X\ Z is open in X), and that a point near which f takes arbitrarily large positive and negative values would have to lie in both of their closures. Clearly If1 is normal on X iff is normal. The following simple lemma will prove useful to us.
5.6.6. LEMMA.Iff and g are normal functions on X defined on X\Z and X \ Z , respectively, and f ( q ) = g(q) for each q in some dense subset S of X \ ( Z u Z’), then Z = Z ’ and f = g . Proof. Since X\(Z u 2’)is dense in X,Sis dense in X. Ifp E Z‘ and q in Sis near p, then Ig(q)l (= If(q)1) is large so that P E Z . Thus Z’ c Z and,
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345
symmetrically, Z G Z'. Hence f - g is defined and continuous on X\Z and 0 on the dense subset S. It follows t h a t f = g.
5.6.7. LEMMA. ZfA is a self-adjoint operator acting on a Hilbert space X, A is affiliaied with some abelian von Neumann algebra. Zf A q d and d is isomorphic to C(X ) , with X an extremely disconnected compact Hausdorffspace, there is a unique self-adjoint function h on X such that h'e is in C ( X ) and represents A E when E is aprojection in d such that A E is a bounded everywheredefined operator, where e in C ( X ) corresponds to E, and (h :e)(p) is h ( p ) fi e( p ) = 1 and 0 otherwise. There is a resolution of the identity { E,} in d such that UF= Fn(H)is a core for A , where Fn = En - E-,,, and A x = J:, A dE, x for each x in F,,(X) and all n, in the sense of norm convergence of approximating Riemann sums. Proof. From Proposition 2.7.10 and Remark 2.7.11, A + il and A - il have range X, null space (0), and inverses T + and T - that are everywhere defined with bound not exceeding 1. Note that (T,(A
+ iI)x,( A - i l l y ) = ( x , ( A - i l l y ) = ( ( A + i l ) x , y ) = ( ( A + il)x, T - ( A - illy),
when x and y are in 9 ( A ) ,since A is self-adjoint. Thus T - = TT . (Recall that A f iZ have range X, so that ( A + i l ) x and ( A - illy represent arbitrary vectors in X.)Again, since A il have range X, we can represent an arbitrary vector as ( A - il)(A + il)x, where x E 9 ( A ) and A x E 9 ( A ) . In this case,
*
( A - il)(A + iI)x = ( A 2 + Z)x = ( A + il)(A - il)x, and T +T - = T - T , . Since T- = T: , T , is normal. Let d be an abelian von Neumann algebra containing I, T , , and T - . If U is a unitary operator in d', for each x in 9 ( A ) , U x = U T + ( A+ i l ) x = T+U(A+ i l ) x so that ( A + iI)Ux = U(A + i l ) x ; and U - ' ( A + iI)U = A + il. Thus U - ' A U = A and A q d . In particular A q d O , where d ois the (abelian) von Neumann algebra generated by I, T , , and T - . From Theorem 5.2.1, d 1C ( X ) , where X is an extremely disconnected compact Hausdorff space. Let g + and g- be the functions in C ( X )corresponding to T , and T - . Let h + and h- be the functions defined as the reciprocals of g + and g - , respectively, at those points where g+ and g - do not vanish. Then h , and h - are continuous where they are defined on X , as is g h + + h - ) ( = h). In a formal sense, h is the function that corresponds to A. We shall see that h is real-valued and find the spectral resolution of A by subjecting h to the same spectral analysis as was performed on the functions of C ( X ) in proving Theorem 5.2.2. Since T , and T - are adjoints of one another, g+ and g- are complex conjugates of one another (in particular, they vanish at the same points of X ) .
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5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
Thus h , and h- are complex conjugates of one another; and h is real-valued. The set Z on which g+ (and g-) vanishes is closed (since g+ is continuous) and nowhere dense; for if it contains a non-null open set it contains its closure, a non-null clopen set. The projection corresponding to this non-null clopen set would have product with T , (and T - ) equal to 0-contradicting the fact that T , (and T - ) have null space (0). Thus each point p in Z is a limit of points q in X\Z (at which h is defined). For each y in 2, A T , T - y = ( A + iZ - iZ)T, T - y = T - y - iT, T-y,
so that A T , T (2)
=
T - - iT, T - . Similarly AT- T , = T ,
+ iT, T - . Hence
2iT+T- = T - - T ,
and (3)
A T + T - = i ( T ++ T - ) .
It follows from (2) that (h(q) + i)-’ = g+(q) (and that (h(q) - i ) - l = g-(q)) forqinX\Z. Hence,foreachqinX\Znearp(inZ),g+(q)isnearOand Ih(q)lis large. Thus h is a self-adjoint function on X . Let 0,be U , u Z, ,where U , is the set of points of X \ Z a t which h exceeds I and Z, has the meaning explained in Definition 5.6.5. We show that 0, is open. Since h is continuous on X\Z and X\Z is open in X , U , is open in X . If p E Z , , there is an open set 0 containing p such that if q E 0 n ( X \ Z ) , then h(q) > 0 -and since Ih(q)Jis large for q (in X\ Z ) near p , we may choose 0 such that h(q) > I for q in 0 n ( X \ Z ) . In this case, 0 n (X\Z) E U , E 0,.If there were ap’ in 0 n Zwithp’ in Z- ,then, from the definition of Z - ,there would be a q in 0 with h(q) strictly negative-contradicting the choice of 0. Thus 0 n Z E Z, ; 0 E 0,;and 0,is open, as asserted. Let X , be X\0;. Again, as in the proof of Theorem 5.2.2, X , contains eachclopen set Y such that h ( p ) ,< I for allp in Y. (For pointsp of Z - , where “ h ( p ) = - co,” we write “ h ( p ) ,< I” as well, so that Y may contain points of Z- .) Indeed, if q E O,, then q E X \ Y, a closed set, so that 0, c X\ Y and Y E X\0; = X , . At the same time, A’, is such a clopen set, for if PE X , n (X\Z), then, since pq! U,, h ( p ) < A. If p E X , n Z , then p E Z\ Z, (= 2 - )and h ( p ) < I (in the extended sense). Thus X, is the largest clopen set on which h ( p ) < I ; and X , n Z = Z - . We proceed now as in the proof of Theorem 5.2.2. Let e, be the characteristic function of X , and Ed be the projection in d corresponding to e,. In this case, { E d }satisfies El < Ed, if A < 1’ and Ed = A a < L , EA..Since Z is nowhere dense, V, e, = 1 and A e, = 0, so that V, Ed = I and A Ed = 0. That is, we have constructed a resolution of the identity { E d } (and this resolution is unbounded if h $ C(X)). Let F be Eb - E,, where a < b. Then eb - e, (=A, the characteristic function of Xb\X,, corresponds to F. Since X b n Z = X , n Z (= Z - ) , we have that &\Xu c X \ Z ; and g + ( p ) g - ( p ) # 0
,
,
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347
when f(p) = 1. For p in X\ 2,
(4) by choice of h. Moreover, there is a positive function k in C ( X ) such that kg+g- =fand kf = k (since g + g - is continuous and vanishes nowhere on the clopen set &\X,). If K in d corresponds to k , then (5)
K T + T - = F.
From our information about X l , if p E Xb\X,, then p 4 2 and a Thus from (4), w+g-f< & J +
+g-)f<
< h(p) < b.
bg+g-f,
and akg+g-f= af< f ( g +
+ g - ) k f = f ( g + + g-)k < bkg+g-f= bf.
Thus aF < i ( T + + T - ) K < bF. (6) Combining (3), (5), and (6), we have (7)
a F < A F < bF.
It follows that AF is bounded; and from (3), (4),and (5), the corresponding element of C ( X )is h -f.With E a projection in d such that A E E a(%) and U a unitaryoperator in d ' ,U-'AEU = U - ' A U E = AE, so that A E E ~Letf, . in C ( X )correspond to F,, and Y,, be the (clopen) subset of Xon whichf, takes the value 1. Since V=; F,, = I , { Y,,}has union dense in X . If e in C ( X )corresponds to E, then (replacing F, above, by F,,) ( h -f,)e corresponds to AF,,E (= AEF,,). Suppose e(p) = 1. There is some n and a q in Y,, near p. In this case, ((h-f,)e)(q)= y q ) and Ih(q)l < IIAp,,,II < 114Ell. Hence-P#Z and 1 " < llAEll. Thus h . e E C ( X ) . Now (h . f , ) e = h (ef,) = (h * elf,. If fi in C ( X ) represents AE, then &, represents AEF,,. Thus Kf,, = (h elf, for all n and fi= h:e. Since (2Fn- I)A(2F,, - I ) = A , we have F,,A G AF,,. Thus, with x in .9(A), F,,? -+ x and AF,,x = F,Ax -+ Ax. Hence =U ; F,,(H)is a core for A. Since (h e)ee, < leel and l ( e - eel) < (h e)(e - eel), we have, from Theorem 5.2.3, that { EEII E(&)}, a resolution of the identity on E ( X ) ,is the resolution of the identity for AEI E ( H ) .With F,, in place of E, Theorem 5.2.2(v) (applied to AJ',,lF,,(i@) and the resolution {F,,EIIF,,(H)})yields the equality, A x = ~ - , , l d E l xfor , x in F,,(%) and all n. If x E 9 ( A ) ,
j:,,
AdE, x =
f
AdE, F,,x = AF,,x -n
-+ Ax.
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Interpreted as an improper integral, we write tm
Ax=[
AdEdx -m
when X E ~ ( A ) . In the circumstances set out in Lemma 5.6.7, we say that h (in Y ( X ) ) represents A (q d ) . 5.6.8. LEMMA.r f d is an abelian von Neumann algebra acting on the Hilbert space X and d is isomorphic to C ( X )with X an extremely disconnected compact Hausdorff space, then each h in Y ( X ) represents some self-adjoint operator A affiliated with d. Proof. From the proof of Lemma 5.6.7, h determines a resolution of the identity { E d }in d and h : f n E C ( X ) ,where f,, = en - e-,, with e,, in C ( X ) representing En.Let A,, in d correspond to h :f,.Noting that (h ?fm)fn = h :f, when n 6 m, we have A,F,, = A,, in this case, where F, in d corresponds tof , . Thus, defining Aox to be A,,x when x E F,,(X), A . is a linear transformation ; F,,(X)(= Q0). From Lemma 5.6.1, A. is preclosed and its with domain =U closure A is self-adjoint with core go.If U is a unitary operator in d' and x,,~F,,(i#'),then UX,,EF,,(X) so that U-'AUx,, = U-'A,,Ux,, = A,,x,, = Ax,,. From Remark 5.6.3, A q d . If h in Y ( x )represents A, then, from Lemma 5.6.7, 6: f n represents AF,, (= A,,). Thus h : f n = h-fn for each n. From Lemma 5.6.6, h = h, since h and h agree on dense subsets of X. Thus h represents A.
5.6.9. LEMMA.If{E,} is a resolution of the identity on a Hilbert space i#' and d is an abelian von Neumann algebra containing { E d }there , is a self-adjoint A affiliated with d such that (9)
Ax=f
AdEdx, -n
for each x in F,,(X)and all n, where F, = En - E - , ;and {Ed}is the resolution of the identity for A (as constructed in Lemma 5.6.7). Proof. Suppose d is isomorphic to C ( X ) with X an extremely disconnected compact Hausdorff space. Let edin C ( X )correspond to E, and let Xdbe the clopen subset of Xon which ed takes the value 1. Let 2- be fl, X, and Z, be X\(U, X,). Then Z - and Z, are closed subsets of X.Both are nowhere dense in X since A En = 0 and V, Ed = 1. Their union Z is a closed nowhere-dense subset of X. IfpEX\Z, let h(p) = inf{A:p EXd}.Given a positive E and a point qoofX\ZatwhichhtakesthevalueA,ifq~X,+,\Xd-,,then Ih(q) - h(qo)l < E . Thus h is continuous on X\Z. By definition, h tends to + 00 at points of Z, and
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349
to - co at points of Z - . Thus h EY(P(X); and, from Lemma 5.6.8, hcorresponds to a self-adjoint operator A affiliated with d.We note that { E , } is the resolution of the identity for A , so that (9) holds, by identifying X , as the largest clopen set on which h takes values not exceeding A. If Y is another such clopen set, e is its characteristic function, and E in d corresponds to e, then Y c X x for each 1' exceeding 1.Thus E < A E,! = E, and Y G X , . H
,',,
5.6.10. LEMMA.I f A is a closed operator on the Hilbert space X, { E , } is a 'U : F,,(X) ( = g o )is a core for A , where resolution of the identity on 2, F,, = En - E-,,, and Ax=J
(10)
AdE,x -n
for each x in F,,(X) and all n, then A is sew-adjoint and { E,} is the resolution of the identity for A . Proof. From (lo), AF,, is bounded, everywhere defined, and is the strongoperator limit of finite real-linear combinations of {E,}. Thus E,AF,, = AF,,E, and AF,, is self-adjoint. From Lemma 5.6.1, A is self-adjoint. If x E 9 ( A ) , there are sequences Inj} (tending to 00) and { x j } such that x j = Fnjxj+ x and A x j + A x , since g o is a core for A . For each n,
F,,Ax = lim F,,AF,,,xj = lim AFnjFnxj= lim AFnxj = AF,,x j
j
i
so that F,,A G AF,, for all n. At the same time AF,,E,x = E,AF,,x + E,Ax and F,,E,x + E,x. Since A is closed, E , x E ~ ( A )and AE,x = E,Ax. Thus E,A c AE, and (2E, - I)A(2EL- I) = A . It follows that {E,} commutes with T , and T - . Let d be the (abelian) von Neumann algebra generated by {E,}, T , , and T - . As noted in Lemma 5.6.7, A q d.From Lemma 5.6.9, there is a self-adjoint operator d affiliated with d such that dx = 1dE, x for each x in F n ( X )and all n. Since a0is a core for both A and d on which they agree, A = A and { E , } is the resolution for A .
p-,,
5.6.1 1. REMARK.The (abelian) von Neumann algebra d ogenerated by T , and T - is the smallest von Neumann algebra with which (the self-adjoint operator) A is affiliated. (See the proof of Lemma 5.6.7 for the main part of this.) We refer to d oas the von Neumann algebra generated by A . H We assemble the foregoing results in the theorem that follows. 5.6.12. THEOREM.If d is an abelian von Neumann algebra acting on a Hilbert space X , Y ( d )is the family of self-adjoint operators affiliated with d, d is isomorphic to C ( X )with X an extremely disconnected compact Hausdorjjf space, and Y ( X ) is the family of self-adjoint functions on X , then:
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5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
(i) there is a one-to-one mapping q of Y ( d )onto , Y ( X ) extending the isomorphism of d with C(X)for which q ( A ) e corresponds to A E for each projection E in d with A E in d,where e in C ( X ) corresponds to E and ( q ( A ) e)(p ) is (cp( A))(p ) or 0 according as e( p ) is 1 or 0 ; (ii) there is a resolution of the identity { E d } in the abelian von Neumann subalgebra d oof d generated by an A in Y ( d )such that
A x = r IdEdx -n
for each x in Fn(%) and all n, where F,, = En - E - , , and =U : Fn(%) is a core for A ; (iii) if{E ; } is a resolution of the identity on i+f such that A x = IdE; x for each x in F:(i+f) and all n, and U=:l FA(#) is a core for A , where F n = E’n - E’- n ’ then Ed = E ; for all A ; ) (iv) i f { E,} is a resolution of the identity in d there is an A in Y ( d for which (1 1) holds; (v) ifel in C ( X )corresponds to Ed and Xdis the clopen set on which ed takes the value 1, then Xdis the largest clopen subset of X o n which q ( A ) takes values not exceeding A (in the extended sense).
p-,,
It will prove convenientto have the scope of our study broadened to include unbounded “normal” operators as well as self-adjoint operators. We say that a closed densely defined operator A is normal when the two self-adjoint operators A*A and AA* (= A**A*) are equal. A spectral theory and function calculus for such operators will make it possible for us to apply complex function theory techniques (see, for example, Stone’s theorem (5.6.36) and Section 9.2). We begin with some preparatory material concerning extensions of unbounded operators. The following simple facts are easily verified. B and C c D,then A
(12)
If A
5
(13)
If A
E B,
(14)
(A
+ C E B + D.
then C A c CB and A C G BC.
+ B)C = AC + BC,
C A + CB G C ( A + B).
In connection with the last assertion of (14), note that we do not have equality in general. This is illustrated by a densely (but not, everywhere-)definedC and A = Z, B = - I . In this case, C ( A + B) is 0 but CA + CB is O(Q(C).It follows from these rules that if C A E AC for each C in some family A then TA c AT for each sum Tof products of operators in 9We cannot speak of the “algebra” generated by $for, as we have just noted, a distributive law fails. However, if 9consists of everywhere-defined operators (in particular, of operators in g ( X ) ) ,we can speak of this algebra.
5.6. UNBOUNDED OPERATORS AND ABELIAN VON NEUMANN ALGEBRAS
351
We may add to (12), (1 3), (14) another easily proved rule. (1 5 ) If { T,} is a net of operators in B(#) tending to Tin the strong-operator
topology and T,A E BT, for each a, where B is closed, then TA E BT. To see this, suppose x ~ 9 ( A )Then . Tax€9 ( B ) , and BT,x = TaAx+ TAX. Now, Tax+ Tx. As B is closed T x E ~ ( Band ) BTx = TAX,from which (15) follows. Combining the results of this discussion, we have the following lemma. 5.6.13. LEMMA.I f A is a closed operator acting on the Hilbert space &‘ and CA c AC for each C in a self-adjoint subset % of B(&‘),then TA G ATfor each T in the von Neumann algebra generated by 5
If A is a closed operator and E is a projection on # such that EA E AE and AE is a bounded everywhere-defined operator on 2,we say that E is a bounding projection for A. If {En}is an increasing sequence of projections each En = I, we say that {En}is a bounding of which is bounding for A and V=; sequence for A.
5.6.14. LEMMA.If E is a bounding projection for a closed densely defined operator A on the Hilbert space 2,then E is bounding for A*, A*A, and AA*; and (AE)* = A*E. If{E,} is a bounding sequencefor A , then U =:, En(&‘)is a core for each of A, A*, A*A, and AA*. Proof. Note that EA is preclosed, densely defined, and bounded, since EA G AE and AE is bounded. Thus EA has closure AE and (EA)* = (AE)* from Theorem 2.7.8(i). If x E E ( Z )and y E 9 ( A ) ,then ( A y , x) = ( y , (EA)*x), so that x ~ 9 ( A *and ) A*x = (EA)*x. It follows that A*E = (EA)*E. But ( I - E ) Z = 0 so that (EA)* = (=)* = (EA)*E = A*E. Now EA* E (AE)* = ( E A ) * = A*E; and E is bounding for A*. Since EA*AG A*AE (= A*EAE), E is bounding for A*A and, similarly, for AA*. It follows that {En}is a bounding sequence for A*, A*A, and AA* if it is for A. If x ~ 9 ( A )then , E,x + x, E n x € 9 ( A ) ,and AEnx = E,Ax -+ Ax. Thus =U ; En(9(A))is a core for A. Since En(#) c 9 ( A ) ,.U : En(&‘) is a core for A (and A*, A*A, AA* as well).
Recall that the statement “ Aq d”includes the assumption that A is closed and densely defined. 5.6.15. THEOREM.I f d is an abelian von Neumann algebra acting on the Hilbert space &‘ and A, Br]d,then:
(i) eachfinite set of operators affiliated with d has a common bounding sequence in d ;
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5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
(ii) A + B is densely defined and preclosed and its closure A $ B q d ; (iii) A B is densely defined and preclosed and its closure A :B q d ; (iv) A - B = B - A and A*A = AA* (= A * : A ) ; (v) (aA $ B)* = dA* $ B*; (vi) ( A : B ) * = B * : A * ; (vii) if A E B, then A = B ; i f A is symmetric, A = A* ; (viii) thefamily N(d) of operators affiliated with d forms a commutative * algebra (with unit I ) under the operations of addition $ and multiplication described in (ii) and (iii). +
Proof. Throughout this argument, U denotes a unitary operator in d ' . Since U*AU = A, we have U*A*U = A*; and A * q d . At the same time U*A*AU = A*A and A*Aq d.If E is a projection in d,(2E - I) is a unitary so that (2E - I)A(2E - I ) = A. Thus EA c AE. From operator in d ( E d'); Theorem 2.7.8(v), A*A is self-adjoint. Let { E L }be its spectral resolution and let F, be En - E-,,. From Theorem 5.6.12(ii), EL€&. As A*AF, is bounded and everywheredefined, AF, is everywhere defined and closed, since A is closed and F, is bounded. The closed graph theorem (1.8.6) tells us that AF, is bounded. (This follows directly, as well, since IIAFnx(l2= (F,x, A*AF,x) < IIA*AF,II Ilxl12.)As {F,,}is an increasing sequence of projections in d with least upper bound I and F,,A E AF,, if X E 9 ( A ) ,F,x + x and AF,x = F,Ax -+ Ax. Thus =U : F,(H) is a core for A and {F,} is a bounding sequence in d for A. Suppose {En}is a bounding sequence in d for { A j } ,j = 1,. . . ,m - 1 and {F,} is a bounding sequence in d for A,,,, where A j € d . Then {E,F,} is a : E,F,(H) is a bounding sequence in d for A l , . . . , A m . In particular, =U common core for A l , . . . ,Am. It follows that both A + B and A* + B* are densely defined. But A* + B* E ( A + B)*, so that ( A + B)* is densely defined and A + B is preclosed (see Theorem 2.7.8(ii)). If {En} is a bounding sequence in d for A , B, A*, and B*, then E,,AB E AE,B E ABE, and AE,BE, E ABE,,. As AE, and BE, are bounded and defined everywhere, AE,,BE, = ABE,. Thus {En}is a bounding sequence for A B and, similarly, for BA and B*A*. In particular B*A* is densely defined. As B*A* E (All)*,(AB)* is densely defined and A B is preclosed. At the same time, ABE,, = AE,,BE, = BE,,AE, = BAE,. Thus A B and B : A agree on =; ! E,(H); and A B = B7 A. As A*A and AA* are selftheir common core adjoint, A*A = A* * A = A :A* = AA*. If X E ~ ( A n 9) ( B ) (= 9 ( A + B)), U X E ~ (+AB ) and U * X E ~ (+AB). Thus U ( 9 ( A + B)) = 9 ( A + B ) and U*(A + B)U = A + B. It follows that U*(A $ B)U = A $ Band A $ B q A If ~ E ~ ( A then B ) ,y ~ 9 ( Band ) lly&(A). Thus U y € 9 ( B )and BUy = UBy E ~ ( A It ) .follows that U y € 9 ( A B ) .Since U*yeQ(AB),U ( 9 ( A B ) )= 9 ( A B ) . As U*ABUy = ABy, U*A :BU = A B and A B q A With {En}bounding for A and A*, E,A* c A*E, and E,A* E (AE,)*. Thus A*E, and (AE,)* are bounded, everywhere-defined extensions of the same
5.6. UNBOUNDED OPERATORS AND ABELIAN VON NEUMANN ALGEBRAS
353
densely defined operator E,,A*. It follows that (AE,)* = A*E,, (or we may cite Lemma 5.6.14). Suppose that { E n }is bounding for B, B*, aA $ B, (aA $ B)*, A :B, ( A B)*, and A* B* ( = B* :A*) as well as for A and A*. Then, from the foregoing,
+
@A* $ B*)E, = UA*E,, B*E,, = &4E,)*
+ (BE,)* = ((aA $ B)E,,)*
= (aA $ B)*E,
and
'
( A B)*E,, = ( ( A B)E,,)* = (AE,,BE,,)* = (BE,,)*(AE,,)*= B*E,,A*E, = (B*' A*)E,,.
Since (aA $ B)* and cSA* $ B* agree on their common core =U ; En(%), they are equal. Similarly ( A :B)* = B* :A* ( = A* :B* ). If A E B and {En}is a bounding sequence in d for both A and B, then AE, E BE, so that AE, = BE,,. Thus A and B agree on their common core =U ; En(%). Hence A = B. If A is symmetric, A E A* and, from the preceding conclusion, A = A*. It is routine to verify identities such as (A: B):
c = A : ( B : c),
by choosing a common bounding sequence for all operators involved. Thus (viii) follows. As noted in (iv) of the preceding theorem, A*A = AA* for each A affiliated with an abelian von Neumann algebra d.By analogy with the case of bounded operators, we expect normal operators to be affiliated with abelian von Neumann algebras. With the aid of the lemmas that follow, we shall prove this. We conclude from this that the multiplication operators corresponding to unbounded (complex-valued)measurable functions (finite almost everywhere) are normal (compare Theorem 5.6.4). Our first lemma is an analogue to Lemma 5.6.1. 5.6.16. LEMMA.I f { F,,} is a bounding sequencefor the closed operator A on the Hilbert space % and AF,, is normal for each n, then A is normal.
Proof. From Lemma 5.6.14, (AF,,)* = A*F,,, so that A*AF,
= A*F,,AF, = (AF,,)*AF,,= AF,(AF,,)* = AF,,A*F,, = AA*F,.
Thus the self-adjoint operators A*A and AA* agree on =U ; each of them. Thus A*A = AA*.
F,(%), a core for
5.6.17. LEMMA.I f B A c A B and 9 ( A ) E 9 ( B ) , where A is a self-adjoint operator and B is a closed operator on the Hilbert space %, then E,B E BE, for each En in the spectral resolution {E,} of A.
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5 . ELEMENTARY VON NEUMANN ALGEBRA THEORY
Proof. We note that B(A + iZ) = BA + iB under the present assumptions. For this, observe that, from (14), BA + iB s B(A + iZ). Suppose x e Q ( B ( A + iZ)). Then x ~ 9 ( A )and A x + i x E g ( B ) . By assumption XE~(A c )9 ( B ) so that A x e Q ( B ) , as well. Thus X E ~ ( B+AiB) and B(A + iZ)x = BAx + iBx. Hence B(A + i l ) c BA + iBand the stated equality follows. Similarly B(A - iZ) = BA - iB. Let T+ and T - be the (bounded, everywhere-defined) inverses to A + iI and A - iZ, respectively. Then, from (12)-(14) and the preceding paragraph,
+
T + B= T+B(A iZ)T+ = T+(BA+ iB)T+ c T + ( A B+ iB)T+
+ il)BT+ c BT, .
= T+(A
Similarly T- B G BT- . From the proof of Lemma 5.6.7, T + = T*_so that Lemma 5.6.13 applies; and T B c BT for each Tin the von Neumann algebra d generated by T+ and T- . In particular E,B c BE, for each 1. 5.6.18. THEOREM.An operator A is normal ifandonly if it is afliliated with an abelian von Neumann algebra. I f A is normal, there is a smallest von Neumann algebra d osuch that A q d o .The algebra d ois abelian. Proof. From Theorem 5.6.15(iv), each operator affiliated with an abelian von Neumann algebra is normal. Assume, now, that A is normal. Since AA*A = A*AAandg(A*A) c 9 ( A ) ,Lemma5.6.17applies.Thus E,A c AE, for each 1,where { E L }is the spectral resolution of A*A; and F,A c AF, for each n, where F, = E, - E-,. In the same way, A*A*A = A*AA* and 9 ( A * A ) = 9 ( A A * ) c_ 9 ( A * ) ,so that F,A* c_ A*F, for each n. As in the proof of Theorem 5.6.15, AF, and A*F, are bounded since A*AF, (= AA*F,) is. Moreover, F,A* c (AF,)*, so that both (AF,)* and A*F, are bounded extensions of the densely defined F,A*. Thus (AF,)* = A*F, (and (A*F,)* = AF,). Note, too, that AF,AFm c AAF, and AFmAF, c AAF,, when n < m. Since AF,AFm and AFmAF, are everywhere defined, AF,AF,,, = AAF, = AFmAF,. At the same time, A*FmAF, = A*AF, = AA*F, = AF,A*F,. Thus {F,, AF,, A*F, :n = 1,2,. ..} generates an abelian von Neumann algebra d o Since . V=; F, = Zand F,A c AF,, =U ; F , ( Z ) (= Qo) is a core for A. If U is a unitary operator in do and x e Q 0 , AUx = AUF,x = AF,Ux = UAF,x = UAx (for some n). From Remark 5.6.3, A q d o (and A * q d o ) . I f A q W , then A*qW and A*AqW. From Remark 5.6.11, A*A generates an abelian von Neumann algebra d ,contained in 9. Thus F, E W so that AF,, A*F, are in 9; and d oc W. I
We refer to d oas the von Neumann algebra generated by (the normal operator) A. 5.6.19. THEOREM.I f d is an abelian von Neumann algebra, cp is an isomorphism of d onto C ( X ) where X i s a compact Hausdorgspace, and A q d,
5.6. UNBOUNDED OPERATORS AND ABELIAN VON NEUMANN ALGEBRAS
355
there isa unique normalfunction cp(A)on Xsuch that cp(AE) = cp(A) 7 q ( E )when E is a boundingprojection in d for A where (cp(A) cp(E))(p)is cp(A)(p)cp(E)(p) ifcp(A)(p) is defined and 0 otherwise. I f N ( X ) is thefamily of normalfunctions on X a n d f , g are in N ( X ) , there are unique normal functionsf, af,f $ g , and f-_ssuch that = A T , ( a f ) ( p )= asp), (f$ gNp) = f T p ) + g(p), and (f g ) ( p ) = A p ) g ( p ) , whenf andg are definedatp. Endowed with the operations ( a , f )-,af, ( f , g ) +f $ g and ( f , g ) +f 7 g , N ( X ) is an associative, commutative algebra with unit 1 and involution f + J The mapping cp, as extended, is a * isomorphism of M ( d ) onto M ( X ) . 4
f+X
Proof. We show first that &I) as,described, is unique. From Theorem 5.6.15, A has a bounding sequence {En}in d. Iff and g are normal functions with the properties ascribed to q ( A ) and bothfand g are defined at p, then f l p ) d E n ) ( p )= d A E n ) ( p )= d p ) d E n ) ( p ) for each n. Thus if cp(E,,)(p)= 1 for some n,f ( p ) = g(p). As { E n }is monotone increasing to I,f and g agree on a dense subset of X; and, from Lemma 5.6.6,
f=s.
If A E Y ( ~ the ) , notation of Theorem 5.6.12(i) agrees with the present notation and the function associated with A there has the properties required of cp(A). Thus cp is defined on Y ( d ) . If A and B are in Y ( d )from , Theorem 5.6.15(i),we can choose a bounding sequence {En}for both A and B. Then AE,,, BE,,, (A + B)E,,, and ABE,, are in d . Moreover (A $ B)E,, = ( A + B)E, = AE,, BE,, and ( A B)E,, = ABE,, = AE,,BE,,, so that
+
V(A
B ) ( P M E ~ ) ( P=) V ( ( A
+ B)En)(p)
+
= cp(A)(pMEn)(p) cp(B)(P)dEn)(P)
and cp(A B)(P)Cp(Efl)(P)= cp((A B)E,,XP) = cp(A)(p)tp(B)(p)cp(Efl)(p) when cp(A), cp(B), q ( A $ B), and cp(A 7 B) are defined at p. Since {En} is monotone increasing to I, cp(A $ B) and q ( A ) + cp(B)agree on a dense subset of Xas do cp(A B) and cp(A)cp(B).Thus cp(A $ B) and cp(A B) are finite, when both &A) and q ( B ) are defined, and, by continuity, are normal extensions of cp(A) + q ( B ) and cp(A)cp(B),respectively. Define cp(A) $ cp(B)and cp(A) cp(B) to be these normal extensions. Since each h in Y ( X )corresponds to some A in Y ( d )from , Theorem 5.6.12, the operations $ and apply to all functions in Y ( X ) .It is easy to verify (by repeated use of Lemma 5.6.6) that 9 ( X ) endowed with these operations (and the indicated multiplication by real scalars) is an associative, commutative algebra (over R) with unit 1. From the preceding discussion, cp is an isomorphism of Y ( d )onto Y ( X ) .
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5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
If A e N ( d ) , then A = A l $ iA2, where A l ( = % A $ A * ) ) and A 2 (= -+i(A $ - A * ) ) are in Y ( d ) .If cp(A,) and cp(A,) are defined on X\ Z1and X\ Z , , respectively, then cp(A icp(A2)is defined on X\(Z1 u Z 2 ) .If p E Z1, then Icp(A,)(q)l and hence Icp(Al)(q) icp(A,)(q)( are large at points q of
+
+
X\(Z1u Z,) near p . A similar comment applies to p in Z 2 , so that cp(A1)+icp(A2) defined on X \ ( Z 1 u Z 2 ) is an element, cp(A,) $ + ( A 2 ) (= cp(A)), of Jw3. If h in N ( X ) is defined on X\ Z , then h-’(O) is a closed set in X.To see this, note that h - ‘(0) is closed in X\Z by continuity of h on X\ Z , and Ihl is large at points of X\Z near a point p of Z so that p $ h - ’ ( O ) - . Thus the interior X o of h-’(O) is a clopen subset of Xand the function g, defined as 1 on X , and as h/lhl on X\(h-’(O) u Z ) , is continuous. As X\((/I-l(0)\Xo)u Z ) is a dense open subset of X , Corollary 5.2.1 1 applies, and g has an extension u in C ( X ) .Forp in X\Z, u(p)lh(p)l = h ( p ) . As l h l ~ Y ( X )Reh , and Imh defined on X\Z have (Re u ) :Ihl and (Im u ) :1/11 as normal extensions. Thus h = hl $ ihi with hl and h2 in Y ( X ) . Choosing A l and A , in Y ( d )such that c p ( A l ) = h l and cp(A,) = h , , we have q ( A l iA2) = h l $ ih, = h. Hence cp maps N ( d )onto
JW3.
Iff in N ( X ) is defined on X\Z, we have just seen that the real and imaginary parts off have normal extensions. Denote these extensions by Ref and Imf. With g in N ( X ) defined on X \ Z , f+ g and fg are defined and continuous on x\(Zu Z ’ ) and have the normal extensions (Ref$ Reg) $ i(Imf$ Img)
(=f$g)
and (Ref: Reg $ - Imf: Im g) $ i(Ref: Im g $ Imf: Reg) ( =f 9). With the indicated operations, N ( X ) becomes an associative, commutative algebra with unit 1 and adjoint operationf+J The mapping cp, as extended from Y ( d ) ,is a * isomorphism of N ( d )onto N ( X ) . If A q d and E is a bounding projection in d for A , we have, now, cp(AE) = cp(A E ) = cp(AFcp(E). In the preceding proof, we note that, with h in N ( X ) , h-’(O), a subset of X\ Z , is closed in X . The same argument establishes that h - ‘ ( X ) ,a subset of X\Z, is closed, hence compact, in X , for each compact subset X of @. It follows that the range of h intersects X in a compact subset (since this intersection is h(h-’(X)),the continuous image of a compact set). Thus if zo is not in the range of h it is at positive distance from that range, and l/(h - zo 1) is a bounded continuous function on X\ Z tending to 0 at each point of Z . Hence zo is not in the range of h if and only if h - z,l has an inverse in N ( X ) and that inverse is in C(X).This observation suggests the concept of spectrum to us and will play the key role in identifying the “spectrum” of an unbounded operator with the range of its representing function. (See Proposition 5.6.20.)
5.6. UNBOUNDED OPERATORS AND ABELIAN VON NEUMANN ALGEBRAS
357
The spectrum sp( T) of a closeddensely defined linear operator Ton a Hilbert space .# is the set of thosecomplex numbers z such that T - zlis not a one-toone mapping of .9(T) onto X. If zo $ sp( T), then T - zoIis a one-to-one linear mapping of 9(T ) onto X and has a linear inverse B (mapping X onto .9(T)). Since the graph of T - zolisclosed, the graph of B is closed. As B is defined on all of X, the closed graph theorem applies and Bis bounded. Thus if zo 4 sp(T), T - zoI has a bounded (everywhere-defined) inverse; and, of course, conversely, zo $ sp( T) if T - zoI has such an inverse. If Tq d,for some abelian von Neumann algebra d$and zo 4 sp(T), then B just constructed is in d. As B is bounded and T - zoI is closed, ( T - zoI)B is closed. Hence I = (T - Z,Z)B = ( T - Z,I) B = B: (T - z~I).
It follows that zo$ sp T if and only if T - zoI has an inverse B in the algebra N ( d )and B lies in d. 5.6.20. PROPOSITION. If A is a normal operator affiliated with the abelian von Neumann algebra d,then sp(A) coincides with the range of cp(A), where cp is the isomorphism of N ( d )onto H ( X ) extending the isomorphism of d with
W). Proof. By definition, zo $ sp(A) if and only if there is a bounded B inverse to A - zoI. If U is a unitary operator in d ‘ ,then U*(A - zoI)U = A - zoI, since A - zoIq d.Hence U*BU = Bfor each such U ;and B E d.Since there is a Bin d such that ( A - zoI) B = I , equivalently, (cp(A) - zol) cp(B) = 1, if and only if zo $ sp(A); and there is such a q(B) in C(X )if and only if zo is not in the range of cp(A), sp(A) is the range of cp(A).
Is there an interpretation of “spectrum relative to A’”(&)”? Equivalently, when does f - zol fail to have an inverse in N ( X ) ? This occurs if and only if f - zol vanishes on some non-null clopen subset of X . Viewed in H ( d )this amounts to the existence of a non-zero projection Eo in d such that ( A - zoI) Eo = 0 or, equivalently, to the existence of a unit vector xo such that ( A - zoI)xo = 0. In this case, we say that zo is in the point spectrum of A . Thus the spectrum of A relative to N ( d )is its point spectrum. If we define a self-adjoint operator A to be positive (and write A 2 0) when ( A x , x ) 3 0 for each x in B(A), the question (answered for bounded A in Theorem 4.2.6) of the relation of this condition to the nature of sp(A) arises. It is easily settled with the help of Proposition 5.6.21. 5.6.21. PROPOSITION. A self-aa’joint operator A is positive i f and only i f a 3 0 when a E sp(A). Proof. We may suppose, from Lemma 5.6.7, that A is affiliated with an abelian von Neumann algebra d. From Theorem 5.6.19, there is an
358
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
isomorphism cp of N ( d )onto N ( X ) mapping d onto C(X), where X is an extremely disconnected compact Hausdorff space. If cp(A) is defined on X\ Z and q(A)(p) c 0 for some p in X\Z, then there is a non-null clopen set X, containingp and contained in X\ 2 such that cp(A)(q)c a c 0 for each q in X,. If Eo is the projection in d corresponding to the characteristic function of X,, then Eo is a non-zero bounding projection for A and AE, < aEo. With x a unit vector in the range of E,, (Ax,x) < a < 0 and A is not positive. Thus sp(A) 2 0 if A 3 0. On the other hand, if q ( A ) has range consisting of non-negative real numbers, its (positive) square root g is a normal (self-adjoint) function on X . If q(B)(inY(X))isg,then B Z = B: B = A(recallTheorem5.6.15(iv));and,with x a unit vector in 9 ( A ) ,X E ~ ( Band ) ( A x , x) = ( B x , Bx) 2 0. Thus A 2 0 if sp(A) b 0. rn It follows now that the set of positive elements in N ( d )(in Y ( d ) forms ) a positive cone and that Y ( d )is a partially ordered vector space relative to the partial ordering induced by this cone. (See the discussion preceding Definition 3.4.5.)The same is true for Y ( X ) . Of course Zis not an order unit for 9 ( d )in the present case. The following lemma, an analogue for N ( X ) of Lemma 5.2.10, will form the basis for a proof that Y ( X ) is a bounded a-lattice in the given ordering as well as providing the basis for our Borel function calculus.
5.6.22. LEMMA.Each Borel function g on an extremely disconnected compact Hausdorff space X agrees with a unique normal function f on the complement of a meager set. The mapping that assignsf to g is a conjugationpreserving homomorphism of the algebra a u ( X )of Borel functions on X onto N ( X ) with kernel consisting of those functions in .%?,,(X) vanishing on the complement of a meager set. Proof. We note first that iff and g are normal functions defined on X\Z and X\ Z', respectively, andflp) = g(p) forp in X\(Z u Z' u M ) , where M is a meager subset of X , then Z = Z' andf= g ;for Z u Z' u M is meager in X , so that X\(Z u Z' u M ) is dense in Xand Lemma 5.6.6applies. Thus there can be at most one normal function agreeing with any function on the complement of a meager set. If g is a Borel function on Xand D,, is the closed disk in C with center 0 and radius n, then g-'(D,) is a Borel subset S,, of X.According to Lemma 5.2.10, there is a clopen set X,, such that (S,\X,) u (X,,\S,) is meager in X . If gn is the (Borel) function equal to g on S, and 0 on X\S,,, then 11g1.1 6 n. Again from Lemma 5.2.10 there is a (unique) continuous functionf, on Xthat agrees with gnon the complement of a meager subset M , of X . Since gnvanishes on X\ S,,f, vanishes on X\(S, u M,,) whichcontains (X\X,,)\(M, u (S,\X,) u (X,,\S,)). As M , u (S,,\X,,) u (X,\S,,) is meager andS, is continuous,f, vanishes on X\X,. Note that S, E S,,, 1 , so that e,(p) 6 en+l(p) for eachp outside a meager set,
5.6. UNBOUNDED OPERATORS AND ABELIAN VON NEUMANN ALGEBRAS
359
where ej is the characteristic function of X j . By continuity, e, < and X,, E A',,+ Again, since g,,+ agrees with gn on S,, and fn and fn+ are continuous,fn+, agreeswithfnonX,,. Inthesameway,n < Ifm(p)Iifp~X,,,\X,,, where n < m, since n < Ig,(q)I if q E S,\ S,,. As Unm= S,, = X , we have
,.
a,
W
( Z = ) X \ (J X,, E ~ ( S , , \ X , , ) U ( X , , \ S , , ) . n= 1
n= 1
Thus Z is closed and meager, hence, nowhere dense in X . Iff(p) is defined as f,(p) when p E X,,, thenfis continuous on X\ Z . For p in Z , X\ X,, is an open set containing p such that, for q in X \ ( Z u A',,), n < 1flq)I. Thus f is normal. MoreoverJand g agree on the complement of Z u (Unm=M,,),a meager subset of X . The mapping assigningfto g is a homomorphism of Bu(X)onto N ( X ) with kernel consisting of those Borel functions on X vanishing on the complement of a meager set.
5.6.23. PROPOSITION. Ifd is an abelian von Neumann algebra and {A,,} is , an increasing sequence of operators in Y ( d )with upper bound A . in Y ( d )then {A,,} has a least upper bound in Y ( d ) . Proof. With d 2 C(X),letf,, in N ( X )represent A,,. If { A , $ - A , } has a least upper bound Bin Y ( d )then , B $ A l is the least upper bound of {A,,}.We may assume, without loss of generality, that each A,, 2 0. If fn is defined on X\Z,,, then Z,, = (Z,,)+, in this case, and Z,, G Z,,+ 1 . If p is not in =U ; Z,, (= Z ) , thenfn(p) is defined for all n and {fn(p)) hasfo(p) as an upper bound. Thus {fn(p)} converges to some g(p). If we define g to be 0 on Z , then g is a Borel function on X . From Lemma 5.6.22, there is anfin N ( X ) agreeing with g on A'\M , where M is a meager subset of X . Hence {f,(p)} converges toflp) on the dense set X \ ( M u Z ) . It follows thatfis an upper bound for {fn} and the least upper bound. If A in Y ( d )is represented byfin Y ( X ) ,then A is the least upper bound of {A,,}. 5.6.24. REMARK.In view of Proposition 5.6.23,we may extend the notion of a-normal homomorphism, in the obvious way, to apply to N ( d ) ,N ( X ) , and to Bu(the algebra of complex-valued Borel functions on C), g U ( X ) With . this extension, we observe that the homomorphism of Lemma 5.6.22 mapping B u ( X ) onto N ( X ) is a-normal. If {g,,) is an increasing sequence of Borel functions on X tending pointwise to the (Borel) function go andf,, is the normal function corresponding to g,,,then cf.}hasf, as its least upper bound in N ( X ) . To see this, supposefn and g,,agree on the complement of the meager set M,,. Then {fn(p)) tends tof,(p)foreachpinX\U,"=, M,,,adensesubset ofX. Ifhin N ( X ) is an upper bound for {fn}, thenf,,(p) < h(p) for all n and all p in the complement of some meager set. Thusf,(p) < h(p) for allp in the complement
360
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
of some meager set, and h $ -fo takes non-negative values on a dense set. Hence fo < h, and fo is the least upper bound in N ( X ) of {h}. 5.6.25. REMARK.With the aid of Lemma 5.6.22, we can define g ( A ) for an arbitrary Borel function g on sp(A) and an arbitrary normal operator A. From Theorem 5.6.18, A, A*, and I generate an abelian von Neumann algebra d o (and A q d o )From . Theorem 5.2.1, d ois isomorphic to C(X), where X is an extremely disconnected compact Hausdorff space. From Theorem 5.6.19, there is an isomorphism q of N ( d o onto ) N ( X ) . If q(A) is defined on X\Z, then 4 defined as 0 on Z and g q(A) on X\Z is in a&%'). From Lemma 5.6.22, there is a function h in N ( X ) agreeing with # on the complement of a meager set inX. Wedefineg(A)tobecp-'(h). Ifsp(A)isasubset oftheBorelset Sandgisa Borel function defined on S, then g ( A ) will denote g,(A), where gr is the restriction of g to sp(A). 0
5.6.26. THEOREM.Ifdois the abelian von Neumann algebra generated by a normal operator A acting on a Hilbert space 2,the mapping g -+ g ( A ) of the algebra B,,(sp(A)) of Borel functions on sp(A) into N ( d o )is a a-normal homomorphism mapping the constant function 1 onto I and the identity transformation I on sp(A) onto A. The mapping S + E(S) of Borel subsets S of @ into d ois a projection-valued measure on @, where E(S) = g(A) and g is the characteristicfunction of S. I f h is in a,the algebra of bounded Borelfunctions on @, then
llh(A>ll < sup{lh(a)l : @I (= llhll). (16) I f x is a vector in 2 and px(S)= (E(S)x,x), then, for each h in 99, (17)
(h(A)x, x> =
j
Wdp,(a).
Q:
With f in a,,,x ~ g ( f ( A ) i)f and only i f (18)
[,
lAa)I2dpx(a) (= Ilf(A)xl12) < 00 ;
and (1 7) is valid withfinplace of h, in this case. Ifq is the extension to N ( d o o)f the isomorphism of d owith C ( X )and v, is the regular Borel measure on X such , X E ~ ( T with ) , Tin that ( B x , ~ )= jx (q(B))(p)dv,(p)for each B in d o then N ( d o )i,f and only if (18')
jxl(cp(T))(P)I~dVx(P) (= II~xIl2)<
I f A is a self-adjoint operator, its spectral resolution is {EA},where EA = I - E((& 00)); and X E ~ M A )if) and only if JTm lf(A)I2 d(EAx,x) < 00. I f XEQCf(A)),(AA)x,x) = J",4WAX,X).
5.6. UNBOUNDED OPERATORS AND ABELIAN VON NEUMANN ALGEBRAS
361
Proof, The mapping g + 8 of g',(sp(A)) into Bu(X)defined in Remark 5.6.25 is a a-normal homomorphism. From Remark 5.6.24, the mapping assigning h (of Remark 5.6.25) in N ( X ) to 8 is a a-normal homomorphism, as is cp. It follows that the mapping g + g ( A ) is a a-normal homomorphism of Bu into N ( d o )If. g is 1, h is the constant function 1 on X and g ( A ) is I . If g is I , h is cp(A); so that i(A) = A . With g the characteristic function of S, a Borel subset of @, g(A)* = g(A) = g ( A ) and g ( A ) = g2(A) = g ( A ) 2 . Thus g ( A ) is a projection E(S) in d o .If g is 0, g ( A ) = 0, so that E ( 0 ) = 0. If g is 1, g(A) = I ; and E(C) = I . If ISj} is a countable disjoint family of Borel subsets of C and g j is the characteristic function of S j , then g1 + * + g, (= h,) is an increasing sequence tending pointwise to the characteristic function h of =U ; Sj (= S ) . Since the mapping g + g ( A ) is a o-normal homomorphism E(Sj)} has least upper bound E(S)- that is I?=, E(Sj)= E(S). With this notation
-
{z=
(h(A)x,x> = (E(S)x,x> = PAS) =
sc
h ( 4dpx(4;
so that (17) is valid for (Borel) step functions h. With h a bounded Borel function on @, ll@ < llhll and the function fin N ( X ) corresponding to filies in C ( X ) . (See Lemma 5 . 2 . 1 0 . ) Moreover, < llhll < llhll. Thus llh(A)ll = IIcp - '(f)ll= llfll < Ilhll.
As each h in 93 is a norm limit of (Borel) step functions, (17) now follows for each such h. Suppose A is self-adjoint and g is the characteristic function of (A, 00). Then 8 is the characteristic function of cp(A)-'((A, a)),an open subset of X\Z (hence, of X)and the function in C ( X )corresponding to 8 is the characteristic function, 1 - e l , of cp(A)-'((A, a))-. Thus E((A, m)) (= g ( A ) ) in d ocorresponds to 1 - ed; and, from Theorem 5.6.12(v), EA= I - E((A,m)). It follows that ((Ed. - E A ) x , x )= px((A,A']) when il< A', and
Im
If(412d(E,x, x> = JIf(A)lWAA)
-a:
for eachfin BU. Hence the last assertions of our theorem reduce to (17) and (18).
Withfin gu,let k, be the characteristic function of If1 - '([O, n ] ) (= S,) and F, be k,(A). Thenf,,(A) =S(A)F,, wheref,, = f.k,, from the first part of this proof. Thus, with x in X, (19)
Ilf(A)F,Xll'
= (If,12(4x,x> =
362
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
and, if m
< n,
(20)
IIAAIFnx - f i ~ ) F m x I I ~ =
J
IAa)I2dpx(a).
Sn\Sm
As {k,,} is increasing and tends pointwise to 1, {F,} is increasing with least upper bound I . Since F,,AA) c f l A ) F , , F,,AA)x =AA)F,,x if X E 9 M A ) ) .Thus f(A)F,,x + f ( A ) x and (18) follows from (19). Conversely, if Jc lf(a)12dpx(a) converges, {AA)Fnx}is a Cauchy sequence from (20), and converges to some vector in X. Since {Fnx}tends to x andAA) is closed, x E 9 ( f ( A ) ) .A completely analogous argument establishes (18'). With x in 9 ( f ( A ) ) , f E L,(C, p x ) c L1(C,p x ) since p x is a finite measure. Thus ( f ( A ) x ,x)
= lim (f(A)F,x, x) = lim
f,(a)dp,(a) = JE
Aa)dp,(a). J C
The projection E(S) appearing in the statement of Theorem 5.6.26 is often referred to as the spectral projection for A corresponding to the Borel subset S of @. 5.6.27. THEOREM. A is a normal operator affiliated with an abelian von Neumann algebra a? acting on a Hilbert space X and $ is a a-normal homomorphism of a,,,the algebra of Borelfunctions on C, into N ( d )such that $(1) = land $ ( I )= A , where 1 is the identity transform on C, then $0= f , ( A ) for each f in g U , wheref, is the restriction o f f to sp(A).
is a-normal, $ is adjoint preserving. Positive elements of = Z, it follows that rl/ maps the algebra B of bounded Borel functions in a,,into d and does not increase norm. Suppose A is bounded and go is the characteristic function of @\ Do,where Do is the closed disk in C with center 0 and radius 211,411. Then 0 < (2(IAll)"go < 111" for each positive integer n. Now $(I$') = (Al", so that 0 < (2IIAll)"~(go)< IAI". Thus Il$(go)ll < 2-" for each positive integer n, and $(go) = 0. If g1 is the characteristic function of Do, then $(gJ = Z, so that $(g,h) = $(h) for each h in a". Let ho denote the restriction of h to Doand let t,bo(h0)be $(h). Then $o is a a-normal homomorphism of au(D0)into M ( d ) mapping the constant function 1 on Doonto Z, lo onto A, and g(Do) into k Since C(Do) is a C*-algebra whose unit is the constant function 1 on Do, Proposition 4.4.7 applies and i,bo(f) = $ocf(io)) =AA),for eachf in C(Do).As noted in the proof of Theorem 5.2.8, the characteristic function hl of the open subset Do\sp(A) of Dois the pointwise limit of an increasing sequence {f,}of positivecontinuous functions on Do.Thus $o(hl) is the least upper bound in d of t,bOcf,),by a-normality of I)~.But t,b0(f,) = f , ( A ) and f , ( A ) = 0, since f, is continuous and vanishes on sp(A). Thus t,bo(h1)= 0. Iffi is the characteristic
Proof. Since
BUhave positive square roots, so that $ is order preserving. As $(1)
5.6. UNBOUNDED OPERATORS AND ABELIAN VON NEUMANN ALGEBRAS
363
function of sp(A) (as a subset of Do), $o(fl) = I and $o(flho) = $o(ho) for each ho in Bu(Do).If $l(kl) = i,b0(k),where k, is the restriction of k in Bu(Do) to sp(A), then is a a-normal homomorphism of B,,(sp(A)) into N ( d ) mapping the constant function 1 on sp(A) onto I , the identity transform l 1 on sp(A) onto A , and B(sp(A)) into d.Theorem 5.2.9 applies to the restriction of $ l to a(sp(A)), and ~)~(f) = A A ) for eachfin B(sp(A)). Each positive g in 9?,,(sp(A)) is the pointwise limit of an increasing sequence of positive functions in B(sp(A)). Since t,bl and the homomorphism f + A A ) of B,,(sp(A)) into N ( d )are a-normal, $l(g) = g(A) for each positive g in B,,(sp(A)). Each h in B,,(sp(A)) is a linear combination of four (or fewer) positive functions in B,,(sp(A)), so that $,(A) = h(A) for each h in B,,(sp(A)). If k €a,, and ko and k, are its restrictions to Doand sp(A), respectively, then $(k) = $o(ko)= ICIl(k,) = k,(A). With A now an arbitrary (normal) operator in N ( d )and E a bounding projection for A in 4 the mapping cp that assigns ( B : E ) I E ( X ) (in N ( d E ) acting on E(X0))to B in N ( d )is a o-normal homomorphism of N ( d )into N ( d E ) .Composing cp with $ yields a o-normal homomorphism t j Z of B,,into N ( d E )mapping 1 onto E l E ( X ) and I onto A l E ( 2 ) . At the same time, the composition of cp with the mappingf+f,(A) of a,, into N ( d )is another such homomorphism of B,,into J ( r ( d E ) .Since A ( E ( S )is bounded, the first part of this proof applies and
( $ ( f ) m E ( m= $
2 0
=f,(AI%w)
=
(f,WE)I~(W*
From Theorem 5.6.15(i), there is a common bounding sequence {En)for A ,
$(f),and f , ( A ) , in N ( d ) ,where f is a given element of a,,.As
:En)I EnW) = ($(f)En)I E n W ) = (f,(A)En)I ~
$ ( f ) E n =f,(A)E,, for each n. Since U=;l f,(4 $(f)= f , ( A ) * w
f
l
(
= ( f , ( A ):E n ) I EnW)
m
9
En(%) is a core for both
$0and
5.6.28. REMARK.The process described in Remark 5.6.25 for forming g ( A ) in d ocould be applied, equally well, to .V(Y), where d is another abelian von Neumann algebra with which A is affiliated and d z C(Y). Theorem 5.6.27 assures us that the operator in N ( d )formed in this way is g ( A ) (and lies in N ( d o ) ) .w 5.6.29. COROLLARY. I f A is a normal operator and f and g are in a,,, then (21)
( f o g ) ( A ) =Ag(A)).
Proof. If d ois the abelian von Neumann algebra generated by A , A*, and cp of B,,into
I, then g(A) E N ( . d oandf+fo ) g is a a-normal homomorphism
364
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
Bu.Composing cp with the a-normal homomorphism h + h(A) of B,, into N ( d o )yields a a-normal homomorphism, f+ (fo g)(A), of 2du into N ( d o ) that maps 1 onto I and I onto g(A). From Theorem 5.6.27, (21) follows. H 5.6.30. PROPOSITION. $ is a a-normal homomorphism of N ( d l )into N ( d 2such ) that $ ( I ) = I, where dland d2are abelian von Neumann algebras, then $ M A ) ) =A$@))for each A in N ( d l )and each f in 9%. Proof. The mapping f+ $ ( A A ) ) of a,, into N ( d 2 )is a a-normal homomorphism mapping 1 onto I and I onto $(A). From Theorem 5.6.27, Il/(f(A))=A $ ( A ) ) for each f i n B,,.
Ifd is an abelian von Neumann algebra acting on the 5.6.31. COROLLARY. Hilbert space X, E is a projection in d,A q d, and fE 2du, then A(A E ) I EW)) = ( A 4 E ) I E(%).
Proof. The mapping B + (B E ) 1 E(H)is a a-normal homomorphism $ of N ( d )onto N ( d E l E ( 8 ' ) )such that $ ( I ) = ,TIE(%) (and ElE(%) is the identity operator on IT(%)). Thus, from Proposition 5.6.30, f((A
'E ) I E ( X ) )=fT$(A))
'Ell E(H).
= $(A41 = MA)
5.6.32. REMARK.If A and B are (unbounded) normal operators whose spectra are contained in the domain of a Borel function g and g has a Borel inverse function5 then g(A) = g ( B ) if and only if A = B; for if g ( A ) = g(B), then, from Corollary 5.6.29, A = ( f o g ) ( A )= M A ) ) = f ( g ( B ) ) = ( f o g ) ( B )= B.
As an application of this comment, we note that if A 2 = B2 with A and B positive operators, then A = B, so that a positive operator has a unique positive square root. We illustrate the use of Corollary 5.6.31 with the observation that flA)xo =A&, when Axo = Axo, where A is a normal operator,fis a Borel function whose domain contains sp(A), and xois a non-zero vector. To see this, let E be the projection with range {x: Ax = Ax} (which is closed since A is a closed operator); and note that, since EA c A E = AE, AAko
=
[MA) ' I~
'
( m x= oA ( A E ) I~
( X N X O
=f(AE(E ( 2 ) ) x o =f ( A ) ~ o . H
As we noted in Theorem 5.6.15, N ( d )becomes a * algebra when the usual operations of operator addition and multiplication are "refined" by passing to closures. In certain instances, the combination of operators in question is closed -for example, A*A is self-adjoint and, hence, closed for each closed densely defined A . In Theorem 5.6.19 we see that N ( X ) is an algebra when the
5.6. UNBOUNDED OPERATORS AND ABELIAN VON NEUMANN ALGEBRAS
365
usual operations of function addition and multiplication are “refined” by passing to a (unique) normal extension. In certain instances, the combination of functions in question is normal -for example,f” is normal iffis. Theorem 5.6.19 tells us, as well, that N ( d )and N ( X ) are isomorphic when endowed with these operations by an isomorphism that extends the isomorphism of d with C ( X ) .With all this in view, the temptation is great to assume that taking the closure of an algebraic combination of operators affiliated with d is unnecessary exactly when the same combination of the corresponding normal functions is normal (without requiring a proper extension). This is not so, as we shall see in the following example. At the same time, this example provides a nice illustration of the isomorphism of N ( d )onto N ( X ) and the way we work with it. 5.6.33. EXAMPLE.Let S be a separable Hilbert space, {en},=1 , 2,... an orthonormal basis for 2, and d the algebra of operators in a(#)having each en as an eigenvector (that is, d is the algebra of bounded diagonal matrices relative to {e,}).In this case, d 2 C ( X )and Xis /?(N), the P-compactification of the numbers { 1,2,. . .}. The points p , corresponding to the pure states T -+ (Te,, e n )of d,n = 1,2,. . . form a dense subset of X , for if 0 = (Te,, en), with Tin d,then T = 0. Thus each function in C ( X )vanishing on {pn}nsN is 0, from which the density of { p , } follows. A function 1 at p1 and 0 at p,,, n = 2,3,. . . ,is therefore 0 at all points of Xother thanp, if it lies in C ( X ) .The projection whose range is generated by el lies in d and corresponds to such a function in C ( X ) .It follows that {pl}is an open subset of Xas is each one-point set formed from a p , . Thus { p , } , , l , 2 , . . . is an open dense subset of X and its complement Z is a closed nowhere-dense subset of X . The function h defined as 6, at p , , where lbnl -+ 00, is normal (and defined on X\Z). Letting b, be n, we have a normal functionfcorresponding to an operator A affiliated with d (and Ae, = ne,). Letting 6, be n1l4 - n,we have a normal function g corresponding to an operator B affiliated with d (and Be, = (n114- n)e,). If x is I,“=n- ‘en,then X E S and v,({ p , } ) = n-’. Thus jxIs(p)IZ~V,(P) = a and j x l ( f + S)(_P)l2MP)= I,“= 1 n - 3 / 2 < 03. From Theorem 5.6.26, XI# 9 ( A ) and X E 9 ( A + B ) . Note for this thatf+ g is normal as defined on X\Z since I(f+ g)(pn)l = n1/4-, co, so that f+ g (=f+ g) corresponds to A 4B. It follows that A + B # A $ B. This same structure provides us with an example of self-adjoint operators A and C affiliated with d such that hfis normal but CA is not closed. With A , f , and x as before, choose h so that h(p,) = n- 3/4. Then C , corresponding to h, is bounded and hfis normal as defined on X\Z. Thus hfcorresponds to C : A . Now m
< 00, n=
1
366
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
so that X E ~ ( CA:) . But x $ 9 ( C A ) since x $ 9 ( A ) . Thus CA # C : A . At the same time, this provides an example of a bounded operator C and a closed operator A such that CA is not closed. (We noted in the first paragraph of the proof of Theorem 5.6.15 that this product, in the reverse order, is automatically closed.) 5.6.34. REMARK.If A and B are positive operators affiliated with the abelian von Neumann algebra d acting on X and d z C ( X ) ,then A and B correspond to positive normal functions f and g defined on X\Z and X\Z ’ , respectively. Hencef + g, defined on X\(Z u Z’),is normal and corresponds to A $ B. In this case, with x in 9 ( A $ B),
0<
j X
IflP)12dVX(P) G
j X
IAP) + S(P>I2&(P) < a*
Thus x ~ 9 ( Aand, ) similarly, x ~ 9 ( B )It. follows that x ~ 9 ( + A B) and that A+B=A$B. Again, with this same notation, but no longer assuming that A and B are positive, if Z n 2’ = fa, then there are disjoint open sets O1 and O2 containing Z and Z’, respectively. Thus 0; c X\B2 and there is a clopen set Y ( = 0;) containing Z such that X\ Ycontains 2’.It follows that g is bounded on Y andf is bounded on X\Y. If E is the projection in d corresponding to the characteristicfunction of Y, then BE and A(Z - E ) are in a(&‘). As A is closed and E is bounded, AE is closed and A E = A :E. Thus ( A $ B ) E = A E $ B E = A E + B E = ( A + B)E,
+
and, similarly, ( A $ B)(Z - E ) = ( A B)(Z - E ) . If x E 9 ( A $ B ) , then Ex and (I - E)x are in 9 ( A $ B ) . Thus Ex and (I - E ) x are in 9 ( A + B ) , and x&(A + B ) . Hence A + B = A $ B. Polynomials in a single variable provide an important case in which we need not pass to a closure. Thus “p(A)” refers to the same operator whether viewed in the customary sense or as a Borel function of A , for a polynomial p . For the purpose of the statement of the following proposition, “p(A)” refers to the operator obtained by forming the Borel function p of A . 5.6.35. PROPOSITION. I f A 9 d,where a2 isan abelian von Neumann algebra acting on the Hilbert space X, andp(z) = a,z” + . . . + a l z + a,, with a, not 0 , then a,A” . . a , A + aol is closed and equal to p ( A ) .
+
+
Proof. Suppose d z C ( X ) and A corresponds to the normal functionf defined on X\Z. Then a n y + . . . + alf + a. is defined on X\Z and normal. Hence it corresponds to p ( A ) . Now x ~ g ( p ( A ) )if and only if S X b n f ” ( P ) + * * + alflP) + aO12 dvx(p) < a* Let Xk be { p :If(p)l < k) -
f
5.6. UNBOUNDED OPERATORS AND ABELIAN VON NEUMANN ALGEBRAS
367
For k large enough and p in X\(Z u &),
so that
Sincef is bounded on xk and v, is a finite measure on X,f E L,,(X, vx). Hence f E Lk(X, v,) for 1 < k < 2n. Asf" is defined on X\ Zand is normal, it represents F . From the foregoing and Theorem 5.6.26 (especially (18')), x ~ 9 ( A kfor ) k = 1,. . . ,n. In particular x ~ 9 ( ; I=) 9 ( A ) . We show, by induction, that = Ak for all k . Assume this for k = 1,. . . ,n - 1. If y ~ g ( A " )then, , from what we have established to this point, y ~ g ( A )Let . {Em}be a bounding sequence in-d forand A. Then F E m = ( F A)Em : = A"-'EmAEm. Hence EmA"y = A"Emy= A"-'EmAEmy = A"-'EmAy. Now EmA"y +A"y and EmAy+Ay. Since A"-' is closed, A ~ E ~ ( A " and ' ) A " ' A y = Any. -~ Hence A" G A " - ' A . But A"-' = A " - ' by inductive assumption. Thus A" E A" and = A", completing the induction. It follows now that x E 9 ( A k )for k = 1, . . . ,n, and
Ak
A"-'
~
~
~
~
~
A"
xEg(a,A"
+
* * *
+ a l A + aoZ),
so that p ( A ) = a,A"
+ . . + a l A + aoZ.
We apply (unbounded-)spectral-resolution considerations to an analysis of the unitary representations of R -the one-parameter unitary groups. We use the notation and definitions in Theorem 4.5.9 and the discussion preceding it. 5.6.36. THEOREM (Stone's theorem). ZfH is a (possibly unbounded)selfadjoint operator on the Hilbert space X, then t + expitH is a one-parameter unitary group on X. Conversely, ift + U, is a one-parameter unitary group on X, there is a (possibly unbounded) self-adjoint operator H on X such that U, = exp itHfor each real t. The domain of H consists ofprecisely those vectors x in X f o r which t -'(U,x - x ) tends to a limit as t tends to 0, in which case this limit is iHx. Zff E Ll(R) and x, y are in 2,then
(.&OX,Y)=
1
At)<ei'Hx,Y>dt.
R
368
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
Proof. From Theorem 5.6.26 and the properties of the function exp it1 on R, exp itH ( = U,) is a unitary operator and U,,,. = U,U,.. To see that t -+ U, is strong-operator continuous, it will suffice to show that IIU,x - -+ 0 as t 0 for each vector x in F,(X), where F,, = En - E - , and { E L )is thespectralresolutionofH. Now HF,,(= H,,)isbounded;and, withxin F n ( X ) (exp , itH)x = (exp itH,,)x,from Corollary 5.6.31. Examining the function representation of exp itH,, we have
A
-+
-+
so that IlU,x - XI[ -+ 0 as t --+ 0 for each x in F,,(X).We see, from (22) as well that IlU, - Ill -+ 0 if His bounded. Suppose now that t -+ U, is a one-parameter unitary group. The argument we shall give to show that U, = exp itHfor some self-adjoint operator Hand all real t constructs the (unbounded) spectral resolution of H. We begin by "extending" the representation f 4 U, of R to a representation cp of '$Iz,(R) (as described in Theorem4.5.9). In the proof of Theorem 3.2.27 (especially, the last paragraph of that proof), we construct a homeomorphism A' of the pure state space of %,,(R) (denoted, there, by &(R)), in its weak* topology, onto the onepoint compactification, { & co},of @.Let $ be the representation of C({@, 00)) on Y? obtained by composing, successively, the isomorphism of C({fi, co}) with C(&(R)), the isomorphism of C(A(R)) with '?2Iz0(R),and cp. From Theorem 4.5.9, cp restricts to an essential representation of d l ( R ) , so that $ is R-essential and gives rise, as in Corollary 5.2.7, to a (possibly unbounded) resolution of the identity { EA}.Withfin L,(R), the Fourier transfom3offis , }). theimageofL,in C ( ( @ co})undertheisomorphismof2l0(R)onto C ( { @ a Employing, in succession, the fact that cp extends t -,U , , the definition of $ and the identification of3just noted, the choice of {En}and Corollary 5.2.7, 3.2(3) of Theorem 3.2.26, an interchange in the order of integration, and Theorems 5.6.12, 5.6.26, we have, for some self-adjoint operator H on X,
(23)
(cp(L/)X,
x) =
=
s, s
flt)(Utx, x> dt = < $ < f i x ,x>
R .fV)d<EAx,x)= S I ( S , f l f ) e i r * d r ) d ( 4 x , x )
for each x in (En - E - , , ) ( X ) , The interchange in the order of integration is justified by noting that f€L1(R), the Bore1 measure on R corresponding to d( Enx,x ) has support in [ - n,n] for the given x , and A -+ exp i t l is bounded
5.6. UNBOUNDED OPERATORS AND ABELIAN VON NEUMANN ALGEBRAS
369
and continuous on R. It follows from (23) that
jRAt)((U p , x> - (eiIHx, x>) dt = 0, for each f in Ll(R), whence the (continuous) functions t t + ((expitH)x, x) coincide when x r ~ ( E, I!-,,)(%). Since
( Utx,x) and
m
lJ ( E n
-E-AX)
n= 1
is a dense submanifold of X, U, = expitH for all real t. As
j$d(E,x,
x) = d w ) x , x>,
we may read from (23) that, forfin Ll(R), aH)x,y) =
j
At)(e"Hx,y>dt.
R
If F,, = En - E-,,, H , = H l F , , ( X ) , and ~ E F , ( X then ), (24)
'
t - [U,x - x]
= t - ' [ e i f H n - F,,]x + iH,,x
( t + 0);
for, employing the function representation of the (commutative) C*-algebra on F,(X)generated by H,, and F,, we have that f - '[exp itH,, - F,,] converges in norm to iH,in g(F,,(X)). In fact, for small t,
With x arbitrary, if t - ' [ U J - x] tends to y in X as t tends to 0, then
'
F,y = lim Fn(t - [U,x - x]) = lim t - [UIFnx- F,x] = iHF,,x. 1-0
1-0
As F,x + x, F,y( = iHFnx)+ y , and H is closed; it follows that y = iHx. If x E 9 ( H ) , then, from Theorem 5.6.26, m
-m
and
XE 9
( H ) and
370
5. ELEMENTARY VON N E U M A N N ALGEBRA THEORY
When t and l are different from 0,
Thus, given a positive E , for sufflciently large n and all non-zero t ,
s
a\[ - n.nl
It-’[ei‘’ - 13 - i l 1 2 d ( E A x , x )d 2
s
l 2d( EAx,x )
R\[ - n.nl
<E. -
2
On the other hand, from (24), It-’[ei‘’ - 11 - i l 1 2 d ( E A x , x )<;
Il(t-’[U, - I ] - iH)Fnxl12= J
E
L
-n
for all small, non-zero t . Hence Ilt-’[U,x-x] -iHxl(+o
(t-+O),
for each x in 9 ( H ) . H 5.6.37. REMARK. Formal differentiation of U,x at 0 would lead us to expect that t - [U,x - x ] tends to iHx for x in 9 ( H ) ,when we keep in mind the eventual expression of U, as exp itH. This process of differentiation can be used as an alternative starting point for the construction of H from U,. The proof we give of Stone’s theorem actually starts in Subsection 3.2, The Banach algebra L,(R) and Fourier analysis. It is formulated in such a way that little needs to be added in order to arrive at a “spectral decomposition” of a unitary representation of a general locally compact abelian group. The representation is extended, again, to a representation of an abelian C*-algebra associated with the group. Theorem 5.2.6 applies and yields a projectionvalued measure on the pure state space of that algebra (the one-point compactification of the dual group). The entire process illustrates the way in which a representation of a given system can be studied through the corresponding representation of an associated C*-algebra. At the same time, it underscores the strong interrelations among Fourier analysis, representations of abelian groups, and representations of abelian C*-algebras. Broadened to the general (non-abelian) case, this area of study is often referred to as “noncommutative harmonic analysis.” H Bibliography: [I, 3, 10, 12, 13, 14, 15, 18, 20, 211 5.7. Exercises 5.7.1. Show that the mapping A operator continuous.
-+
A* of
a(%)into a(#)is weak-
371
5.1. EXERCISES
5.7.2. Show that the weak-operator topology on a(&')is strictly weaker (coarser) than the strong-operator topology on g ( X ) . 5.7.3. Let 9denote the set of all projections from a Hilbert space X onto its closed subspaces, and suppose that F E Y and 0 # F # I. Prove that the mappings
E-EAF,
E-EvF
(9-9)
are not continuous from 9 with the norm topology to 9 with the weakoperator topology. (Compare Exercise 2.8.17.) 5.7.4. Let X be a Hilbert space and K,Ywbe the restrictions of the strong- and weak-operator topologies to the set 9 of projections in W ( X ) . Show that % = Fw. Conclude that if a sequence of projections tends to a projection in the weak-operator topology, it tends to that projection in the strong-operator topology (but compare Exercise 5.7.8(ii)). 5.7.5. Let X be a Hilbert space and S;,S; be the restrictions of the strong- and weak-operator topologies to the set 42 of unitary operators in a(#).Show that Sl=S;.Conclude that if a sequence of unitary operators tends to a unitary operator in the weak-operator topology, it tends to that unitary operator in the strong-operator topology. be an orthonormal basis for 2, 5.7.6. Let X be a Hilbert space, {ya}aEA\ and Y be a bounded subset of g ( X ) .With F a finite subset of A, E a positive number, and So in Y:
{ S :I(@
- SO)Ya,Yd)I
< E,
U,U'EF,
S E W (=
%,A
is a weak-operator open neighborhood of So.Show that { VF,e} is a base for the weak-operator open neighborhoods of So in 9 5.7.7. Let X be a separable Hilbert space and { y l , y 2 , ..} . be an orthonormal basis for X. Show that the equation OD
d(S, T ) =
1
2-("+"')l((S - TlYn,Ym)I
n,m= 1
+
defines a translation-invariant metric d on B ( X ) (that is, d(S + R,T R) = d(S, T ) for each R in g(X)),and the associated metric topology coincides on bounded subsets of g ( X )with the weak-operator topology. 5.7.8. With the notation of Exercise 5.7.4, assume that X is infinite dimensional.
372
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
(i) Show that 9 is weak-operator dense in (B(X)):,the set of positive operators in the unit ball of W ( X ) . [Hint. Note that
[
A
( A I-- A is a projection in &?(X0 X ) when A E(B(X)): .] (ii) Show that 9 is strong-operator closed in a(%), and conclude that there is a sequence of projections tending to an operator in (B(X)):in the weak-operator topology, when X is separable, that does not tend to it in the strong-operator topology. (Compare Exercises 5.7.2 and 5.7.4 and Theorem 5.1.2.) ( A - A2)"2
5.7.9. Let X be a Hilbert space. (i) Show that the mapping A + A B of a(%)into &?(%)is weakoperator continuous for each B in @(H'). (ii) Show that the mapping A + BA of a(%)into g ( 2 ) is weakoperator continuous for each B in g ( X ) . (iii) When X is infinite dimensional, show that the mapping (A, B) + A B of (&?(X)): x (B(H')):into g(%) is not weak-operator continuous. 5.7.10. Let X be a Hilbert space and ( B ( X ) )be , { T E ~ ( %IlTll ) : 6 r}. (i) Show that the mapping ( A , B) -+ AB :
x &?(%)--* &?(X)
is continuous when ( L ~ ( Xx )g)(~X )is provided with the product of the weakoperator topology on and the strong-operator topology on &?(%), and the range a(%)is provided with the weak-operator topology. (Compare Exercise 2.8.33.) (ii) Let % be infinite dimensional and separable. Show that the mapping
A B : (B(%)),x (a(%))l + a(%) is not continuous when the first factor of (a(%)), x is provided with the strong-operator topology and the second is provided with the weakoperator topology.
('4B )
+
5.7.1 1. Let % be a Hilbert space and W be a von Neumann algebra on 2. (i) Show that (W), is weak-operator compact. (ii) Show that (9): is weak-operator compact. (iii) With X infinite dimensional, show that not strong-operator compact.
and ( B ( 2 ) ) :are
5.7.12. With the notation of Exercise 5.7.5, assume that 2 is infinite dimensional.
373
5.7. EXERCISES
[Hint. Follow the (i) Show that 42 is weak-operator dense in pattern of the solution to Exercise 5.7.8(i) and use the facts that each operator on a finite-dimensional Hilbert space has the form VH, where Vis unitary and His positive, and that such an operator Ro is unitary if either of R,*Roor RoRE is I.] (ii) Show that the set of isometries in B ( X )is strong-operator closed. (iii) Conclude that, with % separable, there is a sequence of unitary operators in a(*)that tends to an operator in (B(X))lin the weak-operator topology but not in the strong-operator topology. (iv) Find a sequence of unitary operators that converges in the strongoperator topology to an operator (isometry) that is not a unitary.
5.7.13. Let X be an infinite-dimensional Hilbert space. (i) Show that (%)1 is a weakly compact convex subset of X whose extreme points form a dense subset of it. (ii) Show that (a(%)):is a weak-operator compact convex subset of B ( Z )whose extreme points form a dense subset of it. (iii) Show that (a(%)), is a weak-operator compact convex subset of %!I whose (% extreme ) points form a dense subset of it. 5.7.14. Let X be an extremely disconnected compact Hausdorff space, and let {h: a E A} be a family of real-valued functions in C ( X )bounded above by some constant. (i) Suppose that eachf, is the characteristic function of some clopen subset X , of X. Show that [U,XJ is a clopen set whose characteristic function VacAh is the least upper bound of {fa} and that the interior of nasAX, is a clopen set whosecharacteristic function A fa is the greatest lower bound of {fa} in C(X). (ii) Show that X\
[u
asA
{XEX:fa(X)
1-
>I}
(= X,)
is a clopen subset of X and that if Y is a clopen subset of X with the property that f a ( p ) 6 I for all a in A and all p in Y, then Y G X , . (iii) Let el be the characteristic function of X , . Let k be a constant that bounds {fa} above and such that - k k ; (2) e, 6 el. if I 6 1'; (3) el = A,,,, e,..
p-
(iv) Show that I de, converges in norm (in the sense of approximating Riemann sums) to a functionfin C ( X )and that X, is the largest clopen set on which f takes values not exceeding 1.
374
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
(v) Show thatfis the least upper bound of {h}, and conclude that C ( X ) is a boundedly complete lattice. (vi) Deduce that C ( X )is a boundedly complete lattice if and only if Xis extremely disconnected. 5.7.15. Let X be a compact Hausdorff space. (i) Show that X is extremely disconnected if and only if disjoint open subsets have disjoint closures. (ii) Show that X is extremely disconnected if and only if it satisfies the following two conditions : (a) X is totally disconnected; (b) the family W of clopen subsets of Xpartially ordered by inclusion is a complete lattice. 5.7.16. With the notation of Exercises 3.5.4, 3.5.5, and 3.5.6: (i) show that I, is isometrically isomorphic to the maximal abelian algebra d of multiplication operators Mf( f ~I,) on 12(N,C) via the mapping f + M f ;
(ii) show that the pure state space of the quotient C*-algebra d / V 0(see Exercise 4.6.60) is (naturally homeomorphic to) p(N)\N, where Wo is the image of co under the mapping f + M,. 5.7.17. With the notation of Exercise 5.7.16, let Eo be a projection in the quotient C*-algebra a?/V0. (i) Show that there is a projection Ein d such that Emaps onto Eo under the quotient mapping. (ii) Let Yo be a subset of p(N)\N clopen in the relative topology on p(N)\N. Show that there is a clopen subset Y of p(N) such that Y n (p(N)\N) = Yo. 5.7.18. With the notation of Exercise 3.5.5: (i) show that 0- = (0n N)- for each open subset 0 of p(N); (ii) show that a subset Yo of p(N)\N is clopen in fl(N)\N if and only if it has the form N; n ( p ( N ) \ N ) for some subset No of N, and that Yo is nonempty if and only if No is infinite. 5.7.19. Let cp be a one-to-one mapping of the set of rational numbers onto N. For each real number t , choose a sequence {rl ,r 2 , .. .} of distinct rational numbers tending to t . With the notation of Exercise 5.7.16, let N, be {cp(rl), cp(rd,. . .I and let Y, be N; n (P(WW.
375
5.1. EXERCISES
(i) Show that Yl and Y, are disjoint, non-empty, clopen subsets of j(N)\N when s # t. (ii) Let S be a subset of R and let Ys be the closure of UsesY,. Show that if t # S , then Y , n Ys = Qr. (iii) Show that Y,isnot aclopen subset of P(N)\N for somesubset Sof R. [Hint. “Count” subsets of R and of N and use Exercise 5.7.18(ii).] (iv) Deduce that p(N)\N is totally disconnected but not extremely disconnected and that d / W 0 is not a boundedly complete lattice. 5.7.20. Let X be a complete metric space. We say that an open subset of X is regular when it coincides with the interior of its closure. (i) Show that the interiors of closed (hence of the closures and complements of open) sets in X are regular. (ii) Show that each open subset of Xdiffers from a regular open subset on a meager set. (iii) Show that each Borel subset of Xdiffers from a regular open subset on a meager (Borel) set. [Hint. Follow the pattern of the argument of the first paragraph of the proof of Lemma 5.2.10.1 (iv) Show that there is a unique regular open subset of Xthat differs from a given Borel set on a meager (Borel) set. (v) Let Fobe the family of regular open subsets of X partially ordered by inclusion. Show that Tois a complete lattice. (vi) Let 9 be the family of Borel subsets of X and A the o-ideal of meager Borel subsets of X (a countable union of sets in A is in A and the intersection of a set of A? with any set of 9is in A).Let 9 / Abe the family of equivalence classes of sets in 9under the relation S S‘when S and S‘ differ by a meager set. With Y and 9‘ in 9 / A ,define Y 5 Y’when S G S’ for some Sin Y and S‘in 9’. Show that 5 is a partial ordering of 9 / A(the “quotient” of “inclusion” on % by the ideal A),that each Y in %/A? contains precisely one regular open set, and that the mapping that assigns to each Y in 9 / Athe regular open set it contains is an order isomorphism of 9 / A ? onto go. Conclude that F / A is a complete lattice. (vii) Show that the algebra B(X)of bounded Borel functions on Xis a commutative C*-algebra and that the family A. of functions in a(X)that vanish on the complement of a meager Borel set is a closed ideal in B(X). Conclude that 9 ( X ) / A 0is a commutative C*-algebra. (viii) Let Y be the compact Hausdorff space such that B ( X ) / A oE C(Y). Show that Y is totally disconnected and that the family of clopen subsets of Y, partially ordered by inclusion, form a complete lattice. Conclude that Y is extremely disconnected and that C( Y) (and B ( X ) / A o ) are boundedly complete lattices.
-
376
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
5.7.21. With the notation of Exercise 5.7.20, assume that Xis [0,1] and let p be a state of C(Y). (i) Suppose p ( V=; en)= I;= p(e,) whenever {en}is acountable family of idempotents in C(Y )such that en . en.= 0 unless n = n’.(We say that p is a normal state in this case.) Show that p( V=; f,) ,<=C; p ( f , ) for each countable set { f n } of idempotentsf, in C(Y )(where “a ,< + coy’is envisaged in the inequality of this assertion). (ii) Enumerate the open intervals in [0,1] (= X ) with rational endpoints and let f i , fz,.. . be the idempotents in C(Y) that are the images of their characteristic functions (in ,%(A’)) under the composition of the quotient mapping of g(X)onto B ( X ) / A oand the isomorphism of g ( X ) / A 0 with C( Y ) . For each j in { 1,2,. . .}, let ej be an idempotent in C(Y ) such that 0 < ej G f j . Show that V 7 = l e j = 1. (iii) With the notation of (ii) and given a positive E , show that e j can be chosen such that p(ej) ,< 2-j.5. Conclude that C(Y ) has no normal states. (iv) Deduce that C(r ) is isomorphic to no abelian von Neumann algebra although Y is extremely disconnected. 5.7.22. Let d be an abelian von Neumann algebra acting on a Hilbert space 2,and let A be an operator in d. Suppose d z C(X),where X i s an extremely disconnected compact Hausdorff space, andfin C ( X )represents A . Show that A x = Ax for some unit vector x in 2 and some complex number 2 if and only iff-’@) contains a non-empty clopen subset of X . 5.7.23. Let W be a von Neumann algebra and 9’ be its family of projections. Show that, with p o in Wn,the family of all sets,
{PE@: I(p - po)(Ej>I 6 , j~ { I , . - . n}) (= v ( E l , . .., En,&)), where E > 0 and { E , ,. . . ,En} G 8 is a base for the open neighborhoods of po in the weak* topology on a bounded subset of 9’. 9
5.7.24. Let W be a von Neumann algebra acting on a Hilbert space 2. (i) Let rp be a representation of a C*-algebra ‘iN with image 9, and let V be a unitary operator in 9. Show that there is a unitary operator U in ‘iN such that q ( U ) = V . (ii) Show that the unitary group 9, of W is (pathwise) connected (in its norm topology). 5.7.25. Let S be a locally compact topological space, 9’the a-algebra of Borel sets, and m a a-finite regular Borel measure on S. Let % be the algebra of multiplications by bounded continuous functions on L,(S,m) and d be its weak-operator closure. Show that d is the algebra of multiplications by essentially bounded measurable functions on S.
5.1. EXERCISES
377
5.7.26. Let ( S , Z m ) be a a-finite measure space, f be a measurable function on S, and A be a bounded operator on L,(S, m) such that f . g = Ag almost everywhere for each essentially bounded measurable function g in L2(S,m).Show that f is essentially bounded and that Mf = A. 5.7.27. Let ( S , Z m ) be a a-finite measure space and g be an essentially bounded measurable function on S. (i) Show that m(g-'(D)) = 0 for each closed disk D contained in @\sp(g),where sp(g) is the essential range of g (defined and studied in Example 3.2.16). (ii) Note that each open subset of @ is the union of a countable family of closed disks and conclude that m(g- '(C\sp(g))) = 0. (iii) Let 1 be some point in sp(g) and define go(s) to be g(s) for s in g-'(sp(g)) and 1for s in g-'(C\sp(g)). Show that sp(go) = sp(g). (iv) Let f be a bounded Borel function on sp(g). Show that f ( M g )= f ( M g , ) = M,ogo.[Hint. Use uniqueness of the Borel function calculus.] 5.7.28. Let 9be a von Neumann algebra acting on a Hilbert space X, and such that UA + AU d 2A for each let A be a self-adjoint operator in a(%) self-adjoint unitary operator U in W.Show that A E W ' . 5.7.29. Let Y and 9-be two families of bounded operators on a Hilbert space 2,and suppose that Y c Z? (i) Show that Y' G 9'. (ii) Show that 9" = (9")" (= 9"'). (Compare Theorem 5.3.1.) 5.7.30. Let 2 be a Hilbert space of dimension greater than 1. Find a weak-operator closed subalgebra of B ( X ) such that B # a''. 5.7.31. Let X be a Hilbert space and 210 be a self-adjoint subalgebra of is the strong-operator closure of !No (and that I E a:).
a(%'). Assume that go(#)is dense in 2 but not that I E 210.Show that 2l:
5.7.32. Use the double commutant theorem to re-prove (compare Proposition 5.1.8), in the special case of a von Neumann algebra W (so W is assumed to contain I ) , that the union and intersection of each family of projections in W lie in 9. 5.7.33. Let 'LI be a simple C*-algebra (that is, 9l has no proper two-sided ideals) acting on a Hilbert space X. (i) Show that the center of 2l is { a l :a E C } .
378
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
(ii) Suppose 2l contains a maximal abelian subalgebra of 93(X).Show that 2l acts irreducibly on X.
5.7.34. Find a von Neumann algebra W and a strong-operator dense, selfadjoint subalgebra 210 (containing I ) such that no unitary operator in W other than a scalar is the strong-operator limit of unitary operators in % [Hint. !lo. Consider polynomials on [0,1].) 5.7.35. Let H be a Hilbert space, S a closed subset of R,9the set of selfadjoint operators on X with spectrum in S, and h a real-valued, bounded, continuous function defined on S. With A . in Y: let k be a continuous function on R that takes the value 1 at each point of sp(Ao) and vanishes outside of [ - (IIAoll I), llAoll 11. Let p be hk and q be 1 - k + p .
+
+
(i) Show that p ( A o ) = q(Ao) = h(Ao), h = (1 - h)p + hq, and
44 - h(A0) = (1- h(A))(p(A)- p(A0)) + h(A)(q(A)- q(A0)). (ii) Show that the mapping A + h ( A ) of Y into (the set of normal operators in) 9 ( X )is strong-operator continuous.
5.7.36. Let X be a Hilbert space, S a closed subset of R,and Y the set of self-adjoint operators in a(%)with spectrum in S. (i) Show that the mapping A + IAl is strong-operator continuous on 9 (ii) Let h be a real-valued, continuous function on S (so h is bounded on bounded subsets of S ) . Suppose So is a bounded subset of S such that g is bounded on S\So, where g ( t ) = h(t)/ltl for E in S\So. Show that the mapping A -,h(A) is strong-operator continuous on 9 (iii) Deduce that A -,A"" is a strong-operator continuous mapping on B ( X ) +for each positive integer n.
5.7.37. Let S be a subset of R. Supposefis a function defined on S such that the mapping A + f ( A ) is strong-operator continuous on Y for each Hilbert space H,where 9 'is the set of self-adjoint operators in a(X)with spectrum in S. Show thatfis continuous on S,bounded on bounded subsets of S, and that there is a bounded subset So of S such that {f(t)/ltl: t E S\So} is bounded. 5.7.38. Let 92be a von Neumann algebra acting on a Hilbert space %, and let E be a projection in 9. Find the central carrier of a projection F i n E 9 E relative SO EWE in terms of its central carrier C , relative to 9. 5.7.39. Let W be a von Neumann algebra of infinite linear dimension. Show that there is an orthogonal infinite family of non-zero projections with sum I .
5.7. EXERCISES
379
5.7.40. Let 2I be a C*-algebra acting on a Hilbert space 2, and let {Eb} be an orthogonal family of non-zero projections in the commutant 3' of with sum I . Suppose xo is a generating vector for 2I and Ebx, = a,x,, where Ian1 = IIE~xolland IlXnll = 1. (i) Show that a, # 0 and that x, = 1,a,x,. (ii) Suppose {Eb} is a (countably) infinite family (indexed by positive integers). Choose n ( j ) ( > n ( j - 1)) such that J U , , ~ , J< j - 2 f o r j in N. Let x' be x ; = l j - l x n ( j ) .Show that for each A in the strong-operator closure 2I- of a, Ax0 # (iii) Conclude that a' is finite dimensional if there is a vector xoin 2 such that { A x , : A E%-} = 2. XI.
5.7.41. Let 2I be a C*-algebra that acts topologically irreducibly on a and let {x,, . . . ,x,} and { y ,,. . . ,y,} be sets of vectors in 8. Hilbert space 2, (i) Let H be a self-adjoint operator in g ( 2 )such that H x j = y j f o r j in { 1,. . . ,n } . Show that there is a self-adjoint operator Kin % such that Kxj = y j for j in { 1,. .. ,n} and llKll < llHll. [Hint. Use a diagonalizing orthonormal basis for EHErestricted to [xl ,. . . ,x,, y , ,. . . ,y,], where Eis the projection in g ( 2 )with this subspace as range, and apply Remark 5.6.32.1 (ii) Let B be an operator in a(2)such that Bxj = y j f o r j in { 1 , . . . ,n } . Show that there is an operator A in '3 such that A x j = y j f o r j in { 1,. . .,n} and
[ [ A1) < IlBll. [Hint. With E as in the hint to (i), use the fact that EBE 1 E ( 2 ) has the form VH with V a unitary operator and H a positive operator on E(X).] 5.7.42. Let .X0be a Hilbert space and 2 be the direct sum 1,@ 2,of countably many copies 2,of X 0 .
(i) With the notation of Subsection 2.6, Matrix representations, let W be the subalgebra of g ( 8 )consisting of operators whose matrix has the same element of a(*,) at each diagonal entry and 0 at each off-diagonal entry. (With 2 viewed as a tensor product 2,0 X of 8o with a separable, infinitedimensional Hilbert space X, W is { T Q I : TEg(iW0)}.) Show that 9' consists of those operators whose matrix representations have scalar multiples of I at each entry. (In tensor product form, w' = { I @ S : S e g ( X ) } . ) Show that 9"= 9 and conclude that W is a von Neumann algebra (as well as 9'). Call this sequence 12(ii) Let xl, x2,. . . be a sequence of vectors in 2,. independent when I ;= llxj112 < co and IT=ajxj = 0 for a sequence { a j } in 12(N,C) only if aj = 0 for allj. Note that an 12-independentsequence is linearly independent and find a linearly independent sequence that is not 12independent when 8, is infinite dimensional. (iii) Show that {xl, x2,. . .} is a generating vector for W if and only if xl, x 2 , .. . is 12-independent.
380
5. ELEMENTARY VON NEUMANN ALGEBRA THEORY
(iv) Show that { x l , x 2 , . .} is separating for 9 if and only if [x1,x2,
...I = s o .
5.7.43. Let X be L2([0,13) relative to Lebesgue measure. With d the multiplication algebra of 2 and ‘illthe C*-algebra of multiplications by continuous functions, find a unit vector u that is separating for ‘ill(if A E ‘illand Au = 0, then A = 0) but not for d.[Hint. Use the “Cantor process” to find an open dense subset of [0,1] that has measure $1 5.7.44. Let X‘ be L2([0,13) relative to Lebesgue measure and d be the multiplication algebra of X.
(i) Describe the vectors in X‘ that are generating for d. (ii) Show that the set of generating vectors for d is dense in X (iii) Deduce that a norm limit of generating vectors need not be a generating vector. 5.7.45. Let W be a von Neumann algebra and { E l ,E 2 , .. .} be a countable family of countably decomposable projections in 9. Show that V=; En is a countably decomposable projection in 9. 5.7.46. Let 9 be a countably decomposablevon Neumann algebra acting on a Hilbert space X. Define a metric on W with the property that its associated metric topology coincides with the strong-operator topology on bounded subsets of 9. 5.7.47. Let (S, Z m )be a a-finite measure space, X‘ be L2(S,m),d be the multiplication algebra, andfand g be measurable functions on S finite almost everywhere. Show that:
(i) (ii) (iii) (iv)
M f = M , if and only i f f = g almost everywhere; Mar +, = a M f 4 M , for each scalar a ; M,., = M , M , ; M f 2 0 if and only i f f 2 0 almost everywhere.
5.7.48. Let (S, Z m )be a a-finite measure space, X‘ be L,(S, m),d be the multiplication algebra, g be a measurable function on S finite almost everywhere, and f be a Bore1 function on sp(M,).
(i) Define a concept of “essential range” sp(g) analogous to that of Example 3.2.16 and show that sp(g) = sp(M,). (ii) Show that there is a measurable go on S equal to g almost everywhere such that the range of go is contained in sp(g). (iii) With the notation of (ii), show thatf(M,) = MJeao.
38 1
5.7. EXERCISES
5.7.49. Let ,Y?' be L2(R) relative to Lebesgue measure and A be the (unbounded) multiplication operator corresponding to the identity transform I (the function t + r ) on iw with domain 9 consisting of thosefin L2(iw)such that I .f€L,(R).
(i) Note that A is self-adjoint and that the spectral resolution for A is { E l } ,where El is the multiplication operator corresponding to the characteristic function of (- co,A]. (ii) Let gobe the set of continuously differentiable functions on iw that vanish outside a finite interval. Note that gois a dense linear submanifold of X. Let Do be the operator with domain gothat assigns if' (= idfldt) t o 5 With T the unitary operator defined in Theorem 3.2.31, show that T - l A T f = Dof for each f in go. (iii) Conclude that T - ' A T (= D)with domain T - l ( 9 ) is a self-adjoint extension of Do. (iv) Show that exp(itD) is the unitary operator V , , where ( U , f ) ( p )= f ( p - t ) . How does this relate to Stone's theorem? 5.7.50. Let f be a jointly continuous function of two complex variables defined on S1 x S 2 , where S1 and S2 are subsets of @. Let A be a normal operator such that sp A G S2acting on a Hilbert space X. For each z1 in S1, the mapping z +f(zl, z) is a continuous (hence, Borel) function defined on sp A so that f(zl,A ) is a normal operator on #. Define an operator-valued function g on S1 by g(z) = f(z,A ) .
(i) Suppose A is bounded and So is a compact subset of S1. Show that the restriction of g to So is norm continuous (that is, the mapping z -,g(z) is continuous from So to A?(%) with its norm topology). (ii) Suppose S1is closed and f is bounded (but no longer that A is bounded). Show that g is strong-operator continuous. (iii) Let H be a positive operator on X. Show that exp( - izH) (= V,) is defined for each z in the closed lower half plane C - (= {z: I m z < 0}), that llUzll d 1, and that the mapping z -,V , is strong-operator continuous. 5.7.51. (i) With the notation and hypotheses of Exercise 5.7.50(ii), make the following additional assumptions about f:
(1) for each z2 in S2,z +f(z, z2)is differentiable at each point zo of the interior S(: of S1 with derivative fi(zo, z2), (2) given zo in Sy ,a bounded subset S ; of S2,and a positive E , there are a positive 6, aclosed disk D with center zo in S y , and a positive C , such that for all z 2 in S ;
ICf(Z922) -f(zo,z2)1(z - Zo)-l -fi(zo,z2)l <
E9
382
S. ELEMENTARY VON NEUMANN ALGEBRA THEORY
provided 0 < )z - zoI < 6 and Z E Sy (that is, z + f ( z , z 2 )is differentiable on ,Sy uniformly on bounded subsets of S,) and such that for all z in D\{zo} and z’ in S2 ICf(Z,
z? -f ( z 0 z’)l(z - zo) 9
I < c.
(3) z + f l ( z o , z ) is continuous on S2 for each zo in S y . Show that for each pair of vectors x, y in X, z + ( g ( z ) x , y ) is analytic on Sy with derivative ( f l ( z o , A ) x , y ) at each zo in S y . [Hint. With zo in S y , let z2) be [ f ( Z l , z2) - f(z0 Z 2 ) X Z l - 201- when (z1 z2) E sy x s2 and z 1 # zo, and let h(z0,z2) be f 1 ( z 0 , z 2 ) .Use Exercise 5.7.50(ii) to establish strong-operator continuity of z + h(z, A ) . ] (ii) With the notation and assumptions of Exercise 5.7.5O(iii), show that the function z - +( U , x , y ) is analytic in the open lower half-plane @! (= { z :Imz < 0 } ) for each pair of vectors x, y in 2. (iii) Show that z + (U,x,y) is entire for each pair of vectors x and y in X when H i s bounded. Re-prove the results of (ii) by using a bounding sequence {En}of projections for H and considering first the case where x, y ~ E , , ( 2 ) . Y
9
5.7.52. Let W be a von Neumann algebra acting on a Hilbert space 2 and t -,exp( - itH) (= U,) be a one-parameter unitary group on 2,where H is a self-adjoint operator on 2 with domain 9.Let xo be a unit vector in 9,Make the following assumptions about H, U,, 9, and xo : (1) (2) (3) (4)
U,AU-,EW for each A in H > 0; HxO = 0 ; xo is generating for 9.
W and each t in
[w;
(i) Define U, as in Exercise 5.7.5O(iii) when Z E @ - , and show that Uzxo= xo for each z in C- . With A and A’ self-adjoint operators in W and #,respectively,definef(z) to be ( U,Axo, A’xo) for z in C - . (ii) Show thatf(t) is a real number for real t , and thatfis continuous on C - , analytic on C! , and bounded on Q= - . (iii) Show that U,EW for each real t. 5.7.53. Let H be a self-adjoint operator acting on a Hilbert space 2,d be the von Neumann algebra generated by H , and dobe the von Neumann algebra generated by { U, : t E W}, where U, = exp( - itH). and a0 (i) Assume H is bounded and show that the C*-algebras generated by H (and I ) and by { U, : t E W}, respectively, coincide. (ii) Show that d = do. (iii) With the notation and assumptions of Exercise 5.7.52, show that HVW.
S.7. EXERCISES
383
5.7.54. Let A be a normal operator acting on a Hilbert space 2.
(i) Show that exp iA = I if sp A G (2nn : n E Z} and only if A is selfadjoint and sp A c {2nn :n E Z}. (ii) Show that expitA = I for all t in R if and only if A = 0. (iii) Let A and B be self-adjoint operators on A? such that expitB = expitA for each real t . Show that A = B. (iv) Let t + U, be a one-parameter unitary group acting on A?.Show that there is a unique self-adjoint operator H on 2 such that U, = exp itH for all real t.
5.7.55. With the notation of Exercise 5.7.52, assume conditions (l), (2) and (3), and in place of (4) assume that x, is separating for the center V of W. Show that there is a positive self-adjoint operator K on A? such that K q 3 and W,AW-, = U,AU-, for each A in W and all real t , where W, = exp(- itK) (EW), and such that Kx, = 0. [Hint.Consider the projection E' with range [ a x , ] and the von Neumann algebra a E ' acting on E ' ( X ) . ]
BIBLIOGRAPHY General references [HI P. R. Halmos, “Measure Theory.” D. Van Nostrand, Princeton, New Jersey, 1950; reprinted, Springer-Verlag, New York, 1974. [K] J. L. Kelley, “General Topology.” D. Van Nostrand, Princeton, New Jersey, 1955; reprinted, Sppnger-Verlag, New York, 1975. [Rl W. Rudin, “Real and Complex Analysis,” 2nd ed. McGraw-Hill, New York, 1974.
References [I] W. Ambrose, Spectral resolution of groups of unitary operators, Duke Math. J. 11 (1944), 589-595. [2] J. Dixmier, “Les C*-Algkbres et Leurs Representations.” Gauthier-Villars. Paris, 1964. [English translation: “C*-Algebras.” North-Holland Mathematical Library, Vol. 15. North-Holland Publ., Amsterdam, 1977.1 [3] J. M. G . Fell and J. L. Kelley, An algebra of unbounded operators, Proc. Nat. Acad. Sci. U.S.A. 38 (1952), 592-598. [4] I. M. Gelfand and M. A. Neumark, On the imbedding of normed rings into the ring of operators in Hilbert space, Mat. Sb. I2 (1943), 197-213. [5] J. G. Glimm and R. V. Kadison, Unitary operators in C*-algebras, Pacific J. Math. 10 (1960), 547-556. [6] F. Hansen and G. K. Pedersen, Jensen’s inequality for operators and Lowner’s theorem. Marh. Ann. 258 (1982), 229-241. [7] D. Hilbert, Grundziige einer allgemeinen Theorie der linearen Integralgleichungen IV, Nachr. Akad. Wiss. Gotringen Math.-Phys. K1. 1904, 49-91. [81 R. V. Kadison, Irreducible operator algebras, Proc. Nar. Acad. Sci. U.S.A.43 (1957), 273-276. [91 I. Kaplansky, A theorem on rings of operators, Pacific J. Math. 1 (1951), 227-232. [lo] J. L. Kelley, Commutative operator algebras, Proc. Nat. Acad. Sci. U.S.A.38 (1952), 598-605. [Ill J. von Neumann, Zur Algebra der Funktionaloperationen und Theorie der normalen Operatoren, Math. Ann. 102 (1930), 370-427. [ 121 J. von Neumann, AIlgemeine Eigenwerttheorie Hermitescher Funktionaloperatoren. Mark Ann. 102 (1930), 49-131. [13] J. von Neumann, Uber Funktionen von Funktionaloperatoren, Ann. of Math. 32 (1931), 191-226. [I41 J. von Neumann, Uber adjungierte Funktionaloperatoren, Ann. of Marh. 33 (1932), 294-3 10. 384
BIBLIOGRAPHY
385
[IS] M. Neumark, Positive definite operator functions on a commutative group (in Russian, English summary), Bull. Acad. Sci. URSS Str. Math. [Iz. Akad. Nauk SSSR Ser. Mat.] 7 (1!N3), 237-244. [16] G. K. Pedersen, “C*-Algebras and Their Automorphism Groups,” London Mathematical Society Monographs, Vol. 14. Academic Press, London, 1979. [I71 F. Riesz, “Les Systbmes d’kquations Lineaires 9 une InfinitC d’hconnues.” Gauthier-Villars, Paris, 1913. [18] F. Riesz, Uber die linearen Transformationen des komplexen Hilbertschen Raumes, Acta Sci. Math. (Szeged)5 (1930-1932), 23-54. [19] I. E. Segal, Irreducible representations of operator algebras, Bull. Amer. Math. Sac. 53 (1947), 73-88. [20] M. H. Stone, On one-parameter unitary groups in Hilbert space, Ann. ofMath. 33 (1932), 643-648. [21] M. H. Stone, “Linear Transformations in Hilbert Space and Their Applications to Analysis”. American Mathematical Society Colloquium Publications, Vol. 15. Amer. Math. SOC.,New York, 1932. [221 M. H. Stone, The generalized Weierstrass approximation theorem, Math. Mag. 21 (1948), 167-183, 237-254. [231 M. H. Stone, Boundedness properties in function-lattices; Canad. J. Math. 1 (1949), 176- 186. 1241 S. Stratila and L. Zsidb, “Lectures on von Neumann Algebras.” Abacus Press, Tunbridge Wells, 1979. [251 M. Takesaki, “Theory of Operator Algebras I.” Springer-Verlag, Heidelberg, 1979.
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INDEX OF NOTATION
Algebras and related matters
closed sum of operators, 352 closed product of operators, 352 algebra of convolution operators, 190 positive cone in a;244 set of self-adjoint elements of a, 249 weak-operator closure of a, 328 norm closure of a, 328 norm closure of d l ( R ) , 190 'UI(R) with unit adjoined, 190 algebra of Borel functions on C, 359 algebra of Borel functions on X,358 central carrier, 333 reduced von Neumann algebra, 336 reduced von Neumann algebra, 335 { A X : A E . P } 276 , { A x :A E S " , X E X } 276 , commutant, 325 double commutant, 326 { A * : A E ~ }326 , set of holomorphic functions, 206 unit element, identity operator, 41 isomorphism between algebras, 310 left kernel of the state p , 278 operator, on L2,of convolution byf, 190 positive cone in A, 255 set of self-adjoint elements of A, 255 set of multiplicative linear functionals on a,@), 197 195 algebra of operators affiliated with d , 352 algebra of normal functions on X, 344 set of pure states of d,261 pure states space of A, 261 GNS constructs, 278 dual group of R, 192 spectral radius, 180 spectral radius, 180 restricted von Neumann algebra, 334 restricted von Neumann algebra, 336 spectrum of A , 178, 357 - a Q ) \ { P a J 9
387
388
INDEX OF NOTATION
spectrum of A in 8,178 essential range of j , 185, 380 set of self-adjoint affiliated operators, 349 state space of J?, 257 state space of V, 213 set of self-adjoint functions on X , 344 dual group of TI, 231 T is affiliated with 9, 342 vector state, 256 vector functional, 305 dual group of Z, 230 Direct sums direct direct direct direct direct direct direct direct
sum of Hilbert spaces, 121 sum of Hilbert spaces, 121 sum of Hilbert spaces, 123 sum of vectors, 123 sum of operators, 122 sum of operators, 124 sum of representations, 281 sum of von Neurnann algebras, 336
Inner products and norms <.)
II II
I1 Ilp
II
112
II 111
inner product, 75 norm (on a linear space), 35 bound of a linear operator, 40 of a linear functional, 44 of a conjugate-bilinear functional, 100 of a multilinear functional, 126 norm inLp(1
Linear operators
a(#)+ w x , 'iu) NX)
HT ) 9( Im T
n
positive cone in @(#), 105 set of bounded linear operators on X, 41 set of bounded linear operators from X to OY, 41 domain of T, 154 graph of T, 155 imaginary part of T, 105
INDEX OF NOTATION
inclusion of operators, 155 infimum of projections, 1 1 1 supremum of projections, 11 1 infimum of projections, 11 1 supremum of projections, 1 I 1 multiplication operator, 108, 185, 341 null projection of T, 1 1 8 range projection of T, 118 real part of T, 105 closure of T, 155 Banach adjoint operator, 48 Hilbert adjoint operator, 102, 157 T restricted to C, 14 group of all unitary operators on .@, 282
Linear spaces aX co x C" 06" Iw"
VIyo .K
x+ Y
a multiples of vectors in X , I convex hull of X,4 space of comples n-tuples, 8 space of K n-tuples, 8 space of real n-tuples, 8 quotient linear space, 2 real linear space associated with complex space 'u; 7 vector sum and difference of X and Y , 1
Linear topological spaces, Bnnach spaces, Hilbert spaces closed (sometimes, weak* closed) convex hull, 31 dimension of X, 93 conjugate Hilbert space, 131 orthogonal complement, 87 set of Hilbert-Schmidt functionals, 128 set of Hilbert-Schmidt operators, 141 infimum (intersection) of closed subspaces, 11 1 supremum of closed subspaces, I 1 1 infimum (intersection) of closed subspaces, 1 1 1 supremum of closed subspaces, 1 1 1 weak topology, 28 weak* topology, 31 weak topology on K 30 continuous dual space, 30 subspace generated by xl,. . . ,x., 22 closed subspace generated by X, 22 { x E X : llxll < r ] , 36 Banach dual space, 43 Banach second dual space, 43 orthogonal complement, 87
389
390
INDEX OF NOTATION
Sets and mappings
set-theoretic difference, 1 j-compactification of N, 224 complex field, 1 empty set, 5 inclusion of sets strict inclusion of sets scalar field, Iw or C, 1 minimum of functions, 214 maximum of functions, 214 infimum of functions, 373 supremum of functions, 373 set of positive integers, 68 real field, 1 set of non-negative real numbers, 233 u restricted to %+ circle group 192 additive groups of integers, 230
Tensor products A , C3 ' . . 6 A , tensor product of operators, 145 2,0 . . . Q 2" tensor product of Hilbert spaces, 135 tensor product of vectors, 135 x1 0 . . . 6x,
A Adjoint in an algebra with involution, 237 Banach, 48 Hilbert, 102 of an unbounded operator, 157 Amliated operator, 342, 344 Algebra abelian (commutative) Banach, 180 abelian C*, 210, 269 abelian von Neumann, 310 Banach, 41, 174 of bounded operators, 102, 186, 236, 298, 299, 303, 309 C*, 236 of continuous functions, 175, 210 countably decomposable (von Neumann), 338, 339, 380 division, 180 finite-dimensional C*, 288 LI, 187, 233 L,, 237 maximal abelian, 308 multiplication, 308, 314, 340, 343, 376, 380 von Neumann, 308 normed, 174 operator, 173, 304 quotient, 177, 300 self-adjoint, 237, 282, 309 simple C*, 377 *, 237 of unbounded continuous functions, 355 of unbounded operators, 352, 355 Approximate eigenvector, 178, 179, 183 Approximate identity in C*-algebras, 254, 293 increasing, right, 254 in L , (R), 191
Approximation theorems double commutant, 326 Kaplansky density, 329 Stone-Weierstrass, 219, 221, 235 Archimedian, 297
B Bake category theorem, 60,323 Balanced neighborhood of 0, 13 Balanced set, 8 Banach algebra, 41, 174 Banach dual space, 44 Banach inversion theorem, 61 Banach lattice, 297 Banach module, 302 Banach-Orlicz theorem, 73 Banach space, 36 Bessel's inequality, 90,120 @-compactification,224 Bore1 measure, regular, 53, 54 Bound of a linear functional, 44 of a linear operator, 40 Bounded linear functional, 44 Bounded linear operator, 41 Boundedly complete lattice, see Lattice Bounded multilinear functional, 126 Bounded multilinear mapping, 131 Bounded set (in a normed space), 36 Bounding projection, 351 Bounding sequence, 351 C
C*-algebra, 236 simple, 377 Cauchy criterion, 26 Cauchy-Schwarz inequality, 77, 215, 256 Cayley transform, 327, 328 391
392
INDEX
Central carrier, 332, 333 Character of R, 192, 282 Of Ti,231 of Z,230 Clopen set, 222 Closed graph theorem, 62 Closure (of an operator), 155 Codimension, 2 Commutant, 325 Commutation relations, 181 Commutator, 181 Compact linear operator, 165, 166, 227 Compact self-adjoint operator, 166, 167 Compact support, 201 Complementary subspaces, 1I , 63,88 Complete (linear topological space), 14 Completion of a normed algebra, 174 of a normed space, 38 of a pre-Hilbert space, 80 Complexification of a real Hilbert space, 76, 161 of a real linear space, 66 of a real normed space, 66 Compression, 121, 276 Cone, 212, 245 Conjugate-bilinear functional, 100 bounded, 100 positive, 103 symmetric, 103 Conjugate Hilbert space, 131 Conjugate-linear operator, 15, 65 Convergence of nets in a locally convex space, 25 of series in a normed space, 38 of sums in a locally convex space, 25 Convex combination (finite), 3 Convex hull (of a set), 4 closed, 31 Convex set, 3 Convolution, 187, 230, 231 Core (for an unbounded operator), 155, 349 Countably decomposable von Neumann algebra, 338, 339, 380 Countably decomposable projection, 338, 340, 380 Cyclic projection, 336
Cyclic representation, 276, 278, 279 Cyclic vector, 276 D Definite inner product, 76 Definite state, 289 Derivation, 301, 302 Dimension (of a Hilbert space), 93 Direct sum of Hilbert spaces, 121, 123 of operators, 122, 124 of representations, 281 of von Neumann algebras, 336 Division algebra, 180 Double commutant theorem, 326 Dual group of R, 192 231 Of TI, of H, 230 Dual space algebraic, 2 Banach, 44 continuous, 30, 43 second, 43, 45
E Eigenvalue, 109 Eigenvector, 108 approximate, 178, 179, 183 Equivalence of function representations, 263 of representations, 280 Essential range of an L, function, 185 of a measurable function, 380 Essential supremum, 52 Essential representation, 282 Essentially bounded, 52 Evaluation functional, 21 I Extension by continuity, 14, 15 of pure states, 266, 2% of states, 266, 2% theorems, see Hahn-Banach theorems Extreme point, 31, 32, 33, 34, 163, 164, 373
393
INDEX
Extremely disconnected space, 222, 223, 224, 310, 322, 324, 344, 349, 373, 374, 375. 376
F Face, 32 Factor, 308 Factorization, 2, 42 Factors through, 2,42 Faithful representation, 275, 281 Faithful state, 288 Finite diagonal block (of a matrix), 154 Finite-dimensional space, 22, 23, 24 Finite-dimensional subspace, 22, 23, 24 First category (set of), 322 Fourier coefficients, 95 Fourier series, 95 Fourier transform, 187, 197, 368 inversion of, 198, 199 for L2functions, 201 Function calculus Borel (for bounded normal operators), 319, 321, 322, 324, 377 Borel (for unbounded normal operators), 340,360, 362, 363, 364,366,380 continuous, 239, 240, 271, 272, 273, 274, 340 holomorphic, 206 uniqueness, 273, 322, 362 Function representation, 263, 264 of an abelian C*-algebra, 270 of a Banach lattice, 297 equivalence of, 263 separating, 263 Functional conjugate-bilinear, I 0 0 linear, 2 multiplicative linear, see Multiplicative linear functional real-linear, 7 sublinear, 8 , 9 support, 9, 65 G
Gelfand-Neumark theorem, 275, 281 Generalized nilpotent, 205, 225, 226, 227 Generating set of vectors, 336, 337
Generating vector for a cyclic projection, 336 for a representation, 276 for a von Neumann algebra, 336, 338, 379, 380 GNS construction, 279 (essential) uniqueness of, 279 Gram-Schmidt orthogonalization process, 94 Graph (of an operator), 62, 155 H Hahn-Banach theorems extension type, 7, 9, 10, 21, 22, 44 separation type, 4, 7, 20, 21 Hahn-Jordan decomposition, 219, 258, 259, 265, 290 Half-space (closed or open), 4 Hermite polynomials, 97 Hermitian linear functional on a C*-algebra, 255 on C ( X ) ,215 Hilbert-Schmidt functional, 127 mapping (weak), 131 operator, 141 Hilbert space, 79 conjugate, 131 pre-Hilbert space, 79, 80, 81 Hilbert space isomorphism, 93, 103, 104 Hiilder’s inequality, 71, 188 Holomorphic function (Banach-space valued), 203 Holomorphic function calculus, 206 *Homomorphism, 237 Hull closed convex, 3 I convex, 4 of a set, 211 Hyperplane, 4 I Ideal in a Banach algebra, 177 in a C*-algebra, 251, 252, 254, 277, 300, 301 in C(X),210
394
INDEX
Ideal (conrinued) in L, (R) and related algebras, 187, 190 maximal, 177, 180, 210 Idempotent, I I , 208 Initial topology, 13 Inner product, 75, 76, 79 definite, 76, 79, 277 positive definite, 76 Inner product space, 76 Internal point, 4 Intersection (of projections), 11 1 Invariant mean, 224 Invariant subspace, 121 in Hilbert space, 121 in L, under translations, 233 Inverse (in a Banach algebra), 176 Inversion theorem Banach, 61 for Fourier transforms, 199 Invertible element (in a Banach algebra), 176 Involution, 236 Irreducibility algebraic, 330, 332 topological, 330, 331, 332 Isometric isomorphism, 36 of C*-algebras, 242 of function calculus, 240 natural, from J into J++,45 *Isomorphism, 237
I Jacobi polynomials, 97
K Kaplansky density theorem, 329 Kernel of an ideal in C ( X ) , 21 1 left, of a state, 278 of a representation, 276 Krein-Milman theorem. 32
L Laguerre polynomials, 97 Lattice, 214, 215, 298 Banach, 297 boundedly complete, 222, 223, 374, 375, 376
sublattice, 235 Legendre polynomials, 97 12-independent, 379 Linear combination (finite), 1 Linear dependence, 1 Linear functional, 2 bounded, 44 hermitian, see Hermitian linear functional multiplicative, see Multiplicative linear functional positive, see Positive linear functional weak* continuous, 31 weakly continuous, 29, 30 Linear independence, 1 Linear operator, see Operator; Unbounded operator Linear order isomorphism, 214 Linear topological space, 12 Linear transformation, see Operator; Unbounded operator Locally compact (locally convex space), 24 Locally convex space, 16 finite-dimensional,23, 24 locally compact, 24 Locally convex topology, 16 Locally finite group, 224
M Matrix with operator entries, 148 with scalar entries, 147 Maximal abelian algebra, 308 Maximal ideal, 177, 180, 187, 190, 210 Meager set, 322, 375 Minimal projection, 309 Minkowski’s inequality for sums, 50 for integrals, 53 Multilinear functional, 126 Multilinear mapping, 131 Multiplication algebra, 308, 314, 340, 343, 376, 380 Multiplication operator, 106, 107, 108, 109, 117, 185, 315, 341, 342, 343, 344 Multiplicative linear functional, 180, 183, 187, 269 on C ( X ) ,211, 213 on L , (R), 193
INDEX
on LI (TI,m ) , 232 on II (Z),231 on I. (Z),224 Multiplicity (of an eigenvalue), 167, 227
N Natural image of It: in J * * , 45 Neighborhoods balanced, 13, 18 in a linear topological space, 13 in a locally convex space, 17, 18 in a normed space, 35 in the strong-operator topology, 113 in the weak-operator topology, 305 in the weak* topology, 31 in a weak topology, 28 Nilpotent, 205 generalized, see Generalized nilpotent Non-singular (element of a Banach algebra), 176 Norm, 8, 35 on a Banach algebra, 174 of a bounded operator, 41, 100 on a C*-algebra, 236, 237 on C(S),49 of a conjugate-bilinearfunctional, 100 on a direct sum of Hilbert spaces, 121 of a direct sum of operators, 122 of a Hilbert-Schmidt functional, 128 on a Hilbert space, 77 on lp, 50, 51 on I,, 49 on L,, 53 on L,, 52 of a linear functional, 44 of a matrix with operator entries, 151 of a multilinear functional, 126 of a multilinear mapping, 131 on a normed algebra, 174 on a tensor product of Hilbert spaces, 132, 135 of a tensor product of operators, 146 of a weak Hilbert-Schmidt mapping, 131 Norm-preserving (linear mapping), 36 Norm topology, 35, 66 Normal element of a C*-algebra, 237 Normal function, 344, 355 Normal operator, see Operator; Unbounded operator
395
Normal state, 376 Normed algebra, 174 Normed space, 35 Nowhere-dense set, 60,322 Null function, 52 Null projection (of a linear operator), I18 Null set, 52 Null space (of a linear operator), 2, 118, 171 0
One-parameter unitary group, 282, 367 Open mapping, 59 theorem, 61 Operator, see also Unbounded operator d i a t e d , 342 bounded, 41, 100 compact, 165, 166, 226, 227 compact self-adjoint, 166, 167 conjugate-linear, 15 Hilbert-Schmidt , 141 linear, 2 multiplication, see Multiplication o g erator normal, 103, 319, 321, 322, 350, 354, 357 positive, 103 real-linear, 15 self-adjoint, see Self-adjoint operator unitary, see Unitary operator Operator-monotonic increasing, 250, 294, 295 Order unit, 213, 249, 255, 297 Orthogonal, 87 Orthogonal complement, 88 Orthogonal family of projections, 113 Orthogonal set, 88 Orthonormal basis, 91 Orthonormal set, 88
P Parallelogram law, 80 Parseval’s equation, 91, 120 Partially ordered vector space, 213, 249, 255, 295, 296, 297 archimedian, 297 Plancherel’s theorem, 201, 231, 232 Point spectrum, 357, 376 Polar decomposition, 105, 294
3%
INDEX
Polarization identity, 102 Polynomial Hermite, 97 Jacobi, 97 Laguerre, 97 Legendre, 97 Positive element of a C*-algebra, 244 Positive linear functional on a C*-algebra, 255 on C(X),213 on a partially ordered vector space, 213, 295, 2% Positive nth root, 275 Positive operator, see Operator; Unbounded operator Positive square root, 167, 248, 364 Pre-Hilbert space, 79, 81 Principle of uniform boundedness, 64, 65, 74 Projections, 12, 24, 109, 110 countably decomposable, 338, 340, 380 cyclic, 336 intersection of, 111 join of, 111 meet of, 111 orthogonal, 109 orthogonal family of, 113 cr-finite, 338 spectral, 362 union of, 111 Projection-valued measure, 318, 321, 360 Pure state Of %X), 302, 303 of a C*-algebra, 261, 269 of C ( X ) , 213 extension of, 234,266, 296 of a partially ordered vector space with order unit, 213 space, 261
Radical, 228 Radius of convergence, 204 Radon-Nikodqm derivative, 56 Radon-Nikodqm theorem, 56 Range (of a linear mapping), 2 projection, 118, 171 space, 118 Real-linear functional, 7 Real-linear operator, 15 Real-linear subspace, 7 Reduced atomic representation, 282 Reflexive (Banach space), 45,47, 67, 70, 73, 98 Regular Bore1 measure, 53, 54 Regular (element of a Banach algebra), 176 Representation of a C*-algebra, 275 cyclic, 276, 278, 279 direct sum, 281 equivalent, 280 essential, 282 faithful, 275,281 function, 263, 264 reduced atomic, 282 R-essential, 316 of a * algebra, 282 universal, 281 Resolution of the identity, 31 1 bounded, 311, 313 unbounded, 311, 316, 343, 344, 345, 348, 350 R-essential representation, 316 Riemann-Lebesgue lemma, 197 Riesz decomposition property, 214 Riesz representation theorem, 53 Riesz’s representation theorem, 97 Rodrigues’s formula, 97
Q
S
Quotient Banach algebra, 177 Banach space, 39 C*-algebra, 300 linear space, 2 mapping, 2, 39, 42 norm, 39 normed space, 39
R
Self-adjoint algebra of operators, 237, 282, 309 Self-adjoint element of a C*-algebra, 237 Self-adjoint function, 344 Self-adjoint function representing an operator, 348 Self-adjoint operator, 103, 157, 160, 310, 313, 341, 345, 348
INDEX
Self-adjoint set, 237 Semi-norm, 8, 10, 17, 28 Semi-simple, 228 Separable Banach space, 57, 58, 73,74 Separable Hilbert space, 94 Separable metric space, 57 Separable topological space, 57 Separating family of linear functionals, 28 Separating family of semi-norms, 17 Separating set of vectors, 336, 337 Separating vector, 336, 338, 339, 380 Separation of convex sets, 4 strict, of convex sets, 4 theorems, 4, 7, 20, 21 shift one-sided, 186 two-sided, 186, 227 a-finite projection, 338 von Neumann algebra, 338 a-ideal, 375 u-normal homomorphism, 321, 322, 323, 324, 325, 359, 360, 362, 364 u-normal mapping, 320 Simple C*-algebra, 377 Simple tensor, 135 Singular (element of a Banach algebra), 176, 229 Spectral mapping theorems, 181, 207, 241, 273 Spectral projection, 362 Spectral radius, 180, 185, 205 formula, 202, 204 Spectral resolution, 310, 312, 313, 360 of a representation, 315, 316, 367 Spectral theorem algebraic, 239, 270, 310, 349 for a bounded self-adjoint operator, 310, 313 for an unbounded self-adjoint operator, 345, 348 Spectral value, 178 Spectrum, 178 point spectrum, 357, 376 of an unbounded operator, 357 Square root in a Banach algebra, 233, 234 in a C*-algebra, 248 of a positive operator, 364
397
Stable subspace, 121 State of a C*-algebra, 255 of C(X), 213 definite, 289, 292 extension of, 234, 266, 2% faithful, 288 of a partially ordered vector space with order unit, 213 pure, see Pure state space, 257 vector, 256, 281, 289, 298, 302 Stone’s theorem, 187, 282, 367, 381 Stone-Weierstrass theorem, 219, 221, 235 Strong-operator continuity (of functions), 327, 328, 378 Strong-operator topology, 113, 304, 305, 380 *Subalgebra, 237 Sublattice, 235 Sublinear functional, 8, 9, 65 Subprojection, 110 Subspace closed, generated by a set, 22 complementary, 11, 63, 88 finite-dimensional, 22, 23, 24 generated by a set, 2 invariant, 121 linear, 1 real-linear, 7 reducing, 121 stable, 121 Summable, 25 support of a measure, 219 of a positive linear functional on C ( X ) , 219 Support functional, 9, 65
T Tensor product algebraic, 139 associativity of, 136, 146 of Hilbert spaces, 125, 135 of operators, 145 universal property of, 125, 135, 139 of vectors, 135 Topological divisor of zero, 229
398
INDEX
Topology coarser (weaker), 29 induced by semi-norms, 17 initial, 13 locally convex, 16, 23 norm, 35,66 strong-operator, see Strong-operator topology weak, see Weak topology weak*, 31, 43,45,46,48, 68 weaker (coarser), 29 weak-operator, 304, 305, 306, 371 Totally disconnected, 222, 374 Trace, normalized, 289 Transformation Linear, see Operator; Unbounded operator unitary, 104 Transitivity, 332 Triangle inequalilty, 3 5
U Unbounded operator adjoint of, 157 amliated, 342 closable (preclosed), 155 closed, 155, 357 closure of, 155 core of, 155 densely defined, 155 domain of, 154, 155 extension of, 155 graph of, 155 maximal symmetric, 160 multiplication, see Multiplication operator normal, 340, 350, 353, 354, 360 positive, 357 preclosed (closable), 155 products of, 157, 352 self-adjoint, see Self-adjoint operator spectrum of, 357
sums of, 157, 352 symmetric, 160 von Neumann algebra generated by, 349, 354
Uniform boundedness (principle of), 64, 65,74 Uniform structure in a linear topological space, 14 in a normed space, 35 Uniformly convex, 67, 161 Union (of projections), I 1 1 Unit ball, 36 Unit element, 174, 236 Unitary element of a C*-algebra, 237, 242 Unitary exponential 275, 286, 287, 288, 313, 314 Unitary group, 237, 286, 287 UNtary operator, 103, 313 Unitary representation, 282, 367 Unitary transformation, 104 Universal representation, 281 Unordered sums, 25, 26, 27, 28
v Vector state, 256, 281, 289, 298, 302 von Neumann algebra, 308 generated by an unbounded operator, 349, 354
W Weak-operator topology, 304, 305, 306, 37 1 Weak topology induced by a family of linear functionals, 28,29 the weak topology, 30, 43, 47, 66 Weak* topology, see Topology Weierstrass approximation theorem, 22 1 Wiener’s Tauberian theorems, 233 W-topology (weak* topology), see Topology
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