Lecture Notes in Mathematics 2095
Daniele Angella
Cohomological Aspects in Complex Non-Kähler Geometry
Lecture Notes in Mathematics Editors-in-Chief: J.-M. Morel, Cachan B. Teissier, Paris Advisory Board: Camillo De Lellis (Zurich) Mario Di Bernardo (Bristol) Alessio Figalli (Pisa/Austin) Davar Khoshnevisan (Salt Lake City) Ioannis Kontoyiannis (Athens) Gabor Lugosi (Barcelona) Mark Podolskij (Heidelberg) Sylvia Serfaty (Paris and NY) Catharina Stroppel (Bonn) Anna Wienhard (Heidelberg)
For further volumes: http://www.springer.com/series/304
2095
Daniele Angella
Cohomological Aspects in Complex Non-KRahler Geometry
123
Daniele Angella Dipartimento di Matematica UniversitJa di Pisa Pisa, Italy
ISBN 978-3-319-02440-0 ISBN 978-3-319-02441-7 (eBook) DOI 10.1007/978-3-319-02441-7 Springer Cham Heidelberg New York Dordrecht London Lecture Notes in Mathematics ISSN print edition: 0075-8434 ISSN electronic edition: 1617-9692 Library of Congress Control Number: 2013952956 Mathematics Subject Classification (2010): 32Q99, 53C55, 32Q60, 32C35, 57T15, 32G05, 32G07, 53D05, 53D18 © Springer International Publishing Switzerland 2014 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Duplication of this publication or parts thereof is permitted only under the provisions of the Copyright Law of the Publisher’s location, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the Copyright Clearance Center. Violations are liable to prosecution under the respective Copyright Law. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)
In ricordo di mio nonno, il Maestro Giovanni
Introduction
By a remarkable result by W. L. Chow, [Cho49, Theorem V], see also [Ser56], proı jective manifolds (that is, compact complex submanifolds of CPn WD CnC1 n f0g .C n f0g/, for n 2 N) are in fact algebraic (that is, they can be described as the zero set of finitely many homogeneous holomorphic polynomials). One is hence interested in relaxing the projective condition, looking for special properties on compact manifolds sharing a weaker structure than projective manifolds. For example, a large amount of developed analytic techniques allows to prove strong cohomological properties for compact Kähler manifolds (that is, compact complex manifolds endowed with a Kähler metric, namely, a Hermitian metric admitting a local potential function), [SvD30, Käh33], see also [Wei58], which are, in a certain sense, the “analytic-versus-algebraic”, [Cho49, Theorem V], or the “R-versus-Q”, [Kod54, Theorem 4], version of projective manifolds. On the one side, there is the class of Kähler manifolds, which are in fact endowed with three different structures, interacting each other: a complex structure, a symplectic structure, and a metric structure; it is the strong linking between them that allows to develop many analytic tools and hence to derive the very special properties of Kähler manifolds. On the other side (the Dark Side. . . ), in order to further investigate any of such properties and to understand what of these three structures is actually involved and required, it is natural to look for manifolds not admitting any Kähler structure: a large amount of interesting non-Kähler manifolds has been provided since [Thu76]. In other words, one is led to study complex, symplectic, and metric contributions separately, possibly weakening either the interactions between them, or one of these structures. For example, by relaxing the metric condition, one could ask what properties of a compact complex manifold can be deduced by the existence of special Hermitian metrics defined by conditions similar to, but weaker than, the defining condition of the Kähler metrics (for example, metrics being balanced in the sense of M. L. Michelsohn [Mic82], pluriclosed [Bis89], astheno-Kähler [JY93, JY94], Gauduchon [Gau77], strongly-Gauduchon [Pop13]); by relaxing the complex structure, one is led to study properties of almost-complex manifold, possibly endowed with compatible symplectic structures.
vii
viii
Introduction
In these notes, we are concerned with summarizing some recent results on the cohomological properties of compact complex manifolds endowed with no Kähler structure. Cohomological aspects of manifolds endowed with almost-complex structures, or with other special structures (such as, for example, symplectic, generalized-complex, . . . ) are also considered. We recall that a complex manifold X is endowed with a natural almost-complex structure, that is, an endomorphism J 2 End.TX/ of the tangent bundle of X such that J 2 D idTX , which actually satisfies a further integrability condition, [NN57, Theorem 1.1]. By considering the decomposition into eigen-spaces, just the datum of the almost-complex structure yields a splitting of the complexified tangent bundle, namely, TX ˝ C D T 1;0 X ˚ T 0;1 X ; and hence it induces also a splitting of the bundle of complex differential forms, namely, ^ X ˝R C D
M
^p;q X :
pCqD
Furthermore, on a complex manifold, the integrability condition of such an almost; complex structure yields a further structure on ^ X , namely, a structure of double ; complex ^ X; @; @ , where @ and @ are the components of the C-linear extension of the exterior differential d. Hence, on a complex manifold X , one can consider both the de Rham cohomology HdR .X I C/ WD
ker d im d
and the Dolbeault cohomology H@; .X / WD
ker @ im @
I
whenever X is compact, the Hodge theory assures that they have finite dimension as C-vector spaces. On a compact complex manifold, in general, no natural map .X I C/ exists; on the other hand, the structure of double between H@; .X / and HdR ; complex of ^ X; @; @ gives rise to a spectral sequence E1; ' H@; .X / ) HdR .X I C/ ;
Introduction
ix
from which one gets the Frölicher inequality, [Frö55, Theorem 2]: for every k 2 N, X p;q k .X I C/ dimC H@ .X / : dimC HdR pCqDk
On a complex manifold, a “bridge” between the Dolbeault and the de Rham cohomology is provided, in a sense, by the Bott-Chern cohomology, ; HBC .X / WD
ker @ \ ker @ im @@
;
and the Aeppli cohomology, HA; .X / WD
ker @@ im @ C im @
:
In fact, the identity induces the maps of (bi-)graded C-vector spaces ; .X / HBC KK s KKK s s s KKK ss s KK s s ys % .X I C/ H@; .X / HdR H@; .X / KKK ss KKK ss s KKK ss K% yss HA; .X /
which, in general, are neither injective nor surjective. These cohomology groups have been introduced by R. Bott and S. S. Chern in [BC65], and by A. Aeppli in [Aep65], and studied by several authors in different contexts: among others, B. Bigolin [Big69, Big70], A. Andreotti and F. Norguet [AN71], J. Varouchas [Var86], M. Abate [Aba88], L. Alessandrini and G. Bassanelli [AB96], S. Ofman [Ofm85a,Ofm85b,Ofm88], S. Boucksom [Bou04], J.-P. Demailly [Dem12], M. Schweitzer [Sch07], L. Lussardi [Lus10], R. Kooistra [Koo11], J.-M. Bismut [Bis11b, Bis11a], L.-S. Tseng, and S.-T. Yau [TY11]. We recall that whenever X is compact, the Hodge theory can be performed also for Bott-Chern and Aeppli cohomologies, [Sch07, §2], yielding their finitedimensionality; more precisely, one has that, on a compact complex manifold X of complex dimension n endowed with a Hermitian metric, ; HBC .X / ' Q BC
and
HA; .X / ' Q A ;
where Q BC and Q A are 4th order self-adjoint elliptic differential operators; furthermore, the Hodge--operator associated to any Hermitian metric on X induces an p;q nq;np isomorphism HBC .X / ' HA .X /, for every p; q 2 N.
x
Introduction
; By the definitions, the map HBC .X / ! HdR .X I C/ is injective if and only if every @-closed @-closed d-exact form is @@-exact: a compact complex manifold fulfilling this property is said to satisfy the @@-Lemma; see [DGMS75] by P. Deligne, Ph. A. Griffiths, J. Morgan, and D. P. Sullivan, where consequences of the validity of the @@-Lemma on the real homotopy type of a compact complex manifold are investigated. When the @@-Lemma holds, it turns out that actually all the above maps are isomorphisms, [DGMS75, Lemma 5.15, Remark 5.16, 5.21]: in particular, one gets a decomposition M ; HdR .X I C/ ' H@ .X / such that H@1 ;2 .X / ' H@2 ;1 .X / :
A very remarkable property of compact Kähler manifolds is that they satisfy the @@-Lemma, [DGMS75, Lemma 5.11]: this follows from the Kähler identities, which can be proven as a consequence of the fact that the Kähler metrics osculate to order 2 the standard Hermitian metric of Cn at every point. Therefore, the above decomposition holds true, in particular, for compact Kähler manifolds, [Wei58, Théorème IV.3]. In particular, if X is a compact complex manifold satisfying the @@-Lemma, then, for every k 2 N, X p;q k dimC HdR .X I C/ D dimC HBC .X / : pCqDk
In the first chapter, we study cohomological properties of compact complex manifolds, studying in particular the Bott-Chern and Aeppli cohomologies, and their relation with the @@-Lemma. In fact, we prove an inequality à la Frölicher for the Bott-Chern and Aeppli cohomologies, which provides also a characterization of the compact complex manifolds satisfying the @@-Lemma just in terms of the dimensions of the Bott-Chern cohomology groups, [AT13b, Theorem A, Theorem B]; a key tool in the proof of the Frölicher-type inequality relies on exact sequences by J. Varouchas, [Var86]. More precisely, we state the following result. Theorem (see Theorems 2.13 and 2.14). Let X be a compact complex manifold. Then, for every k 2 N, the following inequality holds: X p;q p;q k .X I C/ : dimC HBC .X / C dimC HA .X / 2 dimC HdR pCqDk
Furthermore, the equality X p;q p;q k dimC HBC .X / C dimC HA .X / D 2 dimC HdR .X I C/ pCqDk
holds for every k 2 N if and only if X satisfies the @@-Lemma. P p;q k Note that the equality pCqDk dimC H@ .X / D dimC HdR .X I C/ for every k 2 N (which is equivalent to the degeneration of the Hodge and Frölicher spectral
Introduction
xi
sequence at the first step, E1 ' E1 ) is not sufficient to let X satisfy the @@-Lemma: in some sense, the above result states that the Bott-Chern cohomology, together with its dual, the Aeppli cohomology, encodes “more information” on the double ; complex ^ X; @; @ than just the Dolbeault cohomology. As a straightforward consequence of the previous theorem, we obtain another proof, see [AT13b, Corollary 2.7], of the following stability result, see [Voi02, Proposition 9.21], [Wu06, Theorem 5.12], [Tom08, §B]. Corollary (see Corollary 2.2). Satisfying the @@-Lemma is a stable property under small deformations of the complex structure, that is, if fXt gt 2B is a complex-analytic family of compact complex manifolds and Xt0 satisfies the @@-Lemma for some t0 2 B, then Xt satisfies the @@-Lemma for every t in an open neighbourhood of t0 in B. The previous results can be generalized to a more algebraic context, see [AT13a]. In particular, one gets applications concerning the cohomologies of symplectic manifolds and, more in general, of generalized complex manifolds, and to the study of the Hard Lefschetz Condition and of the d dJ -Lemma, which we summarize in Appendix: Cohomological Properties of Generalized Complex Manifolds. (See Sects. 1.2 and 1.3 for preliminary results on symplectic and generalized complex structures and on their cohomologies.) We recall that compact Kähler manifolds have special cohomological properties not only in the complex framework but also from the symplectic viewpoint: another important result, other than the Hodge decomposition theorem, [Wei58, Théorème IV.3], is the Lefschetz decomposition theorem, [Wei58, Théorème IV.5], which provides a decomposition of the de Rham cohomology in terms of primitive subgroups of the cohomology. Starting from J.-L. Koszul [Kos85] and J.-L. Brylinski [Bry88], several authors studied symplectic geometry from the cohomological point of view, see, e.g., [Mat95, Yan96, Cav05, TY12a, TY12b, Lin13]. More precisely, J.-L. Brylinski, aimed by drawing a parallel between the symplectic and the Riemannian cases, proposed in [Bry88] a Hodge theory for compact symplectic manifolds, introducing in particular the notion of symplectically harmonic form; O. Mathieu in [Mat95] and D. Yan in [Yan96] proved that any de Rham cohomology class admits a symplectically harmonic representative if and only if the so-called Hard Lefschetz Condition is satisfied. In [TY12a,TY12b], see also [TY11], L.-S. Tseng and S.-T. Yau introduced new cohomologies for symplectic manifolds: among them, in particular, they defined and studied a symplectic counterpart of the Bott-Chern and Aeppli cohomologies, further developing a Hodge theory. By enlarging the study of the (co)tangent bundle to the study of the direct sum of tangent and cotangent bundles, complex structures and symplectic structures can be framed into a unified context, thanks to the notion of generalized complex structure, introduced by N. J. Hitchin in [Hit03] and developed, among others, by M. Gualtieri, [Gua04a, Gua11], and G. R. Cavalcanti, [Cav05], see also [Hit10, Cav07]. The notion of Bott-Chern cohomology, as well as the notion of @@-Lemma, can be reformulated also in the generalized complex setting; in particular, one yields the so-called d dJ -Lemma, see [Cav05]. Looking at symplectic structures as special
xii
Introduction
cases of generalized complex structures, the d dJ -Lemma turns out to be just the Hard Lefschetz Condition. An inequality à la Frölicher, characterizing the validity of the Hard Lefschetz Condition, respectively, of the d dJ -Lemma, holds also for symplectic manifolds, respectively for generalized complex manifolds, see Appendix: Cohomological Properties of Generalized Complex Manifolds. In the second chapter, we consider nilmanifolds and, more in general, solvmanifolds. They are defined as compact quotients of connected simply-connected nilpotent, respectively solvable, Lie groups by co-compact discrete subgroups, and they constitute a fruitful and interesting source of examples in non-Kähler geometry. In fact, on the one hand, non-tori nilmanifolds admit no Kähler structure, [BG88, Theorem A], [Has89, Theorem 1, Corollary], and, on the other hand, focusing on left-invariant geometric structures on solvmanifolds, one can often reduce their study at the level of the associated Lie algebra; this turns out to hold true, in particular, for the de Rham cohomology of completely-solvable solvmanifolds, [Nom54, Hat60], and for the Dolbeault cohomology of nilmanifolds endowed with certain left-invariant complex structures, [Sak76, CFGU00, CF01, Rol09a, Rol11a], see, e.g., [Con06, Rol11a]. More precisely, on a nilmanifold X D nG, the inclusion of the subcomplex composed of the G-left-invariant forms on X (which is isomorphic to the complex .^ g ; d/, where g is the associated Lie algebra) turns out to be a quasiisomorphism, [Nom54, Theorem 1], that is, ' .gI R/ WD H ^ g ; d ! HdR .X I R/ I i W HdR a similar result holds true also for completely-solvable solvmanifolds, [Hat60, Corollary 4.2], and for the Dolbeault cohomology of nilmanifolds endowed with left-invariant complex structures belonging to certain classes, [Sak76, Theorem 1], [CFGU00, Main Theorem], [CF01, Theorem 2, Remark 4], [Rol09a, Theorem 1.10], [Rol11a, Corollary 3.10]. n o As a matter of notation, denote by H]; .gC /, for ] 2 @; @; BC; A , the cohomology of the corresponding subcomplex of G-left-invariant forms on a solvmanifold X D nG, with Lie algebra g, endowed with a G-left-invariant complex structure. The following result states a theorem à la Nomizu also for the Bott-Chern and Aeppli cohomologies, [Ang11, Theorem 3.7, Theorem 3.8, Theorem 3.9]. Theorem (see Theorems 3.5, 3.6, Remark 3.10, and Theorem 3.7). Let X D nG be a solvmanifold endowed with a G-left-invariant complex structure J , and denote the Lie algebra naturally associated to G by g. Suppose that the inclusions of the subcomplexes of G-left-invariant forms on X into the corresponding complexes of differential forms on X yield the isomorphisms '
i W HdR .gI C/ ! HdR .X I C/
and
'
i W H@; .gC / ! H@; .X / I
Introduction
xiii
in particular, this holds true if one of the following conditions holds: X is holomorphically parallelizable; J is an Abelian complex structure; J is a nilpotent complex structure; J is a rational complex structure; g admits a torus-bundle series compatible with J and with the rational structure induced by ; • dimR g D 6 and g is not isomorphic to h7 WD 03 ; 12; 13; 23 .
• • • • •
Then also '
; ; i W HBC .gC / ! HBC .X /
and
'
i W HA; .gC / ! HA; .X /
are isomorphisms. Furthermore, if C .g/ denotes the set of G-left-invariant complex structures on X , then the set U WD
o n ' .gC / ,! H]; .X / J 2 C .g/ W i W H]; J J
is open in C .g/, for ] 2 f@; @; BC; Ag. The above result allows to explicitly compute the Bott-Chern cohomology for the Iwasawa manifold I3 WD H .3I Z Œi/n H.3I C/ and for its small deformations, where 9 80 1 3 1 = < 1z z H.3I C/ WD @ 0 1 z2 A 2 GL.3I C/ W z1 ; z2 ; z3 2 C ; : 0 0 1 and H .3I Z Œi/ WD H.3I C/ \ GL .3I Z Œi/ : The Iwasawa manifold is one of the simplest example of compact non-Kähler complex manifold: as an example of a holomorphically parallelizable manifold, it has been studied by I. Nakamura, [Nak75], who computed its Kuranishi space and classified the small deformations of I3 by means of the dimensions of their Dolbeault cohomology groups. In Sect. 3.2.4, [Ang11, §5.3], we explicitly compute the Bott-Chern cohomology of the small deformations of the Iwasawa manifold, showing that it makes possible to give a finer classification of the small deformations fXt gt2.0;"/C6 of I3 than the Dolbeault cohomology: more precisely, classes (ii) and (iii) in I. Nakamura’s classification [Nak75, §3] are further subdivided into subclasses (ii.a) and (ii.b), 2;2 respectively (iii.a) and (iii.b), according to the value of dimC HBC .Xt /.
xiv
Introduction
Left-invariant complex structures on six-dimensional nilmanifolds have been classified by M. Ceballos, A. Otal, L. Ugarte, and R. Villacampa in [COUV11]. Hence, in Sect. 3.3, we provide the dimensions of the Bott-Chern cohomology for each of these complex structures, as computed in [AFR12] jointly with M. G. Franzini and F. A. Rossi. In view of [AT13b, Theorem A, Theorem B], such dimensions measure the non-Kählerianity of six-dimensional nilmanifolds. Enlarging the class of nilmanifolds to solvmanifolds, several results concerning de Rham cohomology have been studied by A. Hattori [Hat60], G. D. Mostow [Mos54], D. Guan [Gua07], S. Console and A. M. Fino [CF11], and by H. Kasuya [Kas13a,Kas12a,CFK13]; as for Dolbeault cohomology, results have been obtained by H. Kasuya [Kas13b, Kas12a, Kas11, Kas12d, Kas12b]; as for Bott-Chern cohomology, we have obtained some results on Bott-Chern cohomology in joint work with H. Kasuya, [AK12, AK13a]. In Appendix: Cohomology of Solvmanifolds we summarize some of these results, providing the Nakamura manifold as an explicit example. In the third chapter, we do not require the integrability of the almost-complex structure, and we study cohomological properties of almost-complex manifolds, that is, differentiable manifolds endowed with a (possibly non-integrable) almostcomplex structure.1 In this case, the Dolbeault cohomology is not defined. However, following T.-J. Li and W. Zhang, [LZ09], one can consider, for every p; q 2 N, the subgroup n o .p;q/;.q;p/ pCq .X I R/ WD Œ˛ 2 HdR .X I R/ W ˛ 2 .^p;q X ˚ ^q;p X / \ ^pCq X HJ pCq
HdR .X I R/; and the complex counterpart n o .p;q/ pCq pCq HJ .X I C/ WD Œ˛ 2 HdR .X I C/ W ˛ 2 ^p;q X HdR .X I C/ : .p;q/
p;q
If X is a compact Kähler manifold, then HJ .X I C/ ' H@ .X / for every p; q 2 N, [DLZ10, Lemma 2.15, Theorem 2.16]; therefore these subgroups can be considered, in a sense, as a generalization of the Dolbeault cohomology groups to the non-Kähler, or to the non-integrable, case. Two remarks need to be pointed out. Firstly, note that, in general, neither the equality in X pCqDk pq
1
.p;q/;.q;p/
HJ
pCq
.XI R/ HdR
.XI R/; or
X
.p;q/
HJ
pCq
.XI C/ HdR
.XI C/;
pCqDk
The theory developed by T.-J. Li and W. Zhang for almost-complex structures can actually be restated also in the symplectic, [AT12b], and in the D-complex settings, [AR12]. We recall that, in a sense, D-complex geometry is the “hyperbolic analogue” of complex geometry, and that many connections between it and other theory both in Mathematics and in Physics have been investigated in the last years, see, e.g., [HL83, AMT09, CMMS04, CMMS05, CM09, CFAG96, KMW10, ABDMO05, AS05, Kra10, Ros12a, Ros12b].
Introduction
xv
holds, nor the sum is direct, nor there are relations between the equality holding and the sum being direct, see, e.g., Proposition 4.1. Hence, one may be interested in studying compact almost-complex manifolds for which one of the above properties holds, at least for a fixed k 2 N, see [LZ09, DLZ10, DLZ11, FT10, AT11, AT12a, Zha13, ATZ12, DZ12, HMT11, LT12, DL13, DLZ12]. A remarkable result by T. Drˇaghici, T.-J. Li, and W. Zhang, [DLZ10, Theorem 2.3], states that every almost-complex structure J on a compact four-dimensional manifold X 4 satisfies the cohomological decomposition 4 .2;0/;.0;2/ 4 .1;1/ 4 2 HdR X I R D HJ X I R ˚ HJ X IR : Secondly, note that J b^2 X satisfies .J b^2 X /2 D id^2 X ; therefore the above sub2 groups of HdR .X I R/ can be interpreted as the subgroup represented by J -invariant forms, HJC .X / WD HJ
.1;1/
.X I R/ D
˚ 2 Œ˛ 2 HdR .X I R/ W J˛ D ˛ ;
and the subgroup represented by J -anti-invariant forms, HJ .X / WD HJ
.2;0/;.0;2/
.X I R/ D
˚ 2 Œ˛ 2 HdR .X I R/ W J˛ D ˛ :
Note also that if g is any Hermitian metric on X whose associated .1; 1/-form ! WD g.J ; / 2 ^1;1 X \ ^2 X is d-closed (namely, g is an almost-Kähler metric on X ), then Œ! 2 HJC .X /. In fact, T.-J. Li and W. Zhang’s interest in studying such subgroups and C 1 -pureand-full almost-complex structures (that is, almost-complex structures for which the decomposition 2 HdR .X I R/ D HJC .X / ˚ HJ .X /
holds, [LZ09, Definition 2.2, Definition 2.3, Lemma 2.2]) arises in investigating the symplectic cones of an almost-complex manifold, that is, the J -tamed cone ˚ 2 .X I R/ W !x .vx ; Jx vx / > 0 KJt WD Œ! 2 HdR for every vx 2 Tx X n f0g and for every x 2 X g and the J -compatible cone ˚ 2 KJc WD Œ! 2 HdR .X I R/ W !x .vx ; Jx vx / > 0 for every vx 2 Tx X n f0g and for every x 2 X; and J ! D !g : Indeed, they proved in [LZ09, Theorem 1.1] that, given a C 1 -pure-and-full almostKähler structure on a compact manifold X , the J -anti-invariant subgroup HJ .X /
xvi
Introduction
2 of HdR .X I R/ measures the quantitative difference between the J -tamed cone and the J -compatible cone, namely,
KJt D KJc ˚ HJ .X / : A natural question concerns the qualitative comparison between the tamed cone and the compatible cone: more precisely, one could ask whether, whenever an almost-complex structure J admits a J -tamed symplectic form, there exists also a J -compatible symplectic form. This turns out to be false, in general, for nonintegrable almost-complex structures in dimension greater than 4, [MT00, Tom02], see also Theorem 4.6; on the other hand, it is not (yet) known whether, for almost-complex structures on compact four-dimensional manifolds, as asked by S. K. Donaldson, [Don06, Question 2], or for complex structures on compact manifolds of complex dimension greater than or equal to 3, as asked by T.-J. Li and W. Zhang, [LZ09, page 678], and by J. Streets and G. Tian, [ST10, Question 1.7], it holds that KJc is non-empty if and only if KJt is non-empty. We prove in Theorem 4.13 that no counterexample can be found among six-dimensional non-tori nilmanifolds endowed with left-invariant complex structures, [AT11, Theorem 3.3]; note that the same holds true, more in general, for higher dimensional nilmanifolds, as proven by N. Enrietti, A.M. Fino, and L. Vezzoni, [EFV12, Theorem 1.3]. Theorem (see Theorem 4.13). Let X D nG be a six-dimensional nilmanifold endowed with a G-left-invariant complex structure J . If X is not a torus, then there is no J -tamed symplectic structure on X . One can study further cones in cohomology, which are related to special metrics, other than Kähler metrics; a key tool is provided by the theory of cone structures on differentiable manifolds developed by D. P. Sullivan, [Sul76]. In order to compare, in particular, the cone associated to balanced metrics (that is, Hermitian metrics whose associated .1; 1/-form is co-closed, [Mic82, Definition 1.4, Theorem 1.6]) and the cone associated to strongly-Gauduchon metrics (thatis, Hermitian metrics whose associated .1; 1/-form ! satisfies the condition that @ ! dimC X 1 is @-exact [Pop09, Definition 3.1]), we give the following result, [AT12a, Theorem 2.9], which is the semi-Kähler counterpart of [LZ09, Theorem 1.1]. (We refer to Sect. 4.4.3 for the definitions of the cones KbJt and KbJc on a manifold X endowed with an almostcomplex structure J .) Theorem (see Theorem 4.19). Let X be a compact 2n-dimensional manifold endowed with an almost-complex structure J . Assume that KbJc ¤ ¿ (that is, there exists a semi-Kähler structure on X ) and that 0 62 KbJt . Then .n1;n1/
KbJt \ HJ
.X I R/ D KbJc
and .n;n2/;.n2;n/
KbJc C HJ
.X I R/ KbJt :
Introduction
xvii .n;n2/;.n2;n/
2n2 Moreover, if the equality HdR .X I R/ D HJ holds, then .n;n2/;.n2;n/
KbJc C HJ
.n1;n1/
.X I R/CHJ
.X I R/
.X I R/ D KbJt :
In order to better understand cohomological properties of compact almostcomplex manifolds, and in view of the Hodge decomposition theorem for compact Kähler manifolds, it could be interesting to investigate the subgroups .p;q/;.q;p/ .X I R/ for almost-complex manifolds endowed with special structures. HJ For example, we prove the following result, [ATZ12, Proposition 4.1], providing a strong difference between the Kähler case and the almost-Kähler case. Proposition (see Proposition 4.8). The differentiable manifold X underlying the Iwasawa manifold I3 WD H .3I Z Œi/n H.3I C/ admits a non-C 1-pure-and-full almost-Kähler structure. A further study on almost-Kähler structures .J; !; g/ concerns the connections between C 1 -pure-and-fullness and the Lefschetz-type property on 2-forms firstly considered by W. Zhang, that is, the property that the Lefschetz operator ! n2 ^ W ^2 X ! ^2n2 X takes g-harmonic 2-forms to g-harmonic .2n2/-forms, see, e.g., Theorem 4.4; we refer to [ATZ12] for further results. As a tool to study explicit examples, we provide a Nomizu-type theorem for .p;q/;.q;p/ .X I R/ of a completely-solvable solvmanifold X D nG the subgroups HJ endowed with a G-left-invariant almost-complex structure J , [ATZ12, Theorem 5.4], see Proposition 4.2, and Corollary 4.2. A remarkable result by K. Kodaira and D. C. Spencer states that the Kähler property on compact complex manifolds is stable under small deformations of the complex structure, [KS60, Theorem 15]: more precisely, it states that, given a compact complex manifold admitting a Kähler structure, every small deformation still admits a Kähler structure; it can be proven as a consequence of the semicontinuity properties for the dimensions of the cohomology groups of a compact Kähler manifold. Hence, a natural question in non-Kähler geometry is to investigate the (in)stability of weaker properties than being Kähler. As a first result in this direction, L. Alessandrini and G. Bassanelli proved that, given a compact complex manifold, the property of admitting a balanced metric (that is, a Hermitian metric whose associated .1; 1/-form is co-closed) is not stable under small deformations of the complex structure, [AB90, Proposition 4.1]; on the other hand, they proved that the class of balanced manifolds is stable under modifications, [AB96, Corollary 5.7]. Another result in this context is the stability of the property of satisfying the @@Lemma under small deformations of the complex structure, as already recalled, see Corollary 2.2.
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Therefore, it is natural to investigate stability properties for the cohomological .p;q/;.q;p/ decomposition by means of the subgroups HJ .X I R/ on (almost-)complex manifolds .X; J /. More precisely, we consider the Iwasawa manifold I3 WD .p;q/;.q;p/ .X I R/ provide a H .3I Z Œi/n H.3I C/, showing that the subgroups HJ cohomological decomposition for I3 but not for some of its small deformations, Theorem 4.8. We prove the following result, [AT11, Theorem 3.2]. Theorem (see Theorem 4.7). The properties of being C 1 -pure-and-full is not stable under small deformations of the complex structure. More in general, one could try to study directions along which the curves of almost-complex structures on a differentiable manifold preserve the property of being C 1 -pure-and-full. We use a procedure by J. Lee, [Lee04, §1], to construct curves of almost-complex structures through an almost-complex structure J , by means of J -anti-invariant real 2-forms, in order to provide examples, see, e.g., [AT11, Theorem 4.1], see Theorem 4.9. Another problem in deformation theory is the study of semi-continuity properties for the dimensions of the subgroups HJC .X / and HJ .X /. As a consequence of the Hodge theory for compact four-dimensional manifolds, T. Drˇaghici, T.-J. Li, and W. Zhang proved in [DLZ11, Theorem 2.6] that, given a curve fJt gt 2I R of (C 1 -pure-and-full) almost-complex structures on a compact four-dimensional manifold X , the functions I 3 t 7! dimR HJt .X / 2 N
and
I 3 t 7! dimR HJCt .X / 2 N
are, respectively, upper-semi-continuous and lower-semi-continuous. In higher dimension this fails to be true, as we show in explicit examples, [AT12a, Proposition 4.1, Proposition 4.3], see Propositions 4.9 and 4.10. Motivated by such counterexamples, one can study a stronger semi-continuity property on almostcomplex manifolds (namely, that, for every d-closed J -invariant real 2-form ˛, there exists a d-closed Jt -invariant real 2-form t D ˛Co .1/, depending real-analytically in t, for t 2 ."; "/ with " > 0 small enough): we give a formal characterization of the curves of almost-complex structures satisfying such a property, [AT12a, Proposition 4.5], see Proposition 4.11, and we provide also a counterexample to such a stronger semi-continuity property, [AT12a, Proposition 4.9], see Proposition 4.12. The plan of these notes is as follows. In Chap. 1, we collect the basic notions concerning (almost-)complex, symplectic, and generalized complex structures, we recall the main results on Hodge theory for Kähler manifolds, and we summarize the classical results on deformations of complex structures, on currents and de Rham homology, and on solvmanifolds. In Chap. 2, we study cohomological aspects of compact complex manifolds, focusing in particular on the study of the Bott-Chern cohomology, [AT13b, AT13a]. By using exact sequences introduced by J. Varouchas, [Var86], we prove a Frölichertype inequality for the Bott-Chern cohomology, Theorem 2.13, which also provides a characterization of the validity of the @@-Lemma in terms of the dimensions of
Introduction
xix
the Bott-Chern cohomology groups, Theorem 2.14. Finally, we collect some results concerning cohomological aspects of symplectic geometry and, more in general, of generalized complex geometry. In Chap. 3, we study Bott-Chern cohomology of nilmanifolds, [Ang11, AK12], (see also [AK13a, AK13b]). We prove a result à la Nomizu for the Bott-Chern cohomology, showing that, for certain classes of complex structures on nilmanifolds, the Bott-Chern cohomology is completely determined by the associated Lie algebra endowed with the induced linear complex structure, Theorems 3.5, 3.6, and 3.7. As an application, in Sect. 3.2, we explicitly study the Bott-Chern and Aeppli cohomologies of the Iwasawa manifold and of its small deformations. Finally, we summarize some results concerning cohomologies of solvmanifolds. In Chap. 4, we study cohomological properties of almost-complex manifolds, [AT11, AT12a, ATZ12]. Firstly, in Sect. 4.1, we recall the notion of C 1 -pure-andfull almost-complex structure, which has been introduced by T.-J. Li and W. Zhang in [LZ09] in order to investigate the relations between the compatible and the tamed symplectic cones on a compact almost-complex manifold and with the aim to throw light on a question by S. K. Donaldson, [Don06, Question 2]. In particular, we are interested in studying when certain subgroups, related to the almostcomplex structure, let a splitting of the de Rham cohomology of an almost-complex manifold, and their relations with cones of metric structures. In Sect. 4.2, we focus on C 1 -pure-and-fullness on several classes of (almost-)complex manifolds, e.g., solvmanifolds endowed with left-invariant almost-complex structures, semiKähler manifolds, almost-Kähler manifolds. In Sect. 4.3, we study the behaviour of C 1 -pure-and-fullness under small deformations of the complex structure and along curves of almost-complex structures, investigating properties of stability, Theorems 4.7 and 4.9, and of semi-continuity for the dimensions of the invariant and anti-invariant subgroups of the de Rham cohomology with respect to the almostcomplex structure, Propositions 4.9, 4.10, 4.11, and 4.12. In Sect. 4.4, we consider the cone of semi-Kähler structures on a compact almost-complex manifold and, in particular, by adapting the results by D. P. Sullivan on cone structures, [Sul76], we compare the cones of balanced metrics and of strongly-Gauduchon metrics on a compact complex manifold (see Theorem 4.19). This work has been originated from the author’s Ph.D. thesis at Dipartimento di Matematica of Università di Pisa, under the advice of prof. Adriano Tomassini, [Ang13b]. Part of the original results are contained in [AT11, AT12a, Ang11, AT13b, AR12, ATZ12, AC12, AT12b, Ang13a, AFR12, AK12, AT13a] (see also [AC13, AK13a, AK13b]).
Acknowledgments
The existence of these notes is mainly due to the advice, guidance, teachings, patience, suggestions, and support that my adviser Adriano Tomassini has given me during the last years: then my first thanks goes to him for making me grow up a lot. I also wish to thank Jean-Pierre Demailly for his hospitality at Institut Fourier, for his explanations and his kind answers to my questions, and for his support and encouragement. Many thanks to Marco Abate for all his kindness and for his very useful suggestions: his advices improved a lot the presentation of these notes. A particular thanks goes to the director of the Ph.D. school “Galileo Galilei”, Fabrizio Broglia, for his support and his help during my years in Pisa. Thanks also to Ute McCrory, Friedhilde Meyer, and K.Vinodhini, for their kindly assistance in the preparation of the volume. I think mathematics makes sense just when played in two, or more. Hence I would like to thank all my collaborators: Federico Alberto Rossi, Simone Calamai, Weiyi Zhang, Maria Giovanna Franzini, and Hisashi Kasuya. My growth as a mathematician is due to very many useful conversations and discussions (on mathematics, and beyond), sometimes really brief, sometimes everlasting, but always very inspiring, especially with Lucia Alessandrini, Amedeo Altavilla, Claudio Arezzo, Paolo Baroni, Luca Battistella, Leonardo Biliotti, Junyan Cao, Carlo Collari, Laura Cremaschi, Alberto Della Vedova, Valentina Disarlo, Tian-Jun Li, Tedi Drˇaghici, Nicola Enrietti, Anna Fino, Alberto Gioia, Serena Guarino Lo Bianco, Greg Kuperberg, Minh Nguyet Mach, Maura Macrì, Gunnar Þór Magnússon, John Mandereau, Daniele Marconi, Costantino Medori, Samuele Mongodi, Isaia Nisoli, Marco Pasquali, Maria Rosaria Pati, David Petrecca, Massimiliano Pontecorvo, Maria Beatrice Pozzetti, Jasmin Raissy, Sönke Rollenske, Matteo Ruggiero, Matteo Serventi, Marco Spinaci, Cristiano Spotti, Herman Stel, Pietro Tortella, Luis Ugarte, Andrea Villa, and many others. I wish to thank all the members and the staff of the three Departments of Mathematics where I spend part of my life: the Dipartimento di Matematica of the Università di Pisa; the Dipartimento di Matematica e Informatica of the Università xxi
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Acknowledgments
di Parma; and the Institut Fourier in Grenoble. In particular, thanks to Daniele, Matteo, Alberto, Marco, Carlo, Jasmin, Cristiano, Paolo, Simone, Eridano, Pietro, Fabio, Amedeo, Martino, Giandomenico, Luigi, Francesca, Sara, Michele, Minh, Isaia, John, Andrea, David, Ana, Tiziano, Andrea, Maria Rosaria, Giovanni, Marco, Sara, Giuseppe, Matteo, Valentina, Laura, Paolo, Laura, Alessio, Flavia, Stefano, Cristina, Abramo, . . . Very special gracias to Serena, Maria Beatrice and Luca, Andrea, Simone, Paolo, Laura, Matteo, Chiara, Michele: morally, you know you should be considered as a sort of co-authors. Pisa, Italy
Daniele Angella
Contents
1
Preliminaries on (Almost-)Complex Manifolds . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.1 Almost-Complex Geometry and Complex Geometry .. . . . . . . . . . . . . . . . 1.1.1 Almost-Complex Structures .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.1.2 Complex Structures, and Dolbeault Cohomology .. . . . . . . . . . . . 1.2 Symplectic Geometry .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.2.1 Symplectic Structures . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.2.2 Cohomological Aspects of Symplectic Geometry . . . . . . . . . . . . 1.3 Generalized Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.3.1 Generalized Complex Structures .. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.3.2 Cohomological Aspects of Generalized Complex Geometry .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.3.3 Complex Structures and Symplectic Structures in Generalized Complex Geometry . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.4 Kähler Geometry .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.4.1 Kähler Metrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.4.2 Hodge Theory for Kähler Manifolds.. . . . . .. . . . . . . . . . . . . . . . . . . . 1.4.3 @@-Lemma and Formality for Compact Kähler Manifolds . . . 1.5 Deformations of Complex Structures . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.6 Currents and de Rham Homology .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.7 Solvmanifolds .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.7.1 Lie Groups and Lie Algebras.. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.7.2 Nilmanifolds and Solvmanifolds .. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . Appendix: Low Dimensional Solvmanifolds and Special Structures .. . . . . . A.1 Solvmanifolds up to Dimension 4 . . . . . . . . .. . . . . . . . . . . . . . . . . . . . A.2 Five-Dimensional Solvmanifolds . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . A.3 Six-Dimensional Nilmanifolds .. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . A.4 Six-Dimensional Solvmanifolds . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .
1 1 2 4 9 9 12 20 20 23 24 27 27 30 31 35 42 44 44 46 52 52 55 55 63
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2 Cohomology of Complex Manifolds . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.1 Cohomologies of Complex Manifolds . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.1.1 The Bott-Chern Cohomology . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.1.2 The Aeppli Cohomology . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.1.3 The @@-Lemma .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.2 Cohomological Properties of Compact Complex Manifolds and the @@-Lemma . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.2.1 J. Varouchas’ Exact Sequences.. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.2.2 An Inequality à la Frölicher for the Bott-Chern Cohomology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.2.3 A Characterization of the @@-Lemma in Terms of the Bott-Chern Cohomology . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . Appendix: Cohomological Properties of Generalized Complex Manifolds .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .
65 65 66 69 73
3 Cohomology of Nilmanifolds. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.1 Cohomology Computations for Special Nilmanifolds . . . . . . . . . . . . . . . . 3.1.1 Left-Invariant Complex Structures on Solvmanifolds . . . . . . . . 3.1.2 Classical Results on Computations of the de Rham and Dolbeault Cohomologies .. . . . . . . . . . . . . . . . 3.1.3 The Bott-Chern Cohomology on Solvmanifolds .. . . . . . . . . . . . . 3.2 The Cohomologies of the Iwasawa Manifold and of Its Small Deformations.. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.2.1 The Iwasawa Manifold and Its Small Deformations . . . . . . . . . . 3.2.2 The de Rham Cohomology of the Iwasawa Manifold and of Its Small Deformations . .. . . . . . . . . . . . . . . . . . . . 3.2.3 The Dolbeault Cohomology of the Iwasawa Manifold and of Its Small Deformations . .. . . . . . . . . . . . . . . . . . . . 3.2.4 The Bott-Chern and Aeppli Cohomologies of the Iwasawa Manifold and of Its Small Deformations .. . . . 3.3 Cohomologies of Six-Dimensional Nilmanifolds .. . . . . . . . . . . . . . . . . . . . Appendix: Cohomology of Solvmanifolds .. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .
95 95 96
4 Cohomology of Almost-Complex Manifolds . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.1 Subgroups of the de Rham (Co)Homology of an Almost-Complex Manifold .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.1.1 C 1 -Pure-and-Full and Pure-and-Full Almost-Complex Structures .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.1.2 Relations Between C 1 -Pure-and-Fullness and Pure-and-Fullness . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.2 C 1 -Pure-and-Fullness for Special Manifolds . . . . .. . . . . . . . . . . . . . . . . . . . 4.2.1 Special Classes of C 1 -Pure-and-Full (Almost-)Complex Manifolds .. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.2.2 C 1 -Pure-and-Full Solvmanifolds .. . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.2.3 Complex-C 1-Pure-and-Fullness for Four-Dimensional Manifolds.. . . . . . . . . .. . . . . . . . . . . . . . . . . . . .
78 79 81 84 88
97 103 111 111 120 122 126 130 140 151 151 152 161 165 165 168 173
Contents
4.2.4 Almost-Complex Manifolds with Large Anti-invariant Cohomology . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.2.5 Semi-Kähler Manifolds.. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.2.6 Almost-Kähler Manifolds and Lefschetz-Type Property . . . . . 4.3 C 1 -Pure-and-Fullness and Deformations of (Almost-)Complex Structures . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.3.1 Deformations of C 1 -Pure-and-Full Almost-Complex Structures .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.3.2 The Semi-continuity Problem .. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.4 Cones of Metric Structures . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.4.1 Sullivan’s Results on Cone Structures . . . . .. . . . . . . . . . . . . . . . . . . . 4.4.2 The Cones of Compatible, and Tamed Symplectic Structures.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.4.3 The Cones of Semi-Kähler, and Strongly-Gauduchon Metrics . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .
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175 177 184 195 196 212 220 221 222 228
References .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 233 Index . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 247
Chapter 1
Preliminaries on (Almost-)Complex Manifolds
Abstract In this preliminary chapter, we summarize some basic notions and some classical results in (almost-)complex and symplectic geometry. In particular, we start by setting some definitions and notation concerning (almost-)complex structures, Sect. 1.1, symplectic structures, Sect. 1.2, and generalized complex structures, Sect. 1.3; then we recall the main results in the Hodge theory for Kähler manifolds, Sect. 1.4, and in the Kodaira, Spencer, Nirenberg, and Kuranishi theory of deformations of complex structures, Sect. 1.5; furthermore, we summarize some basic definitions and some useful facts about currents and de Rham homology, Sect. 1.6, and about nilmanifolds and solvmanifolds, Sect. 1.7, in order to set the notation for the following chapters. (As a matter of notation, unless otherwise stated, by “manifold” we mean “connected differentiable manifold”, and by “compact manifold” we mean “closed manifold”.)
1.1 Almost-Complex Geometry and Complex Geometry The tangent bundle of a complex manifold X is naturally endowed with an endomorphism J 2 End.TX/ such that J 2 D idTX , satisfying a further integrability property. It is hence natural to study differentiable manifolds endowed with such an endomorphism, the so-called almost-complex manifolds. It turns out that the vanishing of the Nijenhuis tensor NijJ characterizes the almost-complex structures J on X naturally induced by a structure of complex manifold [NN57, Theorem 1.1]. In this section, we recall the notions of almost-complex structure, complex manifold, and Dolbeault cohomology, and some of their properties.
D. Angella, Cohomological Aspects in Complex Non-Kähler Geometry, Lecture Notes in Mathematics 2095, DOI 10.1007/978-3-319-02441-7__1, © Springer International Publishing Switzerland 2014
1
2
1 Preliminaries on (Almost-)Complex Manifolds
1.1.1 Almost-Complex Structures Let X be a (differentiable) manifold. We start by recalling the notion of almostcomplex structure. Definition 1.1 ([Ehr49]). Let X be a differentiable manifold. An almost-complex structure J on X is an endomorphism J 2 End.TX/ such that J 2 D idTX . Let J be an almost-complex structure on X . Extending J by C-linearity to TX ˝ C, we get the decomposition TX ˝ C D T 1;0 X ˚ T 0;1 X ; where T 1;0 X (respectively, T 0;1 X ) is the sub-bundle of TX ˝C given by the i-eigenspaces (respectively, the . i/-eigen-spaces) of J 2 End .TX ˝ C/: that is, for every x 2 X, 1;0 T X x D fvx i Jx vx W vx 2 Tx X g ; 0;1 T X x D fvx C i Jx vx W vx 2 Tx X g : Considering the dual of J , again denoted by J 2 End .T X /, we get analogously a decomposition at the level of the cotangent bundle: T X ˝ C D T 1;0 X ˚ T 0;1 X ; where T 1;0 X (respectively, T 0;1 X ) is the sub-bundle of T X ˝ C given by the i-eigen-spaces (respectively, the . i/-eigen-spaces) of the C-linear extension J 2 End .T X ˝ C/. Extending the endomorphism J to the bundle ^ .T X / ˝ C of complex-valued differential forms, we get, for every k 2 N, the bundle decomposition ^k T X ˝ C D
M
^p T 1;0 X ˝ ^q T 0;1 X :
pCqDk
As a matter of notation, we will denote by C 1 .X I F / the space of smooth sections 2 of a vector bundle F over X , and, will denote for every k 2 N and .p; q/ 2 N , we k 1 k k by ^ X WD C X I ^ .T X / the space of smooth sections of ^ .T X / over p;q p;q 1 p 1;0 q 0;1 XI ^ T X ˝ ^ T X the space of X and by ^ X WD ^J X WD C 1;0 0;1 p q smooth sections of ^ T X ˝ ^ T X over X. Since d ^0 X ˝R C ^1;0 X ˚ ^0;1 X and d ^1 X ˝R C ^2;0 X ˚ ^1;1 X ˚^0;2 X , since every differential form is locally a finite sum of decomposable differential forms, and by the Leibniz rule, the C-linear extension of the exterior differential, dW ^ X ˝ C ! ^C1 X ˝ C, splits into four components:
1.1 Almost-Complex Geometry and Complex Geometry
3
d D A C @ C @ C AN where AW ^; X ! ^C2;1 X ;
@W ^; X ! ^C1; X ;
@W ^; X ! ^;C1 X ;
N ^; X ! ^1;C2 X I AW in terms of these components, the condition d2 D 0 is written as 8 ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ < ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ :
A2 D 0 A@ C @A D 0 A @ C @2 C @ A D 0 A AN C @ @ C @ @ C A AN D 0 : 2
@ AN C @ C AN @ D 0 AN @ C @ AN D 0 AN2 D 0
Remark 1.1. More in general, given a 2n-dimensional manifold X , an almost-c.p.s. structure, [Vai07, Sect. 1], is the datum of an endomorphism K 2 End.TX/ such that K 2 D idTX where 2 f1; 0; 1g: more precisely, the case D 1 corresponds to almost-complex structures, the case D 0 corresponds to almost-subtangent structures, and the case D 1 corresponds to almost-product structures. Among almost-product structures, D-complex structures deserve special interest. By definition, an almost-D-complex structure (also called almost-para-complex structure) on a manifold X is an endomorphism K 2 End.TX/ such that K 2 D idTX and rk T C X D rk T X D 12 dim X , where, for ˙ 2 fC; g, the sub-bundle T ˙ X of TX is the .˙1/-eigen-bundle of K 2 End.TX/. Recall also that a D-holomorphic map between two almost-D-complex manifolds .X1 ; K1 / and .X2 ; K2 / is a smooth map f W X1 ! X2 such that d f ı K1 D K2 ı d f . Recall that an almost-D-complex structure is said to be integrable (and hence it is called a D-complex structure, or also a para-complex structure) if T C X; T C X T C X and ŒT X; T X T X . Equivalently, the integrability condition can be straightforwardly expressed as the vanishing of the Nijenhuis tensor of K, defined as NijK .; / WD Œ; C ŒK; K K ŒK; K Œ; K. Furthermore, for an almost-D-complex structure on an n-dimensional manifold X , the integrability is also equivalent to being naturally associated to a structure on X defined in terms of local homeomorphisms with open sets in Dn and D-holomorphic changes of coordinates, see, e.g., [CMMS04, Proposition 3], where Dn WD Rn C Rn , with 2 D 1, is the algebra of double numbers.
4
1 Preliminaries on (Almost-)Complex Manifolds
Finally, we recall that, given a 2n-dimensional manifold endowed with an almost-D-complex structure K, a D-Hermitian metric on X is a pseudo-Riemannian metric of signature .n; n/ such that g.K; K/ D g.; /. A D-Kähler structure on a manifold X is the datum of an integrable D-complex structure K and a D-Hermitian metric g such that its associated K-anti-invariant form ! WD g .K ; / is d-closed, equivalently, the datum of a K-compatible (that is, a K-anti-invariant) symplectic form on X , see, e.g., [AMT09, Sect. 5.1], [CMMS04, Theorem 1]. We refer, e.g., to [HL83, AMT09, CMMS04, CMMS05, CM09, CFAG96, KMW10, ABDMO05, AS05, Kra10, Ros12a, Ros12b, AR12, Ros13] and the references therein for more notions and results in D-complex geometry and for motivations for its study.
1.1.2 Complex Structures, and Dolbeault Cohomology If X is a complex manifold, then there is a natural almost-complex structure on X: ˚ locally, in a holomorphic coordinate chart U; z˛ DW x 2˛1 C i x 2˛ ˛2f1;:::;dimC X g , with U; fx ˛ g˛2f1;:::;2 dimC X g a (differential) coordinate chart, one defines, for every ˛ 2 f1; : : : ; dimC X g, J
@ @x 2˛1
@ WD ; @x 2˛ loc
J
@ @x 2˛
loc
WD
@ @x 2˛1
I
note that this local definition does not depend on the coordinate chart, by the Cauchy and Riemann equations. Conversely, we recall that an almost-complex structure J on a differentiable manifold X is called integrable if it is the natural almost-complex structure induced by a structure of complex manifold on X . The following theorem by A. Newlander and L. Nirenberg characterizes the integrable almost-complex structures on a manifold X in terms of the Nijenhuis tensor NijJ , defined as NijJ .; / WD Œ; C J ŒJ ; C J Œ; J ŒJ ; J : (See also [KN96, Appendix 8] for the integrability of real-analytic almost-complex structures.) Theorem 1.1 ([NN57, Theorem 1.1]). Let X be a differentiable manifold. An almost-complex structure J on X is integrable if and only if NijJ D 0. By a straightforward computation, the integrability of an almost-complex structure J turns out to be equivalent to the vanishing of the components A and AN of the exterior differential, equivalently, to ^; X; @; @ being a double complex of C 1 .X I C/-modules (see, e.g., [Wel08, Sect. 2.6], [Mor07, Proposition 8.2]).
1.1 Almost-Complex Geometry and Complex Geometry
5
Therefore, for a complex X , one can consider, for every p 2 N, manifold the differential complex ^p; X; @ and its cohomology, defining the Dolbeault cohomology, as the bi-graded C-vector space H@; .X / WD
ker @ im @
:
p;q
For every .p; q/ 2 N2 , denote by AX the (fine) sheaf of germs of .p; q/-forms p on X . For every p 2 N, denote by ˝X the sheaf of germs of holomorphic p-forms p;0 p;1 on X , that is, the kernel sheaf of the map @W AX ! AX . By the Dolbeault and Grothendieck Lemma, see, e.g., [Dem12, I.3.29], one has that p
p;
0 ! ˝X ! AX p
is a fine resolution of ˝X ; hence, one gets the following result. Theorem 1.2 (Dolbeault Theorem, [Dol53]). Let X be a complex manifold. For every p; q 2 N, p;q H@ .X / ' HL q .X I ˝ p / :
This gives a sheaf-theoretic interpretation of the Dolbeault cohomology. On the other hand, also an analytic interpretation can be provided. Suppose X is a compact complex manifold of complex dimension n, and fix g a Hermitian metric on X and vol the induced volume form on X (recall that every complex manifold is orientable, see, e.g., [GH94, pp. 17–18]); denote by ! WD g.J ; / 2 ^1;1 X \ ^2 X the associated .1; 1/-form to g. Recall that g induces a Hermitian inner product h; i on the space ^; X of global differential forms on X , and that the Hodge--operator associated to g is the C-linear map b^p;q X W ^p;q X ! ^nq;np X defined requiring that, for every ˛; ˇ 2 ^p;q X , ˛ ^ ˇN D h˛; ˇi vol : Define @
WD @ W ^; X ! ^;1 X I
the operator @ W ^; X ! ^;1 X is the adjoint1 of @W ^; X ! ^;C1 X with respect to h; i. Define 1
This fact can be generalized for a bi-differential bi-graded algebra of PD -type, [Kas12b, Sect. 2], see also [AK12, Sect. 2.1].
6
1 Preliminaries on (Almost-)Complex Manifolds
WD
h
@; @
i
WD @ @ C @ @W ^; X ! ^; X I
being a 2nd order self-adjoint elliptic differential operator (see, e.g., [Kod05, Theorem 3.16]), one gets the following result. Theorem 1.3 (Hodge Theorem, [Hod89]). Let X be a compact complex manifold endowed with a Hermitian metric. There is an orthogonal decomposition ?
?
^; X D ker ˚ @ ^;1 X ˚ @ ^;C1 X ; and hence an isomorphism H@; .X / ' ker : In particular, dimC H@; .X / < C1. Note that, for any .p; q/ 2 N2 , the Hodge--operator W ^p;q X ! ^nq;np X sends a -harmonic .p; q/-form (that is, 2 ^p;q X is such that D 0) to a -harmonic .n q; n p/-form , where WD Œ@; @ WD @@ C @ @ 2 End .^; X / is the conjugate operator to , and hence, by conjugating, one gets a -harmonic .n p; n q/-form . Hence, one gets the following result. Theorem 1.4 (Serre Duality, [Ser55, Théorème 4]). Let X be a compact complex manifold of complex dimension n, endowed with a Hermitian metric. For every p; q 2 N, the Hodge--operator induces an isomorphism p;q
'
np;nq
W H@ .X / ! H@
.X / :
Since a @-closed form is not necessarily d-closed, Dolbeault cohomology classes do not define, in general, de Rham cohomology classes, that is, in general, on a compact complex manifold, there is no natural map between the Dolbeault cohomology and the de Rham cohomology (as we will see, in the special case of compact Kähler manifolds, or more in general of compact complex manifolds satisfying the @@-Lemma, the de Rham cohomology actually can be decomposed by means of the Dolbeault cohomology groups, [Wei58, Théorème IV.3], [DGMS75, Lemma 5.15, Remark 5.16, 5.21]). Nevertheless, the Frölicher inequality provides a relation between the dimension of the Dolbeault cohomology and the dimension of the de Rham cohomology; it follows by considering the Hodge and Frölicher spectral sequence, which we recall here. The structure of double complex of ^; X; @; @ gives rise to two natural filtrations of ^ X ˝ C, namely (for .p; q/ 2 N2 and for k 2 N,) M M 0 p F ^k X ˝ C WD ^r;s X and 00 F q ^k X ˝ C WD ^r;s X I rCsDk rp
rCsDk sq
1.1 Almost-Complex Geometry and Complex Geometry
7
these filtrations induce two spectral sequences (see, e.g., [McC01, Sect. 2.4], [GH94, Sect. 3.5]), ˚
Er; ; dr D 0 Er; ; 0 dr r2N
and, respectively,
˚00
Er; ; 00 dr
r2N
;
called Hodge and Frölicher spectral sequences (or Hodge to de Rham spectral sequences): one has 0
E1; ' H@; .X / ) HdR .X I C/
00
and
E1; ' H@; .X / ) HdR .X I C/ :
An explicit description of f.Er ; dr /gr2N is given in [CFUG97]: for any p; q 2 N and r 2 N, its terms are p;q
Erp;q '
Xr p;q ; Yr
where, for r D 1, p;q
X1
WD
n
o ˛ 2 ^p;q X W @˛ D 0 ;
p;q
Y1
WD @ ^p;q1 X ;
and, for r 2, n Xrp;q WD ˛ p;q 2 ^p;q X W @˛ p;q D 0 and, for any i 2 f1; : : : ; r 1g ; there exists ˛ pCi;qi 2 ^pCi;qi X o such that @˛ pCi 1;qi C1 C @˛ pCi;qi D 0 ; n Yrp;q WD @ˇ p1;q C @ˇ p;q1 2 ^p;q X W for any i 2 f2; : : : ; r 1g ; there exists ˇ pi;qCi 1 2 ^pi;qCi 1 X o such that @ˇ pi;qCi 1 C @ˇ pi C1;qCi 2 D 0 and @ˇ prC1;qCr2 D 0 ; see [CFUG97, Theorem 1], and, for any r 1, the map dr W Er; ! ErCr;rC1 is given by dr W fŒ˛ p;q 2 Erp;q g.p;q/2N2 7!
˚
@˛ pCr1;qrC1 2 ErpCr;qrC1 .p;q/2N2 ;
see [CFUG97, Theorem 3]. .X I C/, one gets the following As a consequence of 0 E1; ' H@; .X / ) HdR inequality by A. Frölicher.
8
1 Preliminaries on (Almost-)Complex Manifolds
Theorem 1.5 (Frölicher Inequality, [Frö55, Theorem 2]). Let X be a compact complex manifold. Then, for every k 2 N, k dimC HdR .X I C/
X
p;q
dimC H@ .X / :
pCqDk
As a matter of notation, for k 2 N and .p; q/ 2 N2 , we will denote by p;q p;q k bk WD dimR HdR .X I R/, respectively h@ WD dimC H@ .X /, the kth Betti number, respectively the .p; q/th Hodge number of X . Remark 1.2. Other than the Dolbeault cohomology, other cohomologies can be defined for a complex manifold X ; more precisely, since, for every .p; q/ 2 N2 , @@
@C@
^p1;q1 X ! ^p;q X ! ^pC1;q X ˚ ^p;qC1 X
and
.@; @/ @@ ^p1;q X ˚ ^p;q1 X ! ^p;q X ! ^pC1;qC1 X ; are complexes, one can define the Bott-Chern cohomology HBC .X / and the Aeppli ; cohomology HA .X / of X as ; HBC .X / WD
ker @ \ ker @ im @@
and
HA; .X / WD
ker @@ im @ C im @
I
we refer to Sect. 2.1 for further details. In the next chapter, we will provide a Frölicher-type inequality also for the Bott-Chern cohomology, Theorem 2.13, showing that it allows to characterize the compact complex manifolds satisfying the @@-Lemma just in terms of the dimensions of the Bott-Chern cohomology and of the de Rham cohomology, Theorem 2.14. (We refer also to [AT13a], where Dolbeault, Bott-Chern, and Aeppli cohomologies are considered in a more general context.) Remark 1.3. In Complex Analysis, properties concerning the existence of exhaustion functions with convexity properties may have consequences on the vanishing of the cohomology. In fact, the Hörmander theorem, [Hör65, Theorems 2.2.4 and 2.2.5], [Hör90, Theorem 4.2.2, Corollary 4.2.6], states that the Dolbeault p;q cohomology groups H@ .D/ of a strictly pseudo-convex domain D in Cn (that is, a domain admitting a smooth proper strictly pluri-sub-harmonic exhaustion function) vanish for q 1, for any p 2 N. More in general, A. Andreotti and H. Grauert proved in [AG62, Proposition 27] that the Dolbeault cohomology groups H@r;s .D/ of a q-complete domain in Cn (that is, a domain in Cn admitting a smooth proper exhaustion function whose Levi form has at least n q C 1 positive eigen-values, [AG62, Rot55]) vanish for s q, for any r 2 N, see also [AV65a, AV65b, Theorem 5], see also [Ves67]. (Furthermore, given a domain D of a Stein manifold with
1.2 Symplectic Geometry
9
boundary of class C 2 , if H@r;s .D/ D f0g for s q and for any r 2 N, then D is q-complete, [EVS80, Theorem 3.8].) See [AC12, AC13] for other results connecting convexity properties (see [HL12, HL11]) and vanishing of the cohomology, see also [Sha86, Theorem 1], [Wu87, Theorem 1], see also [HL11, Proposition 5.7].
1.2 Symplectic Geometry In this section, we recall some definitions and results concerning symplectic manifolds, that is, differentiable manifolds endowed with a non-degenerate d-closed 2-form. An interesting class of examples of symplectic manifolds is provided by the Kähler manifolds. Moreover, given a differentiable manifold X , its cotangent bundle T X is endowed with a natural symplectic structure (see, e.g., [CdS01, Sect. 2]): in fact, symplectic geometry has applications and motivations in the study of Hamiltonian Mechanics, see, e.g., [CdS01, Part VII].
1.2.1 Symplectic Structures Let X be a compact 2n-dimensional manifold. We start by recalling the notion of symplectic structure. Definition 1.2 ([Wey97, Sect. VI]). Let X be a differentiable manifold. A symplectic form on X is a non-degenerate d-closed 2-form ! 2 ^2 X . The main difference between symplectic geometry and Riemannian geometry is provided by G. Darboux’s theorem. Theorem 1.6 (Darboux Theorem, [Dar82]). Let X be a 2n-dimensional manifold endowed a symplectic with form !. Then, for every x 2 X , there exists a coordinate ˚ chart U; x j j 2f1;:::;2ng , with x 2 U , such that loc
! D
n X
d x 2j 1 ^ d x 2j :
j D1
As for (almost-)complex manifolds, on a symplectic manifold X endowed with a symplectic form !, one has a decomposition of differential forms in symplectictype components, the so-called Lefschetz decomposition; it is a consequence of an sl.2I R/-representation on ^2 X by means of operators related to the symplectic structure.
10
1 Preliminaries on (Almost-)Complex Manifolds
More precisely, define the operators L; ; H 2 End .^ X / as LW ^ X ! ^C2 X ;
˛ 7! ! ^ ˛ ;
W ^ X ! ^2 X ;
˛ 7! ˘ ˛ ; X ˛ 7! .n k/ ^k X ˛
H W ^ X ! ^ X ;
k
(where W ^ X ! ^2 X denotes the interior product with 2 ^2 .TX/, and, for k 2 N, the map ^k X W ^ X ! ^k X denotes the natural projection onto ^k X ). Note that, using the symplectic-?-operator ?! , one can write, [Yan96, Lemma 1.5], D ?! L ?! : The following result holds.2 Theorem 1.7 ([Yan96, Corollary 1.6]). Let X be a manifold endowed with a symplectic structure. Then ŒL; H D 2 L ;
Œ; H D 2 ;
ŒL; D H ;
and hence sl.2I R/ ' hL; ; H i ! End ^ X gives an sl.2I R/-representation on ^ X . (See also [Huy05, Proposition 1.2.2] for a proof.) The above sl.2I R/-representation, having finite H -spectrum,3 induces a decomposition of the space of the differential forms. Theorem 1.8 ([Yan96, Corollary 2.6]). Let X be a manifold endowed with a symplectic structure. Then one has the Lefschetz decomposition on differential forms, ^ X D
M
Lr P^2r X ;
r2N
See, e.g., [Hum78, Sect. 7] for general results concerning sl.2I K/-representations. Recall that an sl.2I R/-representation on a (possibly non-finite dimensional) R-vector space V is called of finite H -spectrum if V can be decomposed into the direct sum of eigen-spaces of H and H has only finitely-many distinct eigen-values, [Yan96, Definition 2.2]. 2 3
1.2 Symplectic Geometry
11
where P^ X WD ker is the space of primitive forms. Remark 1.4. Note that, for every k 2 N, P^k X D ker LnkC1 b^k X ; see, e.g., [Huy05, Proposition 1.2.30(v)]. In general, see, e.g., [TY12b, p. 422], the Lefschetz decomposition of A.k/ 2 ^ X reads as k
X
A.k/ D
rmaxfkn; 0g
1 r .k2r/ L B rŠ
where, for r max fk n; 0g, B
.k2r/
X
WD
ar;`;.n;k/
`2N
1 ` rC` L `Š
! A.k/ 2 P^k2r X
and, for r max fk n; 0g and ` 2 N, ar;`;.n;k/ WD .1/` .n k C 2r C 1/2
r Y
1 n k C 2r C 1 i i D0
` Y
1 2Q: n k C 2r C1Cj j D0
We recall that LbLn2
kD1
^nk2 X
W
n2 M
^nk2 X ! ^nk X
kD1
is injective, [Yan96, Corollary 2.8], and that, for every k 2 N, Lk W ^nk X ! ^nCk X is an isomorphism, [Yan96, Corollary 2.7].
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1 Preliminaries on (Almost-)Complex Manifolds
1.2.1.1 Primitive Currents Let X be a 2n-dimensional compact manifold endowed with a symplectic structure !. Denote by D X WD D2n X the space of currents, and consider the de Rham homology HdR .X I R/ WD H .D X; d/. (See Sect. 1.6 for definitions and results concerning currents and de Rham homology.) Following [Lin13, Definition 4.1], set, by duality, LW D X ! D2 X ;
S 7! S .L / ;
W D X ! DC2 X ;
S 7! S . / ;
H W D X ! D X ;
S 7! S .H / I
note that ŒL; H D 2 L ;
Œ; H D 2 ;
ŒL; D H :
A current S 2 Dk X is said primitive if S D 0, equivalently, if LnkC1 S D 0, see, e.g., [Lin13, Proposition 4.3]; denote by PD X WD PD 2n X the space of primitive currents on X . In [Lin13], Y. Lin proved the following result. Theorem 1.9 ([Lin13, Lemma 4.2, Proposition 4.3]). Let X be a compact manifold endowed with a symplectic structure !. Then hL; ; H i gives an sl.2I R/module structure on D X . In particular, one has the Lefschetz decomposition on the space of currents, M M D X D Lr PD2r X D Lr PD 2nC2r X : r2N
r2N
Finally, if j W Y ,! X is a compact oriented submanifold of X of codimension k (possibly with non-empty boundary), then the dual current ŒY 2 Dk X associated with Y is defined, by setting, for every ' 2 ^k X , Z ŒY .'/ WD j .'/ : Y
If Y is a closed oriented submanifold, then the dual current ŒY is d-closed. According to [TY12a, Lemma 4.1], the dual current ŒY is primitive if and only if Y is co-isotropic.
1.2.2 Cohomological Aspects of Symplectic Geometry Cohomological properties of symplectic manifolds have been studied starting from the works by J.-L. Koszul, [Kos85], and by J.-L. Brylinski, [Bry88]. Drawing a
1.2 Symplectic Geometry
13
parallel between the symplectic and the Riemannian cases, J.-L. Brylinski proposed in [Bry88] a Hodge theory for compact symplectic manifolds .X; !/, introducing a symplectic Hodge-?-operator ?! and the notion of !-symplectically-harmonic form (i.e., a form being both d-closed and d -closed, where the symplectic co-differential is defined as d b^k X WD .1/kC1 ?! d ?! for every k 2 N): in this context, O. Mathieu in [Mat95], and D. Yan in [Yan96], proved that any de Rham cohomology class admits an !-symplectically-harmonic representative if and only if the Hard Lefschetz Condition is satisfied. In [TY12a, TY12b], see also [TY11], L.-S. Tseng and S.-T. Yau introduced new cohomologies for symplectic manifolds .X; !/: among them, in particular, they defined and studied HdC d .X I R/
WD
ker d C d im d d
;
developing a Hodge theory for this cohomology; furthermore, they studied the dual currents of Lagrangian and co-isotropic submanifolds, and they defined a homology theory on co-isotropic chains, which turns out to be naturally dual to a primitive cohomology. In the context of generalized geometry, [Gua04a, Gua11, Cav05, Cav07], the cohomology HdC d .X I R/ can be interpreted as the symplectic counterpart of the Bott-Chern cohomology of a complex manifold, see [TY11]. Inspired also by their works, Y. Lin developed in [Lin13] a geometric measure theoretic approach to symplectic Hodge theory, proving in particular that, on any compact symplectic manifold, every primitive cohomology class of positive degree admits a symplectically-harmonic representative not supported on the entire manifold, [Lin13, Theorems 1.1 and 5.3]. In this section, we recall some notions and results concerning Hodge theory for compact symplectic manifolds; we refer to [Bry88, Mat95, Yan96, Cav05, TY12a, TY12b, Lin13] for further details.
1.2.2.1 Symplectic Hodge Theory Let X be a compact 2n-dimensional manifold endowed with a symplectic form !. By exploiting the parallelism with Riemannian geometry, one can try to develop a Hodge theory also for compact symplectic manifolds, [Bry88]. The first tool that can be introduced is an analogue of the Hodge--operator; hence one can define a symplectic counterpart of the co-differential operator, and define the notion of symplectically-harmonic form, investigating the existence of symplecticallyharmonic representatives in any de Rham cohomology class. n Note that every symplectic manifold is orientable, !nŠ giving a canonical orientation. Denote by I the natural isomorphism of vector bundles induced by !, namely, for every x 2 X , Tx X 3 v 7! I.v/./ WD !.v; / 2 Hom .Tx X I R/ :
14
1 Preliminaries on (Almost-)Complex Manifolds
Then, for every k 2 N, the form ! gives rise to a bi-C 1 .X I R/-linear form on ^k X k denoted by ! 1 , which is skew-symmetric, respectively symmetric, according that k is odd, respectively even, and defined on the simple elements ˛ 1 ^ : : : ^ ˛ k 2 ^k X and ˇ 1 ^ : : : ^ ˇ k 2 ^k X as
! 1
k 1 ˛ ^ : : : ^ ˛ k ; ˇ 1 ^ : : : ^ ˇ k WD det ! 1 ˛ ` ; ˇ m `;m2f1;:::;kg ;
where ! 1 ˛ ` ; ˇ m WD ! I 1 ˛ ` ; I 1 .ˇ m / ˚ for every `; m 2 f1; : : : ; kg. In a Darboux coordinate chart U; x j j 2f1;:::;2ng , the canonical Poisson bi-vector ˘ WD ! 1 2 ^2 TX associated to ! is written as loc P ! 1 D nj D1 @x 2j@ 1 ^ @x@2j . The symplectic-?-operator ?! W ^ X ! ^2n X ; introduced by J.-L. Brylinski, [Bry88, Sect. 2], is defined requiring that, for every k 2 N, and for every ˛; ˇ 2 ^k X , k !n ˛ ^ ?! ˇ D ! 1 .˛; ˇ/ : nŠ By continuing in the parallelism between Riemannian geometry and symplectic geometry, one can introduce the d operator with respect to a symplectic structure ! as d b^k X WD .1/kC1 ?! d ?! for any k 2 N, and interpret it as the symplectic counterpart of the Riemannian d operator with respect to a Riemannian metric. In light of this, J.-L. Brylinski proposed in [Bry88] a Hodge theory for compact symplectic manifolds, conjecturing that, on a compact manifold endowed with a symplectic structure !, every de Rham cohomology class admits a (possibly non-unique) !-symplectically-harmonic representative, namely, a d-closed d -closed representative, [Bry88, Conjecture 2.2.7]. (Note that d d C d d D 0, [Bry88, Theorem 1.3.1], [Kos85, p. 265], provides a strong difference in the parallelism between symplectic geometry and Riemannian geometry; in particular, it follows that a !-symplectically-harmonic representative, whenever it exists, is not unique.) Remark 1.5. For an almost-Kähler structure .J; !; g/ on a compact manifold X (that is, ! 2 ^2 X is a symplectic form on X , and J 2 End .TX/ is an almostcomplex structure on X , and g is a J -Hermitian metric on X such that ! is
1.2 Symplectic Geometry
15
the associated .1; 1/-form to g), the symplectic-?-operator ?! and the Hodge-operator g are related by ?! D J g ; and hence d D .dc /g kC1 where dc WD J 1 d J and .dc /g b^k X WD .1/ g d g for every k 2 N c (note that, when J is integrable, then d D i @ @ ). Moreover, on a compact manifold X endowed with a Kähler structure .J; !; g/, by the Hodge decomposition theorem, [Wei58, Théorème IV.3], the pure-type components with respect to J of the harmonic representatives of the de Rham cohomology classes are themselves harmonic. Hence, it follows that Brylinski’s conjecture holds true for compact Kähler manifolds, [Bry88, Corollary 2.4.3].
Remark 1.6. In the framework of generalized complex geometry, see Sect. 1.3, the d operator associated to a symplectic structure be interpreted as the should symplectic counterpart of the operator dc WD i @ @ associated to a complex structure, [Cav05]. Consider now, for k 2 N, the map Lk W ^nk X ! ^nCk X . Since ŒL; d D 0, it nCk nk induces a map Lk W HdR .X I R/ ! HdR .X I R/ in cohomology. One says that X satisfies the Hard Lefschetz Condition, shortly HLC, if for every k 2 N ;
'
nCk nk .X I R/ ! HdR .X I R/ : Lk W HdR
(HLC)
O. Mathieu in [Mat95], and D. Yan in [Yan96], provided counterexamples to Brylinski’s conjecture, characterizing the compact symplectic manifolds satisfying Brylinski’s conjecture in terms of the validity of the Hard Lefschetz Condition. Furthermore, S.A. Merkulov in [Mer98], see also [Cav05], and V. Guillemin in [Gui01], proved that the Hard Lefschetz Condition on compact symplectic manifolds is equivalent to satisfying the d d -Lemma, namely, to every d-exact d -closed form being d d -exact. See Theorem 1.15 for a summary of several equivalent statements to Hard Lefschetz Condition. Note that, by the Lefschetz decomposition theorem, [Wei58, Théorème IV.5] (see Sect. 1.4), compact Kähler manifolds satisfy the Hard Lefschetz Condition.
1.2.2.2 Symplectic Cohomologies Let X be a compact 2n-dimensional manifold endowed with a symplectic structure !.
16
1 Preliminaries on (Almost-)Complex Manifolds
In [TY12a], L.-S. Tseng and S.-T. Yau introduced also the d C d -cohomology, [TY12a, Sect. 3.2], ker d C d Hd C d .X I R/ WD ; im d d and the d d -cohomology, [TY12a, Sect. 3.3], Hdd .X I R/ WD
ker d d I im d C im d
such cohomologies are, in a sense, the symplectic counterpart of the Bott-Chern and Aeppli cohomologies of complex manifolds, see [TY12a, Sect. 5] and [TY11] for further discussions. Furthermore, they provided a Hodge theory for such cohomologies, proving the following result. Theorem 1.10 ([TY12a, Theorem 3.5, Corollary 3.6]). Let X be a compact manifold endowed with a symplectic structure !. Let .J; !; g/ be an almost-Kähler structure on X . For a fixed > 0, the 4th order self-adjoint differential operator Dd C d WD d d d d C d d d d C d d d d C d d d d C d d C d d : is elliptic, with ker Dd C d D ker d \ ker d \ ker d d . Furthermore, there exist an orthogonal decomposition ^ X D ker Dd C d ˚ d d ^ X ˚ d ^C1 X C d ^1 X and an isomorphism HdC d .X I R/ ' ker Dd C d : In particular, dimR HdC d .X I R/ < C1. An analogous statement holds for the d d -cohomology. Theorem 1.11 ([TY12a, Theorem 3.16, Corollary 3.17]). Let X be a compact manifold endowed with a symplectic structure !. Let .J; !; g/ be an almost-Kähler structure on X . For a fixed > 0, the 4th order self-adjoint differential operator Dd d WD d d d d C d d dd C d d d d d d C d d C d d : C d d is elliptic, with ker Dd d D ker d d \ ker d \ ker d .
1.2 Symplectic Geometry
17
Furthermore, there exist an orthogonal decomposition ^ X D ker Dd d ˚ d ^1 X C d ^C1 X ˚ d d ^ X and an isomorphism Hdd .X I R/ ' ker Dd d : In particular, dimR Hdd .X I R/ < C1.
As for the Bott-Chern and the Aeppli cohomologies, the d C d -cohomology and the d d -cohomology groups turn out to be isomorphic by means of the Hodge--operator associated to any Riemannian metric being compatible with !. Theorem 1.12 ([TY12a, Lemma 3.23, Proposition 3.24, Corollary 3.25]). Let X be a 2n-dimensional compact manifold endowed with a symplectic structure !. Let .J; !; g/ be an almost-Kähler structure on X . The operators Dd C d and Dd d satisfy g Dd C d D Dd d g ; and hence g induces an isomorphism '
g W HdC d .X I R/ ! Hd2n .X I R/ : d Moreover, the cohomology HdC d .X I R/ is invariant under symplectomorphisms and Hamiltonian isotopies, [TY12a, Proposition 2.8]. One has the following commutation relations between the differential operators d, d , and d d , and the elements L, , and H of the sl.2I R/-triple, see, e.g., [TY12a, Lemma 2.3]: d ; L D d ; Œd; D d ; d ; D 0 ; Œd; H D d ; d ; H D d ; Œd; L D 0 ;
dd ; L D 0 ; dd ; D 0 ; dd ; H D 0 :
Hence, by defining the primitive d C d -cohomology as PHdC d .X I R/ WD
ker d \ ker d \P^ X ker d \P^ X D im d d \P^ X im d d bP^ X
(where the second equality follows from [TY12a, Lemma 3.9]), one gets the following result.
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1 Preliminaries on (Almost-)Complex Manifolds
Theorem 1.13 ([TY12a, Theorem 3.11]). Let X be a 2n-dimensional compact manifold endowed with a symplectic structure !. Then there exist a decomposition HdC d .X I R/ D
M
Lr PHd2r .X I R/ C d
r2N
and, for every k 2 N, an isomorphism '
Lk W Hdnk .X I R/ ! HdnCk .X I R/ ; C d C d Analogously, by defining the primitive d d -cohomology as
PHdd .X I R/ WD D
ker d d \P^ X im d C im d \ P^ X
ker d d \P^ X im d CLH 1 d P^1 X C im d P^C1 X
(where the second equality follows from [TY12a, Lemma 3.20]), one gets the following result. Theorem 1.14 ([TY12a, Theorem 3.21]). Let X be a 2n-dimensional compact manifold endowed with a symplectic structure !. Then there exist a decomposition Hdd .X I R/ D
M
Lr PHd2r .X I R/ d
r2N
and, for every k 2 N, an isomorphism '
Lk W Hdnk .X I R/ ! HdnCk .X I R/ ; d d The identity map induces the following natural maps in cohomology: HdC d .X I R/ MMM MMM qqq q q MMM qq q M& q xq Hd .X I R/ HdR .X I R/ MMM q MMM qqq q MMM q qq MM& xqqq Hdd .X I R/
1.2 Symplectic Geometry
19
Recall that a symplectic manifold is said to satisfy the d d -Lemma if every d-exact d -closed form is d d -exact, [DGMS75], namely, if HdC d .X I R/ ! .X I R/ is injective. HdR Remark 1.7. (In view of [DGMS75, Lemma 5.15] and [AT13a, Lemma 1.4]) note that ker d \ im d D im d d
ker d \ im d D im d d :
if and only if
Indeed, since ?2! D id^ X , [Bry88, Lemma 2.1.2], and d d C d d D 0, [Bry88, Theorem 1.3.1], one has ?! ker d D ker d ;
?! im d D im d ;
?! im d d D im d d :
Recalling that a 2n-dimensional compact manifold X endowed with a symplectic form ! is said to satisfy the Hard Lefschetz Condition if and only if for every k 2 N ;
'
nCk nk .X I R/ ! HdR .X I R/ ; Lk W HdR
(HLC)
we summarize in the following result the connection between the d d -Lemma, the Hard Lefschetz Condition, and the existence of !-symplectically harmonic representatives in any de Rham cohomology class. Theorem 1.15 ([Mat95, Corollary 2], [Yan96, Theorem 0.1], [Mer98, Proposition 1.4], [Gui01], [DGMS75, Lemma 5.15, Remarks 5.16, 5.21], [TY12a, Proposition 3.13], [Cav05, Theorem 5.4], [AT13a, Theorem 4.4], [AT12b, Remark 2.3]). Let X be a compact manifold endowed with a symplectic structure !. The following conditions are equivalent: (i) every de Rham cohomology class admits a representative being both d-closed and d -closed (i.e., Brylinski’s conjecture [Bry88, Conjecture 2.2.7] holds on X ); (ii) the Hard Lefschetz Condition holds on X ; (iii) the natural map HdC d .X I R/ ! HdR .X I R/ induced by the identity is injective; (iv) the natural map HdC d .X I R/ ! HdR .X I R/ induced by the identity is actually an isomorphism; (v) X satisfies the d d -Lemma; (vi) for every k 2 Z, it holds dimR HdkC d .X I R/ C dimR Hdkd .X I R/ D 2 dimR k HdR .X I R/; L 2r r (vii) the Lefschetz decomposition ^ X D X on forms induces r2N L P^ L .0;2r/ r a decomposition in cohomology, namely, HdR .X I R/ D r2N L H! ˚ .r;s/ 2rCs r .s/ .s/ 2 HdR .X I R/ W ˇ 2 P^s X . .X I R/, where H! .X I R/ WD L ˇ Note that, by the Lefschetz decomposition theorem, the compact Kähler manifolds satisfy the Hard Lefschetz Condition; in other terms, note that, given a Kähler
20
1 Preliminaries on (Almost-)Complex Manifolds
structure .J; !; g/ on a compact manifold X , one has ?! D J g , [Bry88, Theorem 2.4.1], and hence every de Rham cohomology class admits an !-symplecticallyharmonic representative.
1.3 Generalized Geometry Generalized complex geometry, introduced by N.J. Hitchin in [Hit03] and developed, among others, by M. Gualtieri, [Gua04a, Gua11], and G.R. Cavalcanti, [Cav05], see also [Hit10,Cav07], allows to frame symplectic structures and complex structures in the same context. (In a sense, this add more significance to the term “symplectic”, which was invented by H. Weyl, [Wey97, Sect. VI], substituting the Greek root in the term “complex” with the corresponding Latin root.) We recall here the notion of generalized complex structure, referring to [Cav07, Gua04a, Gua11, Cav05] for more details.
1.3.1 Generalized Complex Structures Let X be a compact differentiable manifold of dimension 2n. ' Note that a complex structure J on X can be seen as an isomorphism J W TX ! TX satisfying the algebraic condition J 2 D idTX and the integrability condition NijJ D 0. Analogously, a symplectic structure ! on X can be seen as an '
isomorphism !././ WD !.; /W TX ! T X satisfying the algebraic condition !././ D !././ and the integrability condition d ! D 0. Therefore, consider the bundle TX ˚ T X . It can be endowed with the natural symmetric pairing hji W TX ˚ T X TX ˚ T X ! R;
hX C jY C i WD
1 . .Y / C .X // : 2
Fixed a d-closed 3-form H on X (possibly, the zero form H D 0), define a suitable bracket for an integrability condition on TX ˚ T X as follows. On the space C 1 .X I TX ˚ T X / of smooth sections of TX ˚ T X over X , define the H -twisted Courant bracket as Œ; H W C 1 X I TX ˚ T X C 1 X I TX ˚ T X ! C 1 X I TX ˚ T X ; ŒX C ; Y C H WD ŒX; Y C LX LY
1 d . X Y / C Y X H 2
(where X 2 End1 .^ X / denotes the interior product with X 2 C 1 .X I TX/ and LX WD ΠX ; d 2 End0 .^ X / denotes the Lie derivative along X 2 C 1 .X I TX/);
1.3 Generalized Geometry
21
the H -twisted Courant bracket can be seen also as a derived bracket induced by the H -twisted differential dH WD d CH ^ , see [Gua04a, Sect. 3.2], [Gua11, Sect. 2]. Consider also the Clifford action4 of TX ˚T X on the space of differential forms with respect to the natural pairing h j i, Cliff TX ˚ T X; h j i ^ X ! ^1 X ˚^C1 X ;
.X C /' D X ' C ^' ;
and extension Cliff ..TX ˚ T X / ˝R C/ .^ X ˝R C/ ! 1its bi-C-linear C1 ^ X ˝R C ˚ ^ X ˝R C . We give now several equivalent definitions of generalized complex structure. Definition 1.3 ([Gua04a, Definitions 4.14, 4.18], [Gua11, Definition 3.1]). Let X be a differentiable manifold and H be a d-closed 3-form on X . An H -twisted generalized complex structure is an endomorphism J 2 End .TX ˚ T X / such that • J 2 D idTX˚T X , and • J is orthogonal with respect to h j i, and • the Nijenhuis tensor NijJ ;H WD ŒJ ; J H C J ŒJ ; H C J Œ; J H C J Œ; H 2 TX ˚ T X ˝R TX ˚ T X ˝R TX ˚ T X of J with respect to the H -twisted Courant bracket vanishes identically. Equivalently, by setting L WD LJ the i-eigen-bundle of the C-linear extension of J to .TX ˚ T X / ˝R C, one gets the following definition. Proposition 1.1 ([Gua04a, Proposition 4.3]). Let X be a differentiable manifold and H be a d-closed 3-form on X . A generalized complex structure on X is identified by a sub-bundle L of .TX ˚ T X / ˝R C such that • L is maximal isotropic with respect to h j i, and • L is involutive with respect to the H -twisted Courant bracket, and • L \ LN D f0g. Equivalently, by choosing a complex form whose Clifford annihilator ˚ L WD v 2 TX ˚ T X ˝R C W v D 0 is the i-eigen-bundle LJ of the C-linear extension of J to .TX ˚ T X / ˝R C), one gets the following definition. We recall that the Clifford algebra associated to TX ˚ T X and h j i is 0 1, k MO .TX ˚ T X/A fv ˝R v hv j vi W v 2 TX ˚ T Xg : Cliff .TX ˚ T X; h j i/ D @
4
k2Z j D1
22
1 Preliminaries on (Almost-)Complex Manifolds
Proposition 1.2 ([Gua04a, Theorem 4.8]). Let X be a differentiable manifold and H be a d-closed 3-form on X . A generalized complex structure on X is identified by a sub-bundle U WD UJ of complex rank 1 of ^ X ˝R C being locally generated by a form D exp .B C i !/ ^˚˝, where B 2 ^2 X , and ! 2 ^2 X , and ˝ D
1 ^ ^ k 2 ^k X ˝R C with 1 ; : : : ; k a set of linearly independent complex 1-forms, such that • ˝ ^ ˝N ^ ! nk ¤ 0, and • there exists v 2 .TX ˚ T X /˝R C such that dH D v , where dH WD d CH ^. According to [Gua04a, Sect. 4.1], [Gua11, Definition 3.7], the bundle UJ as in Proposition 1.2 is called the canonical bundle of the generalized complex structure J . By definition, the type of a generalized complex structure J on X , [Gua04a, Sect. 4.3], [Gua11, Definition 3.5], is the upper-semi-continuous function type .J / WD
1 dimR T X \ J T X 2
on X , equivalently, [Gua11, Definition 1.1], the degree of the form ˝. A point x of a generalized complex manifold is called a regular point if the type of the generalized complex structure is locally constant at x. (See Remark 1.10 for the notion of H -twisted generalized Kähler structure.) A generalized complex structure J on X induces a Z-graduation on the space of complex differential forms on X , [Gua04a, Sect. 4.4], [Gua11, Proposition 3.8]. Namely, define, for k 2 Z, U k WD UJk WD ^nk LN J UJ ^ X ˝R C ; where LJ is the i-eigenspace of the C-linear extension of J to .TX ˚ T X / ˝R C and UJn WD UJ is the canonical bundle of J . For a h j i-orthogonal endomorphism J 2 End .TX ˚ T X / satisfying J 2 D idTX˚T X , the Z-graduation UJ still makes sense, and the condition NijJ ;H D 0 turns out to be equivalent, [Gua04a, Theorem 4.3], [Gua11, Theorem 3.14], to dH W UJ ! UJC1 ˚ UJ1 : Therefore, on a compact differentiable manifold endowed with a generalized complex structure J , one has, [Gua04a, Sect. 4.4], [Gua11, Sect. 3], dH D @J ;H C @J ;H
where @J ;H W UJ ! UJC1
and @J ;H W UJ ! UJ1 :
Define also, [Gua04a, p. 52], [Gua11, Remark at page 97], C1 ˚ UJ1 : dJ H WD i @J ;H @J ;H W UJ ! UJ
1.3 Generalized Geometry
23
1.3.2 Cohomological Aspects of Generalized Complex Geometry Let X be a compact complex manifold endowed with an H -twisted generalized complex structure. L k By considering the decomposition ^ X ˝R C D k2Z UJ and the operators C1 C1 1 dH W UJ ! UJ ˚ UJ , and @J ;H W UJ ! UJ and @J ;H W UJ ! UJ1 , one can study the following cohomologies: GHdRH .X / WD
ker dH ; im dH
and GH@
J ;H
.X / WD
ker @J ;H im @J ;H
and
GH@J ;H .X / WD
@J ;H ; @J ;H
and GHBC .X / WD J ;H
GHAJ ;H .X / WD
ker @J ;H \ ker @J ;H
and
im @J ;H @J ;H ker @J ;H @J ;H im @J ;H C im @J ;H
:
L k Note that, for H D 0, one has GHdR0 .X / D k2Z HdR .X I C/. By [Gua04a, Proposition 5.1], [Gua11, Proposition 3.15], it follows that dimC GH@J ;H .X / < C1 and dimC GH@ .X / < C1. J ;H
one says that X satisfies the @J ;H @J ;H -Lemma if By abuse of notation, UJ ; @J ;H ; @J ;H satisfies the @J ;H @J ;H -Lemma, and one says that X satisfies J J the dH dJ H -Lemma if UJ ; dH ; dH satisfies the dH dH -Lemma. Actually, it turns J out that X satisfies the dH dH -Lemma if and only if X satisfies the @J ;H @J ;H Lemma, [Cav06, Remark at page 129]: indeed, note that ker @J ;H @J ;H D J ker dH dJ H , and ker @J ;H \ ker @J ;H D ker dH \ ker dH , and im @J ;H C im @J ;H D J im dH C im dH . Moreover, the following result by G.R. Cavalcanti holds. Theorem 1.16 ([Cav05, Theorem 4.2], [Cav06, Theorem 4.1, Corollary 2]). A manifold X endowed with an H generalized complex structure J satisfies -twisted J the dH dJ ,! U is a quasi-isomorphism -Lemma if and only if ker d ; d ; d H H H H J of differential Z-graded C-vector spaces. In this case, it follows that the splitting L ^ X ˝R C D k2Z UJk gives rise to a decomposition in cohomology. An application of [DGMS75, Propositions 5.17, 5.21] gives the following result.
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1 Preliminaries on (Almost-)Complex Manifolds
Theorem 1.17 ([Cav05, Theorem 4.4], [Cav06, Theorem 5.1]). A manifold X endowed with an H -twisted generalized complex structure J satisfies the dH dJ Hlemma if and only if the canonical spectral sequence5 degenerates at the first level and the decomposition of complex forms into sub-bundles UJk , varying k 2 Z, induces a decomposition in cohomology. (See Remark 1.10 for dH dJ H -Lemma on H -twisted generalized Kähler manifolds.)
1.3.3 Complex Structures and Symplectic Structures in Generalized Complex Geometry In view of the following generalized Darboux theorem by M. Gualtieri, symplectic structures and complex structures provide the basic examples of generalized complex structures. Theorem 1.18 ([Gua04a, Theorem 4.35], [Gua11, Theorem 3.6]). For any regular point of a 2n-dimensional generalized complex manifold with type equal to k, there is an open neighbourhood endowed with a set of local coordinates such that the generalized complex structure is a B-field transform of the standard generalized complex structure of Ck R2n2k . Hence, in the following examples, we recall the standard generalized complex structure of constant type n (that is, locally equivalent to the standard complex structure of Cn ), the generalized complex structure of constant type 0 (that is, locally equivalent to the standard symplectic structure of R2n ), and the B-field transform of a generalized complex structure, on a 2n-dimensional manifold. See also [Gua04a, Example 4.12]. Example 1.1 (Generalized Complex Structures of Type n, [Gua04a, Examples 4.11 and 4.25]). Let X be a compact 2n-dimensional manifold endowed with a complex structure J 2 End.TX/. Consider the (0-twisted) generalized complex structure JJ WD
J 0 0 J
!
2 End TX ˚ T X ;
where J 2 End.T X / denotes the dual endomorphism of J 2 End.TX/. We recall that, given a manifold X endowed with a generalized complex structure J , the canonical spectral sequence is the spectral sequence being naturally associated to the double complex obtained from UJ ; @J ;H ; @J ;H : more precisely, to the double complex UJ1J 2 ˝C Cˇ 2 ; @JJ ˝R id; @JJ ˝R ˇ , where fˇ m W m 2 Zg is an infinite cyclic multiplicative group generated by some ˇ, [Cav05, Sect. 4.2], see [Bry88, Sect. 1.3], [Goo85, Sect. II.2], [Con85, Sect. II]; (compare also [AT13a, Sect. 1]). 5
1.3 Generalized Geometry
25
The i-eigenspace of the C-linear extension of JJ to .TX ˚ T X / ˝C C is LJJ D TJ0;1 X ˚ TJ1;0 X : The canonical bundle is UJnJ D ^n;0 J X : Hence, one gets that, [Gua04a, Example 4.25], UJJ D
M
p;q
^J X ;
and @JJ D @J
@JJ D @J I
and
pqD
note that dJJ is the operator dcJ WD i.@ @/, [Gua04a, Remark 4.26]. Take an infinite cyclic multiplicative group fˇ m W m 2 Zg generated by some ˇ. The Hodge and Frölicher spectral sequence associated to the canonical double complex UJ1J2 ˝C Cˇ 2 ; @JJ ˝R id; @JJ ˝R ˇ degenerates at the first level if and only if the Hodge and Frölicher spectral sequence associated to the double complex ^J; X; @J ; @J does, [Cav05, Remark at page 76]. X satisfies the d dJJ Lemma if and the d dcJ -Lemma. o n only if X satisfies For ] 2 @; @; BC; A ,
GH]J .X / D Tot H]; .X / D J J
M
p;q
H]J .X / :
pqD
Example 1.2 (Generalized Complex Structures of Type 0, [Gua04a, Example 4.10]). Let X be a compact 2n-dimensional manifold endowed with a symplectic structure ! 2 ^2 X ' Hom .TXI T X /. Consider the (0-twisted) generalized complex structure ! 0 ! 1 J! WD ; ! 0 where ! 1 2 Hom .T X I TX/ denotes the inverse of ! 2 Hom .TXI T X /. The i-eigenspace of the C-linear extension of J! to .TX ˝R C/˚.T X ˝R C/ is LJ! D fX i ! .X; / W X 2 TX ˝R Cg ; which has Clifford annihilator exp.i !/. The canonical bundle is UJn! D C hexp .i !/i :
26
1 Preliminaries on (Almost-)Complex Manifolds
In particular, one gets that, [Cav06, Theorem 2.2], D exp .i !/ UJn !
^ X ˝R C exp : 2i
Note that, [Cav06, Sect. 2.2], dJ! D d : By considering the natural isomorphism 'W
M k2Z
M ˛ 2 ^ X ˝R C 3 ˛ ! 7 exp .i !/ exp ^k X ˝R C ; 2i k
k2Z
one gets that, [Cav06, Corollary 1], ' ^ X ˝R C ' U n ; and ' d D @J! '
and
' dJ! D 2 i @J! ' I
in particular, n .X I C/ : GH@ .X / ' HdR J!
Example 1.3 (B-Transform, [Gua04a, Sect. 3.3]). Let X be a compact 2ndimensional manifold endowed with an H -twisted generalized complex structure J , and let B be a d-closed 2-form. Consider the H -twisted generalized complex structure ! idTX 0 B J WD exp .B/ J exp B where exp B D : B idT X The i-eigenspace of the C-linear extension of J to .TX ˚ T X /˝R C is, [Cav05, Example 2.3], LJ B D fX C X B W X C 2 LJ g ; and the canonical bundle is, [Cav05, Example 2.6], UJn B D exp B ^ UJn :
1.4 Kähler Geometry
27
Hence one gets that, [Cav06, Sect. 2.3], UJ B D exp B ^ UJ ;
and that
@J B D exp .B/ @J exp B
and
@J B D exp .B/ @J exp B : In particular, J satisfies the @J @J -Lemma if and only if J B satisfies the @J B @J B -Lemma.
1.4 Kähler Geometry Note that, given a manifold X endowed with a symplectic form !, there is always a (possibly non-integrable) almost-complex structure J on X such that g WD !.; J / is a Hermitian metric on X with ! as the associated .1; 1/-form, see, e.g., [CdS01, Corollary 12.7] (in fact, the set of such almost-complex structures is contractible, see, e.g., [AL94, Corollary II.1.1.7], [CdS01, Proposition 13.1]; see also [Gro85, Corollary 2.3.C02 ], which proves that the space of almost-complex structures on X tamed by a given 2-form on X is contractible). Instead, the datum of an integrable almost-complex structure with the above property yields a Kähler structure on X . The notion of Kähler manifold has been studied for the first time by J.A. Schouten and D. van Dantzig [SvD30], see also [Sch29], and by E. Kähler [Käh33], and the terminology has been fixed by A. Weil [Wei58]. Kähler structures can be defined in different ways, according to the point of view which is stressed, Sect. 1.4.1. The presence of three different structures (complex, symplectic, and Riemannian) allows to make use of the tools available for any of them; in addition, the relations between such structures make available further tools, which yield many interesting results on Hodge theory, Sect. 1.4.2. Finally, we will study a cohomological property of compact Kähler manifolds, namely, the @@-Lemma, Sect. 1.4.3: other than being a very useful tool in Kähler geometry (compare, e.g., its role in S.-T. Yau’s proof [Yau77, Yau78] of E. Calabi’s conjecture [Cal57]), it provides obstructions to the existence of Kähler structures on differentiable manifolds, by means of the notion of formality introduced by D.P. Sullivan, [Sul77, Sect. 12].
1.4.1 Kähler Metrics Let X be a compact complex manifold of complex dimension n, and denote by J its natural integrable almost-complex structure. Definition 1.4 ([SvD30, Sch29, Käh33, Wei58]). Let X be a compact complex manifold of complex dimension n and J be its natural integrable almost-complex
28
1 Preliminaries on (Almost-)Complex Manifolds
structure. A Kähler metric on X is a Hermitian metric g such that the associated .1; 1/-form ! WD g.J ; / is d-closed (that is, ! is a symplectic form on X ). Remark 1.8. Let X be a complex manifold endowed with a Kähler metric g, and denote the associated .1; 1/-form to g by !. By the Poincaré lemma, see, e.g., [Dem12, I.1.22, Theorem I.2.24], and the Dolbeault and Grothendieck lemma, see, e.g., [Dem12, I.3.29], the property that d ! D 0 is equivalent to ask that, for every x 2 X , there exist an open neighbourhood U in X with x 2 U and a smooth loc function u 2 C 1 .U I R/ such that ! D i @@u in U , that is, the metric has a local potential, [Käh33] (see, e.g., [Mor07, Proposition 8.8]). Remark 1.9. For every n 2 N, the complex projective space CPn admits a Kähler metric, the so-called Fubini and Study metric, [Fub04, Stu05], which is induced by the fibration S1 ,! S2nC1 ! CPn ; more precisely, by using the homogeneous coordinates Œz0 W W zn , one has that the associated .1; 1/-form !FS to the Fubini and Study metric is !FS
n X ˇ ` ˇ2 i ˇz ˇ @@ log D 2
! :
`D0
It follows that complex projective manifolds provide examples of Kähler manifolds. Conversely, by the Kodaira embedding theorem [Kod54, Theorem 4], if X is a compact complex manifold endowed with a Kähler metric ! such that Œ! 2 2 2 .X I R/ \ im H 2 .X I Z/ ! HdR .X I R/ , then there exists a complex-analytic HdR embedding of X into a complex projective space CPN for some N 2 N. In a sense, this suggest that projective manifolds are to Kähler manifolds as Q is to R. Hence, it is natural to ask if every compact Kähler manifold is a deformation of a projective manifold (which is known as the Kodaira problem). Since Riemann surfaces are projective, this is trivially true in complex dimension 1. Furthermore, K. Kodaira proved in [Kod63, Theorem 16.1] that every compact Kähler surface is a deformation of an algebraic surface, as conjectured by W. Hodge; another proof, which does not make use of the classification of elliptic surfaces, has been given by N. Buchdahl, [Buc08, Theorem]. In higher dimension, a negative answer to the Kodaira problem has been given by C. Voisin, who constructed examples of compact Kähler manifolds, of any complex dimension greater than or equal to 4, which do not have the homotopy type of a complex projective manifold, [Voi04, Theorem 2] (indeed, recall that, by Ehresmann’s theorem, if two compact complex manifolds can be obtained by deformation, then they are homeomorphic, and hence they have the same homotopy type). The examples in [Voi04] being, by construction, bimeromorphic to manifolds that can be deformed to projective manifolds, one could ask (as done by N. Buchdahl, F. Campana, S.-T. Yau) whether, in higher dimension, a birational version of the Kodaira problem may hold true; in [Voi06, Theorem 3], C. Voisin provided a negative answer to the birational version of the Kodaira problem, proving that, in any even complex dimension greater that or equal to 10, there exist compact Kähler manifolds X such that, for any compact
1.4 Kähler Geometry
29
Kähler manifold X 0 bimeromorphic to X , X 0 does not have the homotopy type of a projective complex manifold. Positive results under an additional semi-positivity or semi-negativity condition on the canonical bundle have been provided by J. Cao in [Cao12]. In the definition of a Kähler manifold, three different structures are involved: a complex structure, a symplectic structure, and a metric structure. Therefore, changing the point of view allows to give several equivalent definitions of Kähler structure (see, e.g., [Bal06, Theorem 4.17]): we review here two of these characterizations. Firstly, it is straightforward to prove that a Hermitian metric g on a compact complex manifold X is a Kähler metric only if, for every point x 2 X , there if and ˚ j exists a holomorphic coordinate chart U; z j 2f1;:::;ng , with x 2 U , such that g D
n X ı˛ˇ C o .jzj/ d z˛ ˇ d zNˇ
at x ;
˛;ˇD1
that is, g osculates to order 2 the standard Hermitian metric of Cn (see, e.g., [GH94, pp. 107–108], [Huy05, Proposition 1.3.12], [Mor07, Theorem 11.6]). As regards the second characterization, we recall that, on a compact complex manifold X endowed with a Hermitian metric g, there is a unique connection r C such that (i) r C g D 0, (ii) r C J D 0, and (iii) ^0;1 X r C bC 1 .X IC/ D @bC 1 .X IC/ ; such a connection is called the Chern connection of X (see, e.g., [Huy05, Proposition 4.2.14], [Bal06, Theorem 3.18], [Mor07, Theorem 10.3], [Dem12, Sect. V.12]). Let g be a Hermitian metric on a compact complex manifold X , and set ! WD g.J ; / its associated .1; 1/-form, where J is the natural integrable almost-complex structure on X ; consider the Levi Civita connection r LC . One can prove that, for every x; y; z 2 C 1 .X I TX/, d !.x; y; z/ D g
LC rx J y; z C g ryLC J z; x C g rzLC J x; y ;
and 2g
rxLC J y; z D d ! .x; y; z/ d ! .x; Jy; J z/ g .NijJ .y; J z/ ; x/ I
(see, e.g., [Bal06, Theorem 4.16], [Tia00, Proposition 1.5]); in particular, it follows that g is a Kähler metric if and only if r LC J D 0 if and only if the Chern connection is the Levi Civita connection (see, e.g., [Bal06, Theorem 4.17], [Mor07, Proposition 11.8]).
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1 Preliminaries on (Almost-)Complex Manifolds
Remark 1.10. We recall that, given a d-closed 3-form H on a manifold X , an H -twisted generalized Kähler structure on X is a pair .J1 ; J2 / of H -twisted generalized complex structures on X such that (i) J1 and J2 commute, and (ii) the symmetric pairing hJ1 ; J2 i is positive definite. Generalized Kähler geometry is equivalent to a bi-Hermitian geometry with torsion, [Gua04b, Theorem 2.18]. We recall that a compact manifold X endowed with an H -twisted generalized J2 1 Kähler structure .J1 ; J2 / satisfies both the dH dJ H -Lemma and the dH dH -Lemma, [Gua04b, Corollary 4.2]. Any Kähler structure provide an example of a 0-twisted generalized Kähler structure. A left-invariant non-trivial twisted generalized Kähler structure on a (noncompletely solvable) solvmanifold (which is the total space of a T2 -bundle over the Inoue surface, [FT09, Proposition 3.2]) has been constructed by A.M. Fino and A. Tomassini, [FT09, Theorem 3.5]. Furthermore, we note that A. Tomasiello proved in [Tom08, Sect. B] satisfying the d dJ -Lemma is a stable property under small deformations.
1.4.2 Hodge Theory for Kähler Manifolds The complex, symplectic, and metric structures being related on a Kähler manifold, one gets the following identities concerning the corresponding operators (see, e.g., [Huy05, Proposition 3.1.12]); see also [Hod35, Hod89]. (In [Dem86, Theorems 1.1 and 2.12], commutation relations on arbitrary Hermitian manifolds are provided; see also [Gri66], [Dem12, Sect. VI.6.2].) Theorem 1.19 (Kähler Identities, [Wei58, Théorème II.1, II.2, Corollaire II.1]). Let X be a compact Kähler manifold. Consider the differential operators @ and @ associated to the complex structure, the symplectic operators L and associated to the symplectic structure, and the Hodge--operator associated to the Hermitian metric. Then, these operators are related as follows: i h i h (i) @; L D Œ@; L D 0 and ; @ D Œ; @ D 0; h i h i (ii) @ ; L D i @ and Œ@ ; L D i @, and ; @ D i @ and Œ; @ D i @ . nd Therefore, considering i the 2 order self-adjoint elliptic differential operators WD h Œ@; @ , WD @; @ , and WD Œd; d , one gets that
(iii) D D 12 , and commutes with , @, @, @ , @ , L, . The previous identities can be proven either using the sl .2I C/ representation hL; ; H i ! End .^ X ˝ C/, or reducing to prove the corresponding identities on Cn with the standard Kähler structure (which are known as Y. Akizuki and S. Nakano’s identities, [AN54, Sect. 3]) and hence using that every Kähler metric osculates to order 2 the standard Hermitian metric on Cn .
1.4 Kähler Geometry
31
As a consequence, one gets the following theorems, stating a decomposition of the de Rham cohomology of a Kähler manifold related to the complex, respectively symplectic, structure (see, e.g., [Huy05, Corollary 3.2.12], respectively [Huy05, Proposition 3.2.13]). Theorem 1.20 (Hodge Decomposition Theorem, [Wei58, Théorème IV.3]). Let X be a compact complex manifold endowed with a Kähler structure. Then there exist a decomposition HdR .X I C/ '
M
p;q
H@ .X / ;
pCqD
and, for every .p; q/ 2 N2 , an isomorphism p;q
q;p
H@ .X / ' H@ .X / : Theorem 1.21 (Lefschetz Decomposition Theorem, [Wei58, Théorème IV.5]). Let X be a compact complex manifold, of complex dimension n, endowed with a Kähler structure. Then there exist a decomposition HdR .X I C/ D
M
2r 2r2 Lr ker W HdR .X I C/ ! HdR .X I C/ ;
r2N
and, for every k 2 N, an isomorphism '
nCk nk Lk W HdR .X I C/ ! HdR .X I C/ :
1.4.3 @@-Lemma and Formality for Compact Kähler Manifolds The Hodge decomposition theorem and the Lefschetz decomposition theorem provide obstructions to the existence of Kähler structures on a compact complex manifold. In this section, we study another property of compact Kähler manifolds, namely, formality, which provides an obstruction to the existence of a Kähler structure on a compact (differentiable) manifold. Such a property turns out to be a consequence of the validity of the @@-Lemma on compact complex manifolds. Firstly, we need to recall some general notions regarding homotopy theory of differential algebras; we will then summarize some results concerning the homotopy type of Kähler manifolds: by the classical result by P. Deligne, Ph.A. Griffiths, J. Morgan, and D.P. Sullivan, [DGMS75, Main Theorem], the real homotopy type of a Kähler manifold X is a formal consequence of its cohomology ring HdR .X I R/. We recall that a differential graded algebra (shortly, dga) over a field K is a graded K-algebra A (where the structure of K-algebra is induced by an
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1 Preliminaries on (Almost-)Complex Manifolds
inclusion K A0 ) being graded-commutative6 and endowed with a differential dW A ! AC1 satisfying the graded-Leibniz rule.7 A morphism of differential graded algebras F W .A ; dA / ! .B ; dB / is a morphism A ! B of K-algebras such that F ı dA D dB ıF . d Given a dga .A ; d/ over K, the cohomology H .A ; d/ WD ker endowed im d with the zero differential has a natural structure of dga over K; furthermore, every morphism F W .A ; dA / ! .B ; dB / of dgas induces a morphism F W .H .A ; dA / ; 0/ ! .H .B ; dB / ; 0/ of dgas in cohomology; a morphism F W .A ; dA / ! .B ; dB / of dgas is called a quasi-isomorphism (shortly, qis) if the corresponding morphism F W .H .A ; dA / ; 0/ ! .H .B ; dB / ; 0/ is an isomorphism. The de Rham complex .^ X; d/ of a compact (differentiable) manifold X has a structure of dga over R, whose cohomology is the dga HdR .X I R/; 0 . Given a dga .A ; dA / over K, the differential dA is called decomposable if
dA
1 0 1 0 M M A @ Ak A @ Ak A : k2Nnf0g
k2Nnf0g
Given a dga .A ; dA / over K, an elementary extension of .A ; dA / is a dga .B ; dB / over K such that (i) B D A ˝K ^ Vk for Vk a finite-dimensional K-vector space and k > 0, where ^ Vk is the free graded K-algebra generated by Vk , the elements of Vk having degree k, and (ii) dB bA D dA and dB .Vk / A . A dga .M ; dM / over K is called minimal if it can be written as an increasing union of sub-dgas, .K; 0/ D M0 ; dM0 M1 ; dM1 M2 ; dM2 ; [ M ; dM D Mj ; dMj ; j 2N
such that
(i) for any j 2 N, the dga MjC1 ; dMjC1 is an elementary extension of the dga Mj ; dMj , and (ii) dM is decomposable. We recall that a graded K-algebra A is graded-commutative if, for every x 2 Adeg x and y 2 Adeg y , it holds x y D .1/deg xdeg y y x. 7 We recall that a differential dW A ! AC1 on a graded K-algebra A satisfies the graded-Leibniz rule if, for every x 2 Adeg x and y 2 Adeg y , it holds d .x y/ D d x y C .1/deg x x d y. 6
1.4 Kähler Geometry
33
A minimal model for a dga .A ; dA / over K is the datum of a minimal dga qis
.M ; dM / over K and a quasi-isomorphism W .M ; dM / ! .A ; dA / of dgas. / over K are equivalent if there exist an integer Two dgas .A ; dA / and n .B ; dBo of dgas over K with C0 ; dC0 D n 2 N n f0g, a family Cj ; dCj j 2f0;:::;2ng D .B ; dB /, and a family .A ; dA / and C2n ; dC2n C2j ; d C2j C1 C1 NNN q q NNN q q q NNN q q q NN' qis xqq qis C2j ; dC2j C2j C2 ; dC2j C2 of quasi-isomorphisms, varying j 2 f0; : : : ; n 1g. A dga .A ; dA / over K is called formal if it is equivalent to a dga .B ; 0/ over K with zero differential, that is, if it is equivalent to .H .A ; dA / ; 0/. A compact manifold X is called formal if its de Rham complex .^ X; d/ is a formal dga over R. Let .A ; dA / be a dga over K. Given Œ˛12 2 H deg ˛12 A ; dA ; Œ˛34 2 H deg ˛34 A ; dA
Œ˛23 2 H deg ˛23 A ; dA ;
and
such that Œ˛12 Œ˛23 D 0
and
Œ˛23 Œ˛34 D 0 ;
let ˛13 2 Adeg ˛12 Cdeg ˛23 1 and ˛24 2 Adeg ˛23 Cdeg ˛34 1 be such that .1/deg ˛12 ˛12 ˛23 D dA ˛13
and
.1/deg ˛23 ˛23 ˛34 D dA ˛24 I
one can then define the triple Massey product hŒ˛12 ; Œ˛23 ; Œ˛34 i as hŒ˛12 ; Œ˛23 ; Œ˛34 i WD 2
H deg ˛12
.A ; d
A /
h i .1/deg ˛12 ˛12 ˛24 C .1/deg ˛13 ˛13 ˛34 H deg ˛12 Cdeg ˛23 Cdeg ˛34 1 .A ; dA / : .A ; dA / C H deg ˛34 .A ; dA / H deg ˛12 Cdeg ˛23 1 .A ; dA /
H deg ˛23 Cdeg ˛34 1
One can define the higher order Massey product by induction. Fixed m 2 N such that m 4, and given Œ˛12 2 H deg ˛12 A ; dA ;
::: ;
Œ˛m;mC1 2 H deg ˛m;mC1 A ; dA
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1 Preliminaries on (Almost-)Complex Manifolds
such that all the Massey products of order lower than or equal to m 1 vanish, let f˛rs g1 r<s mC1 A be such that X
.1/deg ˛h` ˛h` ˛`k D d ˛hk ;
h<`
for any h; k 2 f1; : : : ; m C 1g with k h < m. Then define the mth order Massey product as "
X
hŒ˛12 ; : : : ; Œ˛m;mC1 i WD
# .1/
deg ˛1k
˛1k ˛k;mC1
1<`<mC1
belonging to a quotient of H .A ; dA /. As a direct consequence of the definitions, the Massey products (of any order) on a formal dga are zero. Now, let X be a compact manifold endowed with a Kähler structure. The Kähler identities allow to prove the following result, known as @@-Lemma (see, e.g., [Huy05, Corollary 3.2.10]), which, in a sense, summarizes many of the cohomological properties of compact Kähler manifolds. Theorem 1.22 (@@-Lemma for Compact Kähler Manifolds, [DGMS75, Lemma 5.11]). Let X be a compact Kähler manifold. Then every @-closed, @-closed, d-exact form is also @@-exact. Proof. We recall the idea of the proof, as can be found, e.g., in [Huy05, Corollary 3.2.10], see also [DGMS75, pp. 266–267]. Let ˛ 2 ^p;q X be a @-closed @-closed d-exact form on X . In particular, by the Hodge decomposition theorem for the de Rham cohomology (see, e.g., [War83, 6.8]), the form˛ is orthogonal to the space of -harmonic forms. Note that, by the Kähler identities, the space of -harmonic forms coincide with the space of -harmonic forms and with the space of -harmonic forms. Since ˛ is @-closed and orthogonal to the space of -harmonic forms, the conjugate version of the Hodge decomposition theorem for the Dolbeault cohomology, [Hod89], yields ˛ D @ for some 2 ^p1;q X . By applying the Hodge decomposition theorem for the Dolbeault cohomology, [Hod89], to the form , one gets a -harmonic form h , a form ˇ 2 ^p1;q1 X , and a form h 2 ^ip1;qC1 X such that D h C @ˇ C @ . By the Kähler identities, one has @; @
D 0 and that h is also -harmonic.
Hence ˛ D @ D @@ˇ @ @. It suffices to prove that @ @ D 0. Indeed, 2 D E since ˛ is @-closed, one has @@ @ D 0, and hence @ @ D @ @; @ @ D D E @@ @; @ D 0. Hence ˛ D @@ˇ is @@-exact. t u Using the differential operator dc WD J 1 d J D i @ @ (where J is the integrable almost-complex structure naturally associated to the structure of complex
1.5 Deformations of Complex Structures
35
manifold on X ), and noting that ker @ \ ker @ D ker d \ ker dc and im @@ D im d dc , the following equivalent formulation can be provided. Theorem 1.23 (d dc -Lemma for Compact Kähler Manifolds, [DGMS75, Lemma 5.11]). Let X be a compact Kähler manifold. Then every d-closed, dc closed, d-exact form is also d dc -exact. Actually, the @@-Lemma holds true for a larger class of compact complex manifolds than the compact Kähler manifolds: indeed, it holds, for examples, for any compact complex manifold that can be blown up to a Kähler manifold, [DGMS75, Theorem 5.22], e.g., for compact complex manifolds in class C of Fujiki, or for Moˇıšezon manifolds; we refer to Sect. 2.1.3 for further results concerning the @@Lemma for compact complex manifolds. If X is a compact Kähler manifold (or, more in general, any compact complex manifold for which the @@-Lemma, equivalently the d dc -Lemma, holds), then one has the following quasi-isomorphisms of dgas:
.^ X; d/
.ker dc ; dbker dc / NNN p NNN ppp p NNN p p p NN' qis xppp qis
ker dc im dc
;0 I
in particular, the dga .^ X; d/ is equivalent to a dga with zero differential, and hence it is formal. This proves the following result by P. Deligne, Ph.A. Griffiths, J. Morgan, and D.P. Sullivan. Theorem 1.24 ([DGMS75, Main Theorem]). Let X be a compact complex manifold for which the @@-Lemma holds (e.g., a compact Kähler manifold, or a manifold in class C of Fujiki). Then the differentiable manifold underlying X is formal (that is, the differential graded algebra .^ X; d/ is formal). In particular, all Massey products (of any order) on a compact complex manifold satisfying the @@-Lemma are zero, [DGMS75, Corollary 1]. This provide an obstruction to the existence of Kähler structures on compact differentiable manifolds.
1.5 Deformations of Complex Structures A natural way to construct new complex structures on a manifold is by “deforming” a given complex structure. Natural questions arise naturally from this construction, concerning, for example, what properties (e.g., the existence of some special metric) remain still valid after such a small deformation. We recall in this section the basic notions and the classical results concerning the K. Kodaira, D.C. Spencer, L. Nirenberg, and M. Kuranishi theory of deformations
36
1 Preliminaries on (Almost-)Complex Manifolds
of complex manifolds, [KS58,KS60,KNS58,Kur62], referring to [Huy05], see also, e.g., [Kod05, MK06]. Let B be a complex (respectively, differentiable) manifold. A family fXt gt 2B of compact complex manifolds is said to be a complex-analytic (respectively, differentiable) family of compact complex manifolds if there exist a complex (respectively, differentiable) manifold X and a surjective holomorphic (respectively, smooth) map W X ! B such that (i) 1 .t/ D Xt for any t 2 B, and (ii)
is a proper holomorphic (respectively, smooth) submersion. A compact complex manifold X is said to be a deformation of a compact complex manifold Y if there exist a complex-analytic family fXt gt 2B of compact complex manifolds, and b0 ; b1 2 B such that Xb0 D Xs and Xb1 D Xt .
A complex-analytic (respectively, differentiable) family X ! B of compact complex manifolds is said to be trivial if X is bi-holomorphic (respectively,
B diffeomorphic) to B Xb ! B for some b 2 B (where B W B Xb ! B denotes the natural projection onto B); it is said to be locally trivial if, for any b 2 B,
b 1 .U /
! U there exists an open neighbourhood U of b in B such that 1 .U / is trivial. The following theorem by Ch. Ehresmann states the local triviality of a differentiable family of compact complex manifolds (see, e.g., [Kod05, Theorems 2.3 and 2.5], [MK06, Theorem 1.4.1]). Theorem 1.25 (Ehresmann Theorem, [Ehr47]). Let fXt gt 2B be a differentiable family of compact complex manifolds. For any s; t 2 B, the manifolds Xs and Xt are diffeomorphic. As a consequence of Ehresmann’s theorem, a complex-analytic family fXt gt 2B of compact complex manifolds with B contractible can be viewed as a family of complex structures on a compact differentiable manifold. We recall some other useful definitions, see, e.g., [Huy05, Sect. 6.2]. Let W X ! B be a complex-analytic family of compact complex manifolds, deformations of X WD 1 .0/. We recall that, given f W .B 0 ; 00 / ! .B; 0/ a morphism of germs with a distinguished point, the pull-back f X WD X B B 0 gives a complexanalytic family of deformations of X . The complex-analytic family W X ! B of deformations of X is called complete if, for any complex-analytic family 0 W X 0 ! B 0 of deformations of X , there exists a morphism f W B 0 ! B of germs with a distinguished point such that X 0 D f X . The complex-analytic family W X ! B of deformations of X is called universal if, for any complex-analytic family
0 W X 0 ! B 0 of deformations of X , there exists a unique morphism f W B 0 ! B of germs with a distinguished point such that X 0 D f X . The complex-analytic family
W X ! B of deformations of X is called versal if, for any complex-analytic family
0 W X 0 ! B 0 of deformations of X , there exists a morphism f W B 0 ! B of germs with a distinguished point such that X 0 D f X and such that d f W T00 B 0 ! T0 S is uniquely determined. The theory of complex-analytic deformations of compact complex manifolds has been introduced by K. Kodaira and D.C. Spencer, [KS58, KS60], and developed also by L. Nirenberg, [KNS58], and M. Kuranishi, [Kur62, Kur65], see also
1.5 Deformations of Complex Structures
37
[Kod05, MK06]. In recalling the main results of this theory, we follow the approach in [Huy05], based on the construction of a differential graded Lie algebra structure on C 1 X I T 1;0 X ˝ ^0; X , see also [Man04]. Let X be a compact manifold endowed with an integrable almost-complex structure J . Every section s 2 C 1 X I TJ1;0 X ˝ ^0;1 X near to the zero section J determines an almost-complex structure J 0 , defined in such a way that ^1;0 J 0 X is the 0;1 0 X ! ^ X ; it turns out that J is integrable if and only if the graph of sW ^1;0 J J Maurer and Cartan equation @s C
1 Œs; s D 0 2
(MC)
holds (see, e.g., [Huy05, Lemma 6.1.2]), where 0;p 0;q • Œ; W C 1 X I TJ1;0 X ˝ ^J X C 1 X I TJ1;0 X ˝ ^J X ! C 1 X I TJ1;0 X 0;pCq ˝ ^J X is defined as
X ˝ ˛; N Y ˝ ˇN WD X ˝ ˇN ^ LY ˛N C Y ˝ ˛N ^ LX ˇN C ŒX; Y ˝ ˛N ^ ˇN ;
where LW ' WD W d ' C d . W '/ is the derivative of ' along W ; locally, in ˚ Lie a chart with holomorphic coordinates zj j , one has
w ˝ d zN`1 ^ ^ d zN`p ; w0 ˝ d zNm1 ^ ^ d zNmq ^ loc D w; w0 ˝ d zN`1 ^ ^ d zN`p ^ d zNm1 ^ ^ d zNmq I
0;p 0;pC1 X is defined as • @W C 1 X I TJ1;0 X ˝ ^J X ! C 1 X I TJ1;0 X ˝ ^J N WN WD Z; N ' WN 1;0 WN ; ' ZN 1;0 ' Z; N WN @' Z; ; where X 1;0 WD X i J X˚ isthe .1; 0/-component of X ; locally, in a chart with holomorphic coordinates zj j , one has
@ @ ˝˛ @z`
loc
D
@ ˝ @˛ : @z`
Hence, to study complex-analytic families of infinitesimal deformations of a compact complex manifold X , it suffices to study complex-analytic families fs.t/gt2.0;"/Cm C 1 X I T 1;0 X ˝ ^0;1 X (where " > 0 is small enough) with s.0/ D 0. Consider the power series expansion in t of s.t/, s.t/ DW
X k2N
sk .t/ ;
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1 Preliminaries on (Almost-)Complex Manifolds
where sk .t/ 2 C 1 X I T 1;0 X ˝ ^0;1 X is homogeneous of degree k in t, and s0 .t/ D 0. Then the Maurer and Cartan equation (MC) can be rewritten, for every t 2 .0; "/, as the system (
@s1 .t/ D 0 @sk .t/ D
P 1 j k1
I sj .t/; skj .t/ for k 2
in particular, s1 .t/ defines a class in H 0;1 .X I X /, where X denotes the sheaf of the germs of holomorphic vector fields on X ; up to the action of Diff.X /, one has that s1 .t/ is uniquely determined by its class in H 0;1 .X I X / (see, e.g., [Huy05, Lemma 6.14]). Fix now a Hermitian metric g on X . Consider the decomposition T 1;0 X ˝ ^0;1 X D T 1;0 X ˝ ker b^0;1 X ˚ T 1;0 X ˝ @ ^0;0 X ˚ T 1;0 X ˝ @ ^0;2 X ; and the corresponding projections H@ W T 1;0 X ˝^0;1 X ! T 1;0 X ˝ker b^0;1 X ;
P@ W T 1;0 X ˝^0;1 X ! T 1;0 X ˝@^0;0 X:
In order that s.t/ satisfies (MC), for every t 2 .0; "/, one should have 0 @sk .t/ D P@ @
X
1 sj .t/; skj .t/ A :
1 j k1
Hence, one gets @s.t/ C Œs.t/; s.t/ D H@ .Œs.t/; s.t// : Therefore, define the map obsW H 0;1 .X I X / ! H 0;2 .X I X / ˚ as follows. Let Xj ˝ !N k j 2f1;:::;ng be a basis of H 0;1 .X I X /. Given D k2f1;:::;mg P j j k t X ˝ ! N , denote t D tk j 2f1;:::;ng , and define s1 .t/ WD and sk .t/ j j 2f1;:::;ng k k2f1;:::;mg k2f1;:::;mg P s .t/; skj .t/ for k 2; hence, define the such that @sk .t/ WD P@ 1 j k1 j P formal power series s.t/ WD k2N sk .t/. Define obs ./ WD H@ .Œs.t/; s.t// :
1.5 Deformations of Complex Structures
39
Hence, one has then that fs.t/gt2.0;"/Cm C 1 X I TJ1;0 X ˝ ^0;1 X (where J " > 0 is small enough) defines an infinitesimal family of compact complex manifolds if obs .s1 .t// D 0 for every t 2 .0; "/ (indeed, for " > 0 small enough, the formal power series converges, see, e.g., [Kod05, Sect. 5.3], [MK06, Sect. 2.3]). One gets the following result by M. Kuranishi. Theorem 1.26 ([Kur62, Theorem 2]). Let X be a compact complex manifold. Then X admits a versal complex-analytic family of deformations. Fixed a Hermitian metric on X , such a family of deformations, which is called the Kuranishi space Kur.X / of X , is parametrized by Kur.X / D
˚ 2 H 0;1 .X I X / W kk 1; obs./ D 0 :
Remark 1.11. A compact complex manifold X is called non-obstructed if Kur.X / is non-singular. In particular, if H 0;2 .X I X / D f0g, then X is non-obstructed. There are other interesting cases in which the Kuranishi space turns out to be non-singular: as announced by F.A. Bogomolov, [Bog78], and proven by G. Tian, [Tia87], and, independently, by A.N. Todorov, [Tod89, Theorem 1], this happens for Calabi-Yau manifolds (that is, compact complex manifolds X of complex dimension n endowed with a Kähler structure .J; !; g/ and with a nowhere vanishing 2 ^n;0 X such that (i) r LC D 0,where r LC denotes the Levi Civita connection n.nC1/ n associated to g, and (ii) ^ N D .1/ 2 in !nŠ ). In [dBT12], P. de Bartolomeis and A. Tomassini introduced the notion of quantum inner state manifold, [dBT12, Definition 2.2], as a possible generalization of Calabi-Yau manifolds, proving that, under a suitable hypothesis, the moduli space of quantum inner state deformations of a compact Calabi-Yau manifold is unobstructed, [dBT12, Theorem 3.6]. On the other hand, in [Rol11b], S. Rollenske studied the Kuranishi space of holomorphically parallelizable nilmanifolds, proving that it is cut out by polynomial equations of degree at most equal to the step of nilpotency of the nilmanifold, [Rol11b, Theorem 4.5], and it is smooth if and only if the associated Lie algebra is a free 2-step nilpotent Lie algebra, [Rol11b, Corollary 4.9]. Remark 1.12. We refer to [Rol11b, Rol09b, Rol09a] by S. Rollenske and to [Kas12c] by H. Kasuya for results concerning deformations of nilmanifolds and, respectively, solvmanifolds. It could be interesting to study what properties of a complex manifold are, in a sense, compatible with the construction of small deformations of the complex structure. In such a context, a property P concerning compact complex manifolds is called open under (holomorphic) deformations of the complex structure (or stable under small deformations of the complex structure) if, for every complex-analytic family fXt gt 2B of compact complex manifolds, and for every b0 2 B, if Xb0 has the property P, then Xb has the property P for every b in an open neighbourhood of b0 . A property P is called is called closed under (holomorphic) deformations of the complex structure if, for every complex-analytic family fXt gt 2" .0/ of compact
40
1 Preliminaries on (Almost-)Complex Manifolds
complex manifolds, where " .0/ WD fz 2 Cm W jzj < "g for some m 2 N n f0g and for some " > 0, if Xt has the property P for every t 2 " .0/ n f0g, then also X0 has the property P. Remark 1.13 (Gunnar Þór Magnússon8). In a sense, the notion of closedness is considered in the Zariski sense. In fact, as suggested to us by Gunnar Þór Magnússon, by considering, for example, moduli space of complex tori of complex dimension 2, and by using [Kod63, Theorem 16.1] (compare also [Cao12, Main Theorem]), one can construct examples of complex-analytic families fXt gt 2B of compact complex manifolds such that there exists a converging sequence fbk gk2N B with b1 WD limk!C1 bk 2 B for which Xbk is Moˇıšezon for every k 2 N, while Xb1 is not. We recall here the following classical result by K. Kodaira and D.C. Spencer, stating that admitting a Kähler metric is a stable property under deformations of the complex structure. Theorem 1.27 ([KS60, Theorem 15]). Let fXt gt 2B be a differentiable family of compact complex manifolds. If Xt0 admits a Kähler metric for some t0 2 B, then Xs admits a Kähler metric for every s in an open neighbourhood of t0 in B. Moreover, given any Kähler metric ! on Xt0 , one can choose an open neighbourhood U of t0 in B and a Kähler metric !s on Xs for any s 2 U such that !s depends differentiably in s and !t0 D !. Proof (sketch). We just sketch the idea of the proof, as one can found, e.g., in [Sch07, Lemme 3.3], see also [Voi02, Théorème 9.23] and [KS60, Sect. 6]. Consider a family of compact complex manifolds fXt gt . By the Ehresmann k theorem, [Ehr47], for every k 2 Z, the function t 7! dimC HdR .Xt I C/ is locally constant. By the theory of families of elliptic differential operators (see, e.g., p;q [Kod05, Sect. 7.1]), for every .p; q/ 2 Z2 , the functions t 7! dimC HBC .Xt / and p;q t 7! dimC HA .Xt / are upper-semi-continuous, see, e.g., [Sch07, Lemme 3.2]. Recall also that, by the inequalityP à la Frölicher for the Bott-Chern cohomology, p;q p;q [AT13b, Theorem A], it holds that pCqDk dimC HBC .Xt / C dimC HA .Xt / k 2 dimC HdR .Xt I C/ for any k 2 Z and for any t. If we assume that Xt0 is Kähler for some t0P , then in particular Xt0 satisfies the p;q @@-Lemma. Therefore, for any k 2 Z, it holds pCqDk dimC HBC .Xt0 / C dimC p;q k HA .Xt0 / D 2 dimC HdR .Xt0 I C/. In particular, from the facts remarked above, p;q it follows that, for any .p; q/ 2 Z2 , the functions t 7! dimC HBC .Xt / and t 7! p;q dimC HA .Xt / are locally constant at t0 . 1;1 .Xt / is locally constant In particular, we will use that the function t 7! dimC HBC at t0 . In fact, by the theory of families of elliptic differential operators (see, e.g., [Kod05, Sect. 7.1]), this implies that the map hs W ^2 X ˝ C ! ker Q BC s 1;1 ^
8
Xs
The author would like to thank Luis Ugarte for having pointed out the subject, and Junyan Cao and Gunnar Þór Magnússon for useful discussions on the matter.
1.5 Deformations of Complex Structures
41
depends smoothly in s in a neighbourhood of t0 , [KS60, Theorem 7], see also [Kod05, Theorem 7.4]. (For simplicity, we identify the smooth structures on Xt0 and Xs , for s in a neighbourhood of t0 , by means of the diffeomorphism given by the Ehresmann theorem, [Ehr47].) Hence define, for s in a neighbourhood of t0 , !s WD
1 hs ! C hs ! : 2
For any s, the form !s is a real d-closed .1; 1/-form on Xs . Furthermore, the family f!s gs depends smoothly in s. Since !t0 D !, it follows that the form !s is positive for s in a neighbourhood of Xt0 . In particular, !s is a Kähler form on Xs depending smoothly in s in a neighbourhood of t0 . t u Remark 1.14. In [Hir62], it is proven that admitting a Kähler structure is not a closed property under deformations of the complex structure: indeed, H. Hironaka provided an explicit example of a complex-analytic family of compact complex manifolds of complex dimension 3 such that (i) one of the complex manifold is non-Kähler (indeed, it carries a positive 1-cycle algebraically equivalent to zero), and (ii) the others are Kähler and, in fact, bi-regularly embedded in a projective space (and hence projective, [Moı66, Theorem 11]), [Hir62, Theorem]. (Note that, in complex dimension 2, the Kähler property is closed under deformations of the complex structure, since a compact complex surface is Kähler if and only if its 1st Betti number is even, by [Kod64, Miy74, Siu83], or [Lam99, Corollaire 5.7], or [Buc99, Theorem 11].) It is not known whether the limit of compact Kähler manifolds admits some special structure; J.-P. Demailly and M. Pˇaun conjectured that, given a complex-analytic family fXt gt 2S of compact complex manifolds such that one of the fibers, Xt0 , is endowed with a Kähler structure, then there exists a countable union S 0 ¨ S of analytic subsets in the base such that Xt admits a Kähler structure for t 2 S n S 0 , [DP04, Conjecture 5.1]; they also guessed that a “natural expectation” is that the remaining fibres, Xt for t 2 S 0 , are in class C of Fujiki, [DP04, p. 1272]. In [Pop13, Pop09, Pop10], D. Popovici studied limits of projective and Moˇıšezon manifolds under holomorphic deformations of complex structures, proving, in particular, that the limit of projective manifolds is Moˇıšezon under some additional conditions either on Hodge numbers or on some special Hermitian metrics called strongly-Gauduchon metrics, [Pop13]. C. LeBrun and Y.S. Poon [LP92], and F. Campana [Cam91] showed that being in class C of Fujiki is not a stable property under small deformations of the complex structures, [LP92, Theorem 1], [Cam91, Corollary 3.13], studying twistor spaces. It is conjectured that being in class C of Fujiki is a closed property under deformations of the complex structure, see, e.g., [Pop11, Standard Conjecture 1.17]. We refer to [Pop11] for a review on the behaviour under holomorphic deformations of properties concerning, e.g., the existence of various types of Hermitian metrics on compact complex manifolds. See also Corollary 2.2, Theorem 4.7 for some results concerning stability or instability of special properties of complex manifolds.
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1 Preliminaries on (Almost-)Complex Manifolds
1.6 Currents and de Rham Homology In this section, we recall the basic notions and results concerning currents on (differentiable) manifolds and de Rham homology: they turn out to be a useful tool to study the geometry of complex manifolds (as an example, we recall F.R. Harvey and H.B. Lawson’s intrinsic characterization of Kähler manifolds by means of currents, [HL83, Proposition 12, Theorem 14], or M.L. Michelsohn’s intrinsic characterization of balanced manifolds by means of currents [Mic82, Theorem 4.7], see also Theorem 4.18, or J.P. Demailly and M. Pˇaun’s characterization of compact complex manifolds in class C of Fujiki by means of Kähler currents[DP04, Theorem 3.4]). We refer, e.g., to [dR84, Chap. 3], [Dem12, Sect. I.2], and [Fed69] (see also [Ale98, Ale10]) for further details. Let X be a m-dimensional oriented differentiable manifold. s For every compact set L X and for every s 2 N, define the semi-norm L on ˚ ^ X as follows: chosen U; x j j 2f1;:::;mg a coordinate chart with U L, and given loc
' D
X
'I d x i1 ^ ^ d x ik 2 ^ X ;
fi1 ;:::;ik gf1;:::;mg i1 <
set Ls .'/ WD sup L
sup
sup
fi1 ;:::;ik gf1;:::;mg .˛1 ;:::;˛m /2Nm ˛1 CC˛m s i1 <
ˇ ˇ ˇ ˇ @˛1 CC˛m 'I ˇ ˇ ˇ .@x 1 /˛1 .@x m /˛m ˇ 2 R :
Consider ^ X endowed with the topology induced by the family of semi-norms Ls , varying L among the compact sets in X , and s 2 N: the manifold X being secondcountable, ^ X has a structure of a Fréchet space. Let ^c X be the topological subspace of ^ X consisting of differential forms with compact support in X . For any k 2 N, the space of currents of dimension k (or degree mk), denoted by Dk X WD Dmk X ; is defined as the topological dual space of ^kc X ; the space D X is endowed with the weak- topology. Two basic examples of currents are the following. • If Z is a (possibly non-closed) k-dimensional oriented compact submanifold of X , then Z 2 Dk X ŒZ WD Z
is a current of dimension k.
1.6 Currents and de Rham Homology
43
• If ' 2 ^k X , then Z T' WD
' ^ 2 Dk X X
is a current of degree k. The exterior differential dW ^ X ! ^C1 X induces a differential on D X by duality: dW D X ! D1 X is defined, for every T 2 Dk X , as d T WD .1/kC1 T .d / : In particular, if Z is a k-dimensional oriented closed submanifold of X , then d ŒZ D .1/mkC1 Œb Z, where b is the boundary operator; if ' 2 ^k X , then d T' D Td ' . By definition, the de Rham homology HdR .X I R/ of X is the homology of the differential complex .D X; d/. By means of a regularization process, [dR84, Theorem 12], (see also [Dem12, Sects. 2.D.3 and 2.D.4],) one can prove, [dR84, Theorem 14], that dR HdR .X I R/ ' H2n .X I R/ :
Since, for every k 2 N, the sheaf AkX of germs of k-forms is a sheaf of CX1 module over a paracompact space (where CX1 denotes the sheaf of germs of smooth functions over X ), and by the Poincaré lemma for forms, see, e.g., [Dem12, I.1.22], one has that 0 ! RX ! AX ; d is a fine (and hence acyclic, see, e.g., [Dem12, Corollary IV.4.19]) resolution of the constant sheaf RX , and hence ker dW ^ X ! ^C1 X L H .X I RX / ' DW HdR .X I R/ ; im .dW ^1 X ! ^ X / see, e.g., [Dem12, IV.6.4]. Analogously, the regularization process [dR84, Theorem 12] allows to prove the analogue of the Poincaré lemma for currents, see, e.g., [Dem12, Theorem I.2.24], k and hence, the sheaf DX of germs of currents of degree k being fine for every k 2 N 1 since it is a sheaf of CX -module over a paracompact space, one has that ;d 0 ! RX ! DX
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1 Preliminaries on (Almost-)Complex Manifolds
is a fine (and hence acyclic, see, e.g., [Dem12, Corollary IV.4.19]) resolution of the constant sheaf RX over X , and hence HL .X I RX / '
ker dW D X ! DC1 X dR DW H2n .X I R/ ; im .dW D1 X ! D X /
see, e.g., [Dem12, IV.6.4]. If X is compact, then it follows that the map T W ^ X ! D X is injective and a quasi-isomorphism of differential complexes: indeed, fixed a Riemannian metric g on X , if ˛ is a -harmonic form (i.e., a d-closed d -closed form), then T˛ .˛/ D k˛k2 . Suppose now that X is a 2n-dimensional manifold endowed with an almostcomplex structure J 2 End.TX/. Considering the induced endomorphisms J 2 End .^ X / and J 2 End ^c X , one can define J 2 End .D X / by duality. In the same way as J 2 End .^ X / defines a bi-graduation on ^ X ˝ C, one has that J 2 End .D X / defines the splitting D X ˝ C D
M
Dp;q X I
.p;q/2N2
note that Dp;q X WD Dnp;nq X is the topological dual of ^p;q X \ ^c X ˝R C , for every .p; q/ 2 N2 .
1.7 Solvmanifolds Nilmanifolds and solvmanifolds provide an important class of examples in nonKähler geometry. Indeed, on the one hand, in studying their properties, one often can reduce to study left-invariant objects on them, which is the same to study linear objects on the corresponding Lie algebra (this allows, for example, to reduce the study of the de Rham cohomology of a nilmanifold to the study of the cohomology of a complex of finite-dimensional vector spaces, [Nom54, Theorem 1]); on the other hand, they do not admit too strong structures, e.g., they do not admit any Kähler structure. In this section, we recall the main definitions and results concerning the theory of nilmanifolds and solvmanifolds, setting also the notation for the following chapters.
1.7.1 Lie Groups and Lie Algebras We briefly recall the notions of Lie groups and Lie algebras and the relations between them; see, e.g., [War83, Kir08, TO97, FOT08], for further results on the subject.
1.7 Solvmanifolds
45
A Lie group (respectively a Lie group of class C k , for k 2 N [ f1; !g, respectively a complex Lie group) is9 a smooth (respectively C k , respectively complex) manifold endowed with a structure of group given by differentiable (respectively C k , respectively holomorphic) maps W G G ! G and ./1 W G ! G. A homomorphism of Lie groups (respectively, of Lie groups of class C k , respectively of complex Lie groups) is a smooth (respectively C k , respectively holomorphic) map being also a homomorphism of groups. Remark 1.15. According to A.M. Gleason’s, and D. Montgomery and L. Zippin’s theorems, [Gle52, MZ52], answering to Hilbert’s fifth problem, every topological Lie group (that is, a Lie group of class C 0 ) admits a unique real-analytic structure with respect to which it is a Lie group of class C ! . A Lie algebra over a field K (with characteristic char K ¤ 2) is a K-vector space g endowed with an operation denoted by the bracket Œ; W g g ! g being bi-K-linear, skew-symmetric (i.e., for every x; y 2 g, it holds Œx; y C Œy; x D 0), and satisfying the Jacobi identity (i.e., for every x; y; z 2 g, it holds ŒŒx; y ; z C ŒŒy; z ; x C ŒŒz; x ; y D 0). An homomorphism of Lie algebras between the Lie algebra g endowed with the bracket Œ; g and the Lie algebra h endowed with the bracket Œ; h is a K-linear map 'W g ! h preserving the brackets (namely, such that ' Œ; g D Œ'./; './h ). Given a K-vector space V , the K-vector space End.V / endowed with the bracket Œx; y WD x ı y y ı x has a structure of Lie algebra over K. Given a Lie algebra g endowed with the bracket Œ; , define the adjoint representation adW g ! End.g/ ;
ad
g 3 x ! adx WD Œx; 2 End.g/ I
in other words, the Jacobi identity states that adW g ! End.g/ is a representation of the Lie algebra g, that is, a homomorphism of Lie algebras. Let G be a Lie group. For any g 2 G, consider the left-translation homomorphism of Lie groups Lg W G ! G defined by Lg .h/ WD g h, where is the group operation on G. An object on G is called G-left-invariant if it is invariant under the actions of left-translations Lg varying g 2 G. The Lie bracket of vector fields yields a structure of Lie algebra on the space of G-left-invariant vector fields on G, see, e.g., [War83, Proposition 3.7]: such a Lie algebra is the Lie algebra naturally associated to the Lie group G. By S. Lie’s and E. Cartan’s theorem, [Lie80, Car30], the functor that associates to every Lie group its naturally associated Lie algebra 9
Let C be a category with finite products, and consider Grp the category of groups; a group-object G in C is an object of C such that Hom.; G/W C ! Grp is a contravariant functor. In such a notation, a Lie group (respectively a Lie group of class C k , for k 2 N [ f1; !g, respectively a complex Lie group) is a group-object in the category of differentiable manifolds with differentiable maps (respectively in the category of manifolds of class C k with maps of class C k , respectively in the category of complex manifolds with holomorphic maps). A homomorphism of Lie groups between G and H is a morphism f W G ! H of manifolds inducing, for every manifold X, a homomorphism of groups between Hom.X; G/ ! Hom.X; H /.
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1 Preliminaries on (Almost-)Complex Manifolds
is an equivalence between the full sub-category of connected simply-connected Lie groups and the full sub-category of finite-dimensional Lie algebras, see, e.g., [Kir08, Corollary 3.44]. ˚ Let g be a Lie algebra. Define the descending central series gŒn n2N of g as (
gŒ0 WD g I gŒn WD g; gŒn1 for n 2 N n f0g
a Lie algebra is called nilpotent if there exists n 2 N such that gŒn D f0g, and the ˚ step of nilpotency of g is defined as nilstep.g/ WD inf n 2 N W gŒn D f0g ; a Lie group is called nilpotent if its associated Lie algebra is nilpotent.˚ Let g be a Lie algebra. Define the descending derived series gfng n2N of g as (
gf0g WD g I gfng WD gfn1g ; gfn1g for n 2 N n f0g
a Lie algebra is called solvable if there exists n 2 N such that gfng D f0g; a Lie group is called solvable if its associated Lie algebra is solvable. One has that g is solvable if and only if Œg; g is nilpotent, see, e.g., [Kir08, Corollary 5.32]; in particular, every nilpotent Lie algebra is also solvable. Let g be a solvable Lie algebra, and consider the adjoint representation adW g 3 x 7! Œx; 2 End.g/. One says that g is completely-solvable (or of type (R)) if, for any x 2 g, all the eigen-values of the endomorphism adx 2 End.g/ are in R. One says that g is of rigid type (or of type (I)) if, for any x 2 g, all the eigen-values of the endomorphism adx 2 End.g/ are in i R, [Vin94, p. 65].
1.7.2 Nilmanifolds and Solvmanifolds A nilmanifold, [Mal49, p. 278], respectively solvmanifold, X D n G is a compact quotient of a connected simply-connected nilpotent, respectively solvable, Lie group G by a co-compact discrete subgroup (see, e.g., [TO97, Definitions 2.1.1 and 3.1.1]). A solvmanifold X D n G is called completely-solvable if its naturally associated Lie algebra is completely-solvable, that is, if, for any g 2 G, all the eigenvalues of Adg 2 End.g/ are real, equivalently, if, for any X 2 g, all the eigenvalues of adX 2 End.g/ are real. In particular, nilmanifolds and solvmanifolds are homogeneous spaces, that is, differentiable manifolds endowed with a transitive action of a Lie group. Recall that, given an homogeneous manifold X together with a transitive action W G ! HomDiff .X I X / of a Lie group G, then X is diffeomorphic to the quotient Hx nG where x 2 X and Hx WD fg 2 G W .g/.x/ D xg is the isotropy group of G at x, see, e.g., [War83, Theorem 3.62].
1.7 Solvmanifolds
47
Remark 1.16 ([Aus61,Has06]). We notice that in the literature, different definitions of solvmanifolds are also considered: for example, L. Auslander defines a solvmanifold as a “homogeneous space of a connected solvable Lie group”, [Aus61, p. 398], see also [Aus73a, Aus73b]; K. Hasegawa defines a solvmanifold X as “a compact homogeneous space of solvable Lie group, that is, a compact differentiable manifold on which a connected solvable (nilpotent) Lie group G acts transitively”, [Has06,
p. 132]; as noticed in [Has06, p. 132], one can assume that X D D GQ , where GQ is a connected simply-connected Lie group (namely, the universal covering group of G) Q Note that a closed subgroup of a solvable nonand D is a closed subgroup of G. nilpotent group is not necessarily discrete; however, every compact solvmanifold in
the Auslander sense has a solvmanifold GQ with discrete isotropy subgroup as a finite covering, [Aus61]. Remark 1.17. Nilmanifolds and solvmanifolds nG are aspherical, that is,
j . nG / D f0g for j 2; (in fact, they are Eilenberg and MacLane spaces of type K. I 1/ with D ). Furthermore, they are uniquely determined, up to diffeomorphism, by their fundamental group, as proven by A.I. Mal’tsev for nilmanifolds, [Mal49, Corollary, p. 293], and by G.D. Mostow for solvmanifolds, [Mos54, Mos57, Theorem A], see also, e.g., [Rag72, Corollary 2, p. 34, Theorem 3.6], [Aus73a, pp. 235, 244], [GOV97, Theorems II.1.3(i) and II.2.6]. Obviously, every nilmanifold is in particular a solvmanifold; on the other hand, there exist solvmanifolds which are not diffeomorphic to any nilmanifold, since their fundamental group is not nilpotent.10 In fact, given a solvmanifold nG , if 1 . nG / is nilpotent, then nG is diffeomorphic to some nilmanifold, see, e.g., [GOV97, Corollary II.2]; if 1 . nG / is Abelian, then nG is diffeomorphic to a torus, [Mos54, Mos57, Theorem 1], see, e.g., [GOV97, Corollary II.2]. More precisely, one can prove that a compact aspherical homogeneous manifold having nilpotent fundamental group is diffeomorphic to some nilmanifold, which is unique up to diffeomorphism, see, e.g., [GOV97, Theorem II.2.7]. Remark 1.18. Note that nilmanifolds and solvmanifolds are parallelizable,11 [TO97, Theorem 2.3.11]. Given a 2n-dimensional solvmanifold X D n G, consider .g; Œ; / the Lie algebra naturally associated to the Lie group G; given a basis fe1 ; : : : ; e2n g of g, Lie algebra structure of g is characterized by the structure constants ˚ kthe c`m `;m;k2f1;:::;2ng R: namely, for any k 2 f1; : : : ; 2ng, dg e k DW
X
k c`m e` ^ em ;
`;m
10
We recall that the following theorem by A.I. Mal’tsev: a group is isomorphic to a discrete co-compact subgroup in a simply-connected connected nilpotent Lie group if and only if it is finitely-generated, nilpotent, and torsion-free, [Mal49, Corollary 1], see, e.g., [Rag72, Theorem 2.18]. 11 Recall that a manifold is called parallelizable if its tangent bundle is trivial.
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1 Preliminaries on (Almost-)Complex Manifolds
˚ where e 1 ; : : : ; e 2n is the dual basis of g of fe1 ; : : : ; e2n g and dg W g ! ^2 g is defined by g 3 ˛ 7! dg ˛.; / WD ˛ .Œ; / 2 ^2 g : To shorten the notation, as in [Sal01], we will refer to a given solvmanifold X D n G writing the structure equations of its Lie algebra: for example, writing (or g WD 04 ; 12; 13 ; ) X WD 04 ; 12; 13 ; we mean that X D n G and there exists a basis of the Lie algebra g naturally asso ˚ ciated to G, let us say fe1 ; : : : ; e6 g, whose dual will be denoted by e 1 ; : : : ; e 6 , such that the structure equations with respect to such basis are 8 d e1 D d e2 D d e3 D d e4 D 0 ˆ ˆ < ; d e 5 D e 1 ^ e 2 DW e 12 ˆ ˆ : 6 d e D e 1 ^ e 3 DW e 13 where we also shorten e AB WD e A ^ e B . The following theorem by A.I. Mal’tsev characterizes the nilpotent Lie algebras g for which the naturally associated connected simply-connected Lie group admits a co-compact discrete subgroup, and hence such that there exists a nilmanifold with g as Lie algebra; (see also, e.g., [Rag72, Theorem 2.12]). Theorem 1.28 ([Mal49, Theorem 7]). In order that a simply-connected connected nilpotent Lie group contain a discrete co-compact Lie group it is necessary and sufficient that the Lie algebra of this group have rational constant structures with respect to an appropriate basis. As regards the existence of discrete co-compact subgroups in solvable Lie groups, one has the following obstruction provided by J. Milnor. (Recall that a Lie group G, with associated Lie algebra g, is called unimodular if, for all X 2 g, it holds tr ad X D 0; see also [Mil76, Lemmas 6.1, 7.1, and 6.3] for other equivalent formulations.) Lemma 1.1 ([Mil76, Lemma 6.2]). Any connected Lie group that admits a discrete subgroup with compact quotient is unimodular and in particular admits a bi-invariant volume form . Dealing with G-left-invariant objects on X , we mean objects induced by objects on G being invariant under the left-action of G on itself given by left-translations. By means of left-translations, G-left-invariant objects will be identified with objects on the Lie algebra g. For example, a G-left-invariant complex structure J 2 End.TX/ on X is uniquely determined by a linear complex structure J 2 End.g/ on g satisfying the integrability condition NijJ D 0, [NN57, Theorem 1.1], where
1.7 Solvmanifolds
49
NijJ .; / WD Œ; C J ŒJ ; C J Œ; J ŒJ ; J 2 ^2 g ˝R g I we will denote the set of G-left-invariant complex structures on X by ˚ C .g/ WD J 2 End .g/ W J 2 D idg and NijJ D 0 : By the Leibniz rule, the map dg W ^1 g ! ^2 g induces a differential operator dW ^ g ! ^C1 g giving a graded differential algebra .^ g ; d/, and hence a differential complex .^ g ; d/, which is usually called the Chevalley and Eilenberg complex of g, [CE48]; we will denote by HdR .gI R/ WD H .^ g ; d/ the cohomology of such a differential complex. In general, on a solvmanifold, the inclusion .^ g ; d/ ,! .^ X; d/ induces an injective map in cohomology, i W HdR .gI R/ ,! HdR .X I R/ (see [Rag72, Remark 7.30], [TO97, Theorem 3.2.10], and compare [CF01, Lemma 9] and Lemma 3.2 for the Dolbeault and, respectively, Bott-Chern cohomologies), which is not always an isomorphism, as the example in [dBT06, Corollary 4.2, Remark 4.3] shows. On the other hand, the following theorem by K. Nomizu says that the de Rham cohomology of a nilmanifold can be computed as the cohomology of the subcomplex of left-invariant forms (some results in this direction have been provided also by Y. Matsushima in [Mat51, Theorems 5 and 6]). Theorem 1.29 ([Nom54, Theorem 1]). Let X D n G be a nilmanifold and denote the Lie algebra naturally associated to G by g. The complex .^ g ; d/ is a minimal model for .^ X; d/. In particular, the map .^ g ; d/ ! .^ X; d/ of differential complexes is a quasi-isomorphism: '
i W HdR .gI R/ ! HdR .X I R/ :
The proof rests on an inductive argument, which can be performed since every nilmanifold can be seen as a principal torus-bundle over a lower dimensional nilmanifold, see [Mal49, Lemma 4], [Mat51, Theorem 3]; see also [Rag72, Corollary 7.28]. A similar result holds also in the case of completely-solvable solvmanifolds, as proven by A. Hattori, as a consequence of the Mostow structure theorem,12 [Mos54, Mos57].
12
We recall the following, referring, e.g., to [TO97, Theorem 1.2]; see also [Rag72, Theorem 3.3, Corollary 3.5] for a different proof. Theorem 1.30 (Mostow Structure Theorem, [Mos54, Mos57, Theorem 2]). Any solvmanifold can be naturally fibred over a torus with a nilmanifold as a fibre. More precisely, let nG be a solvmanifold, and consider the maximal connected nilpotent subgroup N of G. Then (i) N is a closed subgroup in G, (i) N \ is a lattice in N , and (i) N nG is a torus. Hence, one gets the Mostow bundle N \ nN D nN ,! nG ! N nG :
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1 Preliminaries on (Almost-)Complex Manifolds
Theorem 1.31 ([Hat60, Corollary 4.2]). Let X D n G be a completely-solvable solvmanifold and denote the Lie algebra naturally associated to G by g. The map .^ g ; d/ ! .^ X; d/ of differential complexes is a quasi-isomorphism: '
.gI R/ ! HdR .X I R/ : i W HdR
The previous result holds true also for solvmanifolds satisfying the so-called Mostow condition, [Mos54, Theorem 8.2, Corollary 8.1], see also [Rag72, Corollary 7.29]. For some results concerning the de Rham cohomology of (non-necessarily completely-solvable) solvmanifolds, see [Gua07, CF11, Kas13a, Kas12a, CFK13]. We refer also to Sect. 3.1.2 for further classical results concerning the computation of the de Rham and Dolbeault cohomologies of nilmanifolds and solvmanifolds. In some cases, we will see that the study of (properties of) geometric structures on a solvmanifold is reduced to the study of the corresponding (properties of) geometric structures on the associated Lie algebra (see, e.g., Theorem 4.13, Proposition 4.2, Theorem 4.6). To this aim, we need the following trick by F.A. Belgun (see also [FG04, Theorem 2.1]). Lemma 1.2 (F.A. Belgun’s Symmetrization Trick, [Bel00, Theorem 7]). Let X D n G be a solvmanifold, and denote the Lie algebra naturally associated R to G by g. Let be a G-bi-invariant volume form on G such that X D 1, whose existence follows from J. Milnor’s lemma [Mil76, Lemma 6.2]. Up to identifying G-left-invariant forms on X and linear forms over g through left-translations, define the F. A. Belgun’s symmetrization map W ^ X ! ^ g ;
Z .˛/ WD
˛bm .m/ : X
One has that b^ g D idb^ g ; and that d ı D ı d : In particular, the symmetrization map induces a map W .^ X; d/ ! .^ g ; d/ of differential complexes, and hence a map W HdR .X I R/ ! HdR .gI R/ in cohomology. Since b^ g D idb^ g , if the inclusion .^ g ; d/ ,! .^ X; d/ is a quasi-isomorphism (for example, if X is a nilmanifold, by [Nom54, Theorem 1],
(See also [Boc09, Theorem 3.6], which gives a sufficient condition for the Mostow bundle to be a principal bundle.)
1.7 Solvmanifolds
51
or a completely-solvable solvmanifold, by [Hat60, Corollary 4.2]), then the map W .^ X; d/ ! .^ g ; d/ turns out to be a quasi-isomorphism. K. Nomizu’s theorem [Nom54, Theorem 1], A. Hattori’s theorem [Hat60, Corollary 4.2], and F.A. Belgun’s theorem [Bel00, Theorem 7] suggest that nilmanifolds, and, more in general, solvmanifolds, may provide a very useful and interesting class of examples in non-Kähler geometry. On the other hand, another reason for this statement is given by the following result by Ch. Benson and C.S. Gordon, and by K. Hasegawa: it answers the Weinstein and Thurston problem for nilmanifolds, namely, it characterizes the nilmanifolds admitting a Kähler structure. Theorem 1.32 ([BG88, Theorem A]). Let X be a nilmanifold endowed with a symplectic structure ! such that the Hard Lefschetz Condition holds. Then X is diffeomorphic to a torus. Actually, one can prove that any 2n-dimensional nilmanifold X endowed with a 1 2n1 symplectic structure ! such that the map Œ!n1 W HdR .X I R/ ! HdR .X I R/ is an isomorphism is diffeomorphic to a torus, [LO94], see, e.g., [FOT08, Theorem 4.98]. A minimal model proof of Ch. Benson and C.S. Gordon’s theorem [BG88, Theorem A] is due to G. Lupton and J. Oprea, [LO94, Theorem 3.5]. Remark 1.19. For some results on Hard Lefschetz Condition for solvmanifolds, obtained by H. Kasuya, see [Kas13a, Kas09]. Theorem 1.33 ([Has89, Theorem 1, Corollary]). Let X be a nilmanifold. If X is formal, then X is diffeomorphic to a torus. Remark 1.20. For some results on formality for solvmanifolds, obtained by H. Kasuya, see [Kas13a, Kas12d, Kas09]. In particular, since compact Kähler manifolds satisfy the Hard Lefschetz Condition, [Wei58, Théorème IV.5], and are formal, [DGMS75, Main Theorem], it follows that a nilmanifold admits a Kähler structure if and only if it is diffeomorphic to a torus (compare also [Han57, Theorem II, Footnote 1]). More in general, compact completely-solvable Kähler solvmanifolds are tori, as proven by A. Tralle and J. Kedra in [TK97, Theorem 1], solving a conjecture by Ch. Benson and C.S. Gordon, [BG90, p. 972]. In fact, the following result by K. Hasegawa gives a complete characterization of Kähler solvmanifolds. Theorem 1.34 ([Has06, Main Theorem]). Let X be a compact homogeneous space of solvable Lie group, that is, a compact differentiable manifold on which a connected solvable Lie group acts transitively. Then X admits a Kähler structure if and only if it is a finite quotient of a complex torus which has a structure of a complex torus-bundle over a complex torus. In particular, a completely-solvable solvmanifold has a Kähler structure if and only if it is a complex torus. Remark 1.21. For some results on Hodge decomposition and @@-Lemma for solvmanifolds, obtained by H. Kasuya, see [Kas11, Kas12b].
52
1 Preliminaries on (Almost-)Complex Manifolds
Remark 1.22. In this context, another class that could provide several interesting examples is given by complex orbifolds13 of the type XQ D X = G, where X is a complex manifold (possibly, a nilmanifold or a solvmanifold14) and G is a finite group of biholomorphisms of X (compare the Bochner linearization theorem, [Boc45, Theorem 1], see also [Rai06, Theorem 1.7.2]). Orbifolds of such a globalquotient-type have been considered and studied, e.g., by D.D. Joyce in constructing examples of compact seven-dimensional manifolds with holonomy G2 , [Joy96b] and [Joy00, Chaps. 11–12], and examples of compact eight-dimensional manifolds with holonomy Spin.7/, [Joy96a, Joy99] and [Joy00, Chaps. 13–14]. We refer to [Sat56, Bai56, Bai54, Ang13a] for results concerning the de Rham, Dolbeault, and Bott-Chern cohomologies of orbifolds.
Appendix: Low Dimensional Solvmanifolds and Special Structures In this section, we summarize some results concerning low-dimensional solvable Lie algebras, especially focusing on their classification and on the geometric properties of the possibly associated solvmanifolds. As regards nilpotent Lie algebras, we recall that, up to isomorphisms, the nilpotent Lie algebras of dimension less than or equal to 6 are finitely-many; on the other hand, in dimension 7, there are infinitelymany non-isomorphic nilpotent Lie algebras. We refer mainly to [Boc09] for the classification, and to [TO97,Sal01,Mac13] for results on the geometric properties of nilmanifolds and solvmanifolds. (As regards classification of nilpotent Lie algebras up to dimension 7, see also, e.g., [Gon98] and the references therein.)
A.1 Solvmanifolds up to Dimension 4 In Table 1.1, we list the isomorphism classes of solvable Lie algebras of dimension less or equal to 4 and whose connected simply-connected Lie group admit a lattice (at least for some parameter), [Boc09, Sect. 5, Theorem 5.3, Table 1], [Boc09, Sect. 6, Theorem 6.2, Table 2], [Boc09, Table 8]. The notation follows [Mub63c,
13 The notion of orbifold has been introduced by I. Satake in [Sat56], with the name of V-manifold, and has been studied, among others, by W.L. Baily, [Bai56, Bai54]. We recall that a complex orbifold of complex dimension n is a singular complex space whose singularities are locally isomorphic to quotient singularities Cn =G , for finite subgroups G GL.nI C/, [Sat56, Definition 2]. We refer, e.g., to [Joy07, Joy00, Sat56, Bai56, Bai54] for more results concerning complex orbifolds and their cohomology. 14 Recall that nilmanifolds and solvmanifolds are Eilenberg and MacLane spaces of type K. I 1/; in particular, they are not simply-connected.
Completely-solvable Solvable Nilpotent Completely-solvable
.13; 23; 0; 0/ .23; 13; 0; 0/
.24; 34; 0; 0/ 14; p 1 24; .pC1/1 34; 0
g1 3:4 ˚ g1 g03:5 ˚ g1
12 p < 0
Abelian Nilpotent
.0; 0; 0; 0/ .23; 0; 0; 0/
4g1 g3:1 ˚ g1
g4:1 p;p1 g4:5
Solvable
.23; 13; 0/
g03:5
Completely-solvable
.13; 23; 0/
Type Abelian Abelian Abelian Nilpotent
g1 3:4
Conditions
Structure equations .0/ .0; 0/ .0; 0; 0/ .23; 0; 0/
Name g1 2g1 3g1 g3:1
2 1
2 2
4 3
1
1
b1 1 2 3 2
[Boc09, p. 35] [Boc09, p. 35]
[Boc09, p. 34] [Boc09, p. 35]
Lattice Torus Torus Torus [Boc09, Theorem 5.4] [Boc09, p. 25], [TO97, Theorem 1.9] [Boc09, p. 25], [TO97, Theorem 1.9] Torus [Boc09, p. 34] Torus Primary Kodaira surface
Hyper-elliptic surface
Complex?
Torus
X
X
X Kähler
Kähler
X X X X
X
X
X X
X
HLCa ?
X
(continued)
Formal? X Kähler X
Symplectic?
X
Table 1.1 Isomorphism classes of solvable Lie algebras of dimension less or equal to 4 and whose connected simply-connected Lie group admit a lattice (at least for some parameter), [Boc09, Sect. 5, Theorem 5.3, Table 1], [Boc09, Sect. 6, Theorem 6.2, Table 2], [Boc09, Table 8]
1.7 Solvmanifolds 53
a
Solvable
Solvable
p>0
.23; 34; 24; 0/
Type
Completely-solvable
Conditions
Structure equations .2p/1 14; p 1 24 34; 24 p 1 34; 0 .23; 24; 34; 0/ 1
1
1
b1
[Boc09, p. 36]
[Boc09, p. 35]
[Boc09, p. 35]
Lattice
Inoue surface of type S C Secondary Kodaira surface
Inoue surface of type S 0
Complex?
Symplectic?
X
X
X
Formal?
HLCa ?
By [Boc09, Theorem 9.2], a four-dimensional symplectic solvmanifold does not satisfy the Hard Lefschetz Condition if and only if it is a non-torus nilmanifold; in particular, the Hard Lefschetz Condition does not depend on the symplectic structure for four-dimensional solvmanifolds
g04:9
g1 4:8
2p;p g4:6
Name
Table 1.1 (continued)
54 1 Preliminaries on (Almost-)Complex Manifolds
1.7 Solvmanifolds
55
Boc09]. (For the classification of three-dimensional solvable Lie algebras and, respectively, of three-dimensional solvmanifolds, see also, e.g., [TO97, Theorems 1.6 and 1.9]. For results concerning four-dimensional solvmanifolds, see also, e.g., [TO97, Theorem 1.10].)
A.2 Five-Dimensional Solvmanifolds In Table 1.2, we list the 9 isomorphism classes of five-dimensional nilpotent Lie algebras, [Boc09, Table 9], [Mub63a], and the 19 isomorphism classes of fivedimensional solvable non-nilpotent unimodular Lie algebras, [Boc09, Tables 10–14] (a list of the 24, respectively 33 classes of five-dimensional solvable non-nilpotent decomposable, respectively indecomposable Lie algebras has been obtained by G.M. Mubarakzjanov in [Mub63a]).
A.3 Six-Dimensional Nilmanifolds V.V. Morozov classified in [Mor58] the six-dimensional nilpotent Lie algebras, up to isomorphism, in 34 different classes, see also [Mag86], see also [Boc09, Table 15], see also [Gon98, Sect. 3]. In [Sal01], S.M. Salamon identified the 18 classes of six-dimensional nilpotent Lie algebras admitting a linear integrable complex structure, [Sal01, Theorems 3.1–3.3, Proposition 3.4], and provided an estimate of the moduli spaces of complex structures and symplectic structures, [Sal01, Eqs. (28) and (31)], see [Sal01, Sect. 5]: up to equivalence, the linear integrable nonnilpotent complex structures have been classified by L. Ugarte and R. Villacampa in [UV09], the linear integrable Abelian complex structures have been classified by A. Andrada, M.L. Barberis, and I.G. Dotti in [ABDM11], and lastly the linear integrable nilpotent non-Abelian complex structures have been classified by M. Ceballos, A. Otal, L. Ugarte, and R. Villacampa in [COUV11]. The 26 classes of six-dimensional nilmanifolds admitting left-invariant symplectic structures have been classified by M. Goze and Y. Khakimdjanov, [GK96], see also [Sal01, Sect. 5]. There are 16 classes of nilmanifolds admitting both left-invariant complex structures and left-invariant symplectic structures (only one admitting a Kähler structure). Hence, there are five classes of six-dimensional nilmanifolds admitting neither left-invariant complex structures nor left-invariant symplectic structures, [Sal01, Theorem 5.1]: in fact, G.R. Cavalcanti and M. Gualtieri proved that such classes admit left-invariant generalized complex structures, [CG04, Theorems 4.1 and 4.2], see also [CG04, Table 1]. In Table 1.3, we list the classes of six-dimensional nilpotent Lie algebras following the notation [Boc09, Tables 4 and 15], and [Mag86,CFGU97,CG04,Sal01]. Moreover, in Table 1.4, we recall the classification of linear integrable complex structures on six-dimensional nilpotent Lie algebras up to equivalence by M. Ceballos, A. Otal, L. Ugarte, and R. Villacampa, [COUV11].
g5:9
p;2p
g1 5:8
g5:7
p;q;r
g04:9 ˚ g1
g1 4:8 ˚ g1
p;p1 g4:5 ˚ g1 2p;p g4:6 ˚ g1
g2 4:2 ˚ g1
g03:5 ˚ 2g1
g1 3:4 ˚ 2g1
g5:6
g5:5
g5:4
g5:3
g5:2
g5:1
g4:1 ˚ g1
g3:1 ˚ 2g1
5g1
Name
p<0
p>0
12
Conditions
1 35; 15 25; 25; p .2 C p/1 45; 0
25; 0; 35; 45; 0 p 1
pqr ¤ 0; p C q C r D 1
.15; p 1 25; q 1 35; r 1 45; 0/ 1 r q p 1;
Structure equations 0; 0; 0; 0; 0 23; 0; 0; 0; 0 24; 34; 0; 0; 0 35; 45; 0; 0; 0 25; 35; 45; 0; 0 25; 45; 24; 0; 0 24 35; 0; 0; 0; 0 34 25; 35; 0; 0; 0 34 25; 35; 45; 0; 0 13; 23; 0; 0; 0 23; 13; 0; 0; 0 1 14; 24 34; 34; 0; 0 2 14; p 1 24; .p C 1/1 34; 0; 0 .2p/1 14; p1 24 34; 24 p 1 34; 0; 0 23; 24; 34; 0; 0 23; 34; 24; 0; 0 X
cplt-sol
cplt-sol
cplt-sol
sol
cplt-sol
sol
cplt-sol
cplt-sol
sol
–
2
1
–
–
3
0; 2; 4
X
X
X
X
–
X
X
X
X X
X
–
X
X
X
X
X
X
2
cplt-sol
nil
X
X
3
4
nil
nil
X
X
2
2
nil
nil
X
X X
Formal?
Latticeb ?
X
–
b2
3
3
4
5
b1
nil
nil
nil
Ab
Typea
[Boc09, Pr 7.2.3]
[Boc09, Pr 7.2.2, Table 3]
[Boc09, Pr 7.2.1]
[Boc09, p. 36]
[Boc09, p. 36]
[Boc09, p. 36]
[Boc09, p. 36]
[Boc09, Th 7.11]
[Boc09, p. 36]
[Boc09, p. 36]
Note
Table 1.2 Isomorphism classes of five-dimensional nilpotent Lie algebras [Boc09, Table 9], see also [Mub63a], and isomorphism classes of five-dimensional solvable non-nilpotent unimodular Lie algebras [Boc09, Tables 10–14, 3], see [Boc09, Sect. 7]
56 1 Preliminaries on (Almost-)Complex Manifolds
g4 5:23
g1 5:20
g5:19
p;2p2
g05:18
25 35; 15 45; 45; 35; 0
p 1 1525; 15p 1 25; p 1 35 r 1 45; r 1 35 C p 1 45; 0
23 12 15; 25; 2535; 14 45; 0
23.1Cp/1 15; 25; p 1 35; .2p C 2/1 45; 0 23 45; 25; 35; 0; 0
p;p;r
g5:17
g5:16
15 25; 25; 35 q 1 45; q 1 35 C 45; 0
15 25; 25; 35 45; 45; 0
25; 0; 45; 35; 0
15; .1 C 2q/1 25; q 1 35 r 1 45; r 1 35 q 1 45; 0
15 25; 25 35; 35; 13 45; 0
1;q
g1 5:15
g05:14
g5:13
12q;q;r
g3 5:11
p ¤ 1
r ¤0
q¤0
1 q 0; q ¤ 12 ; r ¤ 0
cplt-sol
cplt-sol
cplt-sol
sol
sol
sol
cplt-sol
sol
sol
cplt-sol
–
2
–
1
1
–
1
2
1
–
–
1
–
2
0
–
2
3
0
–
X
X
X
X
X
X
–
X
–
?
?
–
?
?
–
[Boc09, Pr 7.2.16] (continued)
[Boc09, Pr 7.2.16-17, Table 3]
[Boc09, Pr 7.2.16]
[Boc09, Pr 7.2.15, Table 3]
[Boc09, Pr 7.2.11-1213-14, Table 3] ([Har96])
[Boc09, Pr 7.2.10] ([Har96])
[Boc09, Pr 7.2.9, Table 3]
[Boc09, Pr 7.2.7-8, Table 3]
[Boc09, Pr 7.2.5-6, Table 3] ([Har96])
[Boc09, Pr 7.2.4] ([Har96])
1.7 Solvmanifolds 57
b
a
2
14; 24 35; 34 C 25; 0; 0
14; 25; 34 C 35; 0; 0
24 32 15; 34 C 1 1 25; 34 35; 45; 0 3
1 15; 23 25; 35; 35 23 C 12 45; 0
1
Structure equations 23 .2p/1 15; p 1 25 C 35; 25 p 1 35; .4p/1 45; 0 23 "1 45; 35; 25; 0; 0 " 2 f1; 1g
p¤0
Conditions
sol
cplt-sol
cplt-sol
cplt-sol
sol
sol
Typea
2
2
–
–
2
–
b1
1
1
–
–
1
–
b2
X
X
X
Latticeb ?
?
X
–
–
?
–
Formal?
[Boc09, Pr 7.2.20, Table 3]
[DF09, Theorem 3.1, Claim 3, Table 3]
[Boc09, Pr 7.2.19]
[Boc09, Pr 7.2.16]
[Boc09, Pr 7.2.16-18, Table 3]
[Boc09, Pr 7.2.16]
Note
“Ab” means Abelian; “nil” means nilpotent; “cplt-sol” means completely-solvable; “sol” means solvable “X” means that there exists a lattice at least for some values of the parameters; “ ” means that there is no lattice for any value of the parameters
g2;0 5:35
1;1 g5:33
g5:303
4
g5:282
3
g0;" 5:26
p;4p g5:25
Name
Table 1.2 (continued)
58 1 Preliminaries on (Almost-)Complex Manifolds
Name
6g1 g3:1 ˚ 3g1 g5:4 ˚ g1 g5:1 ˚ g1 2g3:1 g6N:4 g6N:5
g5:5 ˚ g1 g4:1 ˚ 2g1 g6:N12 g6:N 3 g6:N1 g6:N 6
g6:N 7 g5:2 ˚ g1 g5:3 ˚ g1 g5:6 ˚ g1
g6:N 8 g6:N 9 g6:N10
g6:N13
g16:N14
] [Boc09]
B01 B02 B03 B04 B05 B06 B07
B08 B09 B10 B11 B12 B13
B14 B15 B16 B17
B18 B19 B20
B21
B22
.0; 0; 0; 0; 0; 0/ .23; 0; 0; 0; 0; 0/ .24 35; 0; 0; 0; 0; 0/ .35; 45; 0; 0; 0; 0/ .23; 0; 0; 56; 0; 0/ .0; 0; 0; 0; 12; 1324/ .0; 0; 0; 0; 13 24; 14 C 23/ .3425; 35; 0; 0; 0; 0/ .24; 34; 0; 0; 0; 0/ .0; 0; 0; 13; 0; 1425/ .0; 0; 0; 13; 23; 12/ .0; 0; 12; 13; 0; 15/ .0; 0; 0; 13; 14 23; 12/ .0; 0; 0; 13; 14; 23/ .25; 35; 45; 0; 0; 0/ .25; 45; 24; 0; 0; 0/ .34 25; 35; 45; 0; 0; 0/ .0; 0; 12; 13; 12; 25/ .0; 0; 12; 13; 23; 15/ .0; 0; 12; 0; 13 24; 14 C 23/ .0; 0; 0; 13; 12; 14 25/ .0; 0; 0; 13; 23; 14 25/
Structure equations [Boc09]
3
3
3 3 3
3 3 3 3
4 4 4 3 3 3
6 5 5 4 4 4 4
b1
5
5
5 5 5
6 5 5 5
7 7 6 8 6 6
15 11 9 9 8 8 8
b2
6
6
6 6 6
8 6 6 6
8 8 6 12 8 8
20 14 10 12 10 10 10
b3
3
3
3 3 3
3 4 3 4
3 3 3 2 3 3
1 2 2 2 2 2 2
nilstep
X X X
X
X X
X X X
X X
X X X X X X
X X X X X X X
X X X X X X X X X X X
Type 2
Type 3 (complex)
X
X
X X X
X X X X
X X X X X X
X X X X X X X
Type 1
X X X
X X X
X X X X X
X X X X X X
Type 0 (symplectic)
M14
M13
M08 M09 M10
M07 – – –
– – M12 M03 M01 M06
– – – – – M04 M05
[Mag86]
Other lists
CFGU19
CFGU18
CFGU13 CFGU14 CFGU15
CFGU12 CFGU21 CFGU16 CFGU22
CFGU09 CFGU17 CFGU20 CFGU07 CFGU10 CFGU11
CFGU01 CFGU8 CFGU03 CFGU06 CFGU02 CFGU04 CFGU05
[CFGU97]
CG22
CG09
CG20a CG19 CG24b
CG11 CG17 CG18 CG16
CG27 CG26 CG25 CG13 CG12 CG10
CG34 CG33 CG32 CG30 CG29 CG28 CG31
[CG04]
S16
S14
S20 S19 S18
S22 S13 S17 S12
S27 S26 S25 S24 S23 S21
S34 S33 S32 S31 S30 S29 S28
[Sal01]
(continued)
.0; 0; 0; 12; 23; 14 35/
.0; 0; 0; 12; 13 C 14; 24/ .0; 0; 0; 12; 14; 13 C 42/ .0; 0; 0; 12; 13 C 42; 14 C 23/ .0; 0; 0; 12; 13; 14 C 35/
.0; 0; 0; 12; 13; 24/ .0; 0; 0; 12; 14; 15/ .0; 0; 0; 12; 14; 24/ .0; 0; 0; 12; 14; 15 C 24/
.0; 0; 0; 0; 12; 14 C 25/ .0; 0; 0; 0; 12; 15/ .0; 0; 0; 0; 12; 15 C 34/ .0; 0; 0; 12; 13; 23/ .0; 0; 0; 12; 13; 14/ .0; 0; 0; 12; 13; 14 C 23/
.0; 0; 0; 0; 0; 0/ .0; 0; 0; 0; 0; 12/ .0; 0; 0; 0; 0; 12 C 34/ .0; 0; 0; 0; 12; 13/ .0; 0; 0; 0; 12; 34/ .0; 0; 0; 0; 12; 14 C 23/ .0; 0; 0; 0; 13C42; 14C23/
Structure equations [Sal01]
Table 1.3 Isomorphism classes of six-dimensional nilpotent Lie algebras [Boc09, Tables 4, 15, and 16], see also [Sal01, Sect. 5], [CG04, Table 1], see [Boc09, Sect. 8]
1.7 Solvmanifolds 59
g6:N15
g6:N17
g6:N16
g6:N11 g16:N18
g1 6:N18
g6:N 2 g6:N19
g6:N 20
g6:N 21
g6:N 22
B24
B25
B26
B27 B28
B29
B30 B31
B32
B33
B34
b
.0; 0; 0; 13; 23; 14 C 25/ .0; 0; 12; 13; 12; 14 25/ .0; 0; 12; 13; 0; 14 25/ .0; 0; 0; 13; 14 23; 15 24/ .0; 0; 12; 13; 14; 23/ .0; 0; 12; 13; 23; 14 25/ .0; 0; 12; 13; 23; 14 C 25/ .0; 0; 12; 13; 14; 15/ .0; 0; 12; 13; 14; 15 23/ .0; 0; 12; 13; 14 23; 15 24/ .0; 0; 12; 23; 24; 15 34/ .0; 0; 12; 23; 24C13; 15 34/
Structure equations [Boc09]
2
2
2
2 2
2
2 2
3
3
3
3
b1
2
2
3
3 3
4
4 4
4
5
5
5
b2
2
2
4
4 4
6
6 6
4
6
6
6
b3
5
5
5
5 5
4
4 4
4
4
4
3
nilstep
X
X
X
X
Type 2
Type 3 (complex)
X
X
X
X X
X
X
X
X
X
Type 1
The structure equations in [CG04] are written as (0, 0, 0, 12, 14, 23 C 24) The structure equations in [CG04] are written as (0, 0, 0, 12, 14 C 23, 13 C 42)
g1 6:N14
B23
a
Name
] [Boc09]
Table 1.3 (continued)
X
X X
X
X X
X
X
X
Type 0 (symplectic)
M22
M21
M20
M02 M19
M18
M11 M18
M16
M17
M15
M14
[Mag86]
Other lists
CFGU32
CFGU31
CFGU30
CFGU28 CFGU29
CFGU26
CFGU23 CFGU26
CFGU27
CFGU25
CFGU24
CFGU19
[CFGU97]
CG07
CG02
CG08
CG01 CG03
CG05
CG04 CG06
CG23
CG14
CG15
CG21
[CG04]
S01
S02
S04
S03 S05
S07
S06 S08
S09
S10
S11
S15
[Sal01]
.0; 0; 12; 13; 14 C 23; 34 C 52/
.0; 0; 12; 13; 14; 34 C 52/
.0; 0; 12; 13; 14 C 23; 24 C 15/
.0; 0; 12; 13; 14; 15/ .0; 0; 12; 13; 14; 23 C 15/
.0; 0; 12; 13; 23; 14 25/
.0; 0; 12; 13; 23; 14/ .0; 0; 12; 13; 23; 14 C 25/
.0; 0; 0; 12; 14 23; 15 C 34/
.0; 0; 0; 12; 14; 15 C 23/
.0; 0; 0; 12; 14; 15 C 23 C 24/
.0; 0; 0; 12; 23; 14 C 35/
Structure equations [Sal01]
60 1 Preliminaries on (Almost-)Complex Manifolds
5
4
3
h8 WD .0; 0; 0; 0; 0; 12/
h9 WD .0; 0; 0; 0; 12; 14 C 25/
h10 WD .0; 0; 0; 12; 13; 14/
12
14
3
h7 WD .0; 0; 0; 12; 13; 23/
11
13
4
4
4
h6 WD .0; 0; 0; 0; 12; 13/
h5 WD .0; 0; 0; 0; 13 C 42; 14 C 23/
h4 WD .0; 0; 0; 0; 12; 14 C 23/
10
09
08
07
06
05
04
03
5
4
h2 WD .0; 0; 0; 0; 12; 34/
01
h3 WD .0; 0; 0; 0; 0; 12 C 34/
6
h1 WD .0; 0; 0; 0; 0; 0/
00
02
b1
Algebra
]
6
7
11
8
9
8
8
9
8
15
b2
8
8
14
12
12
10
10
10
10
20
b3
J WD
J WD
J WD
J WD
J WD
0; 0; ! 12 C ! 11N C ! 12N 0; ! 11N ; ! 12 C ! 12N 0; 0; ! 11N 0; ! 11N ; ! 12N C ! 21N 0; ! 11N ; ! 12 C ! 21N
J WD .0; 0; 0/ J1D WD 0; 0; ! 11N C D ! 22N ; J2D WD 0; 0; ! 12 C ! 11N C ! 12N C D ! 22N ; J1 WD 0; 0; ! 11N C ! 22N J2 WD 0; 0; ! 11N ! 22N J1 WD 0; 0; ! 11N C ! 12N C 14 ! 22N J2D WD 0; 0; ! 12 C ! 11N C ! 12N C D ! 22N ; J1D WD 0; 0; ! 11N C ! 12N C D ! 22N ; J2 WD 0; 0; ! 12 .; D/ J3 WD 0; 0; ! 12 C ! 11N C ! 12N C D ! 22N ;
Complex structure(s)
(continued)
˚ .; D/ 2 .0; x C i y/ 2 R C W y 0 ; 4y 2 < 1 C 4x o n 2 [ .; i y/ 2 R C W 0 < 2 < 12 ; 0 y < 2 n o 2 [ .; i y/ 2 R C W 12 2 < 1; 0 y < 1 2 n o 2 [ .; i y/ 2 R C W 2 > 1; 0 y < 21
D 2 R n f0g D 2 0; 14
ImD > 0
ImD D 1
Conditions . 0; c 0; B 2 C; D 2 C/ 2
S.B; c/ WD c 4 2 jBj2 C 1 c 2 C jBj2 1
Table 1.4 M. Ceballos, A. Otal, L. Ugarte, and R. Villacampa’s classification of linear integrable complex structures on six-dimensional nilpotent Lie algebras up to equivalence [COUV11]
1.7 Solvmanifolds 61
3
3
3
3
3
h11 WD .0; 0; 0; 12; 13; 14 C 23/
h12 WD .0; 0; 0; 12; 13; 24/
h13 WD .0; 0; 0; 12; 13 C 14; 24/
h14 WD .0; 0; 0; 12; 14; 13 C 42/
h15 WD .0; 0; 0; 12; 13 C 42; 14 C 23/
15
16
17
18
19
26
25
24
hC 26 WD .0; 0; 12; 13; 23; 14 C 25/
2
3
3
h16 WD .0; 0; 0; 12; 14; 24/
h 19 WD .0; 0; 0; 12; 23; 14 35/
22
23
21
20
b1
Algebra
]
Table 1.4 (continued)
4
5
5
5
5
5
6
6
b2
6
6
6
6
6
6
8
8
b3 Complex structure(s) J B WD 0; ! 11N ; ! 12 C B ! 12N C jB 1j ! 21N ; J B WD 0; ! 11N ; ! 12 C B ! 12N C jB 1j ! 21N ; J .B; c/ WD 0; ! 11N ; ! 12 C B ! 12N C c ! 21N ; J .B; c/ WD 0; ! 11N ; ! 12 C B ! 12N C c ! 21N ; J1 WD 0; ! 11N ; ! 21N J2c WD 0; ! 11N ; ! 12N C c ! 21N ; .B; c/ J3 WD 0; ! 11N ; ! 12 C B ! 12N C c ! 21N ; J B WD 0; ! 11N ; ! 12 C B ! 12N ; J1 WD 0; ! 13 C ! 13N ; i ! 12N ! 21N J2 WD 0; ! 13 C ! 13N ; i ! 12N ! 21N J1 WD 0; ! 13 C ! 13N ; i ! 11N C i ! 12N ! 21N J2 WD 0; ! 13 C ! 13N ; i ! 11N i ! 12N ! 21N jBj D 1; B ¤ 1
c ¤ jB 1j ; .c; jBj/ ¤ .0; 1/ ; S.B; c/ > 0
c¤1
c ¤ jB 1j ; .c; jBj/ ¤ .0; 1/ ; S.B; c/ D 0
c ¤ jB 1j ; .c; jBj/ ¤ .0; 1/ ; S.B; c/ < 0
ImB ¤ 0
B 2 R n f0; 1g
Conditions . 0; c 0; B 2 C; D 2 C/ 2
S.B; c/ WD c 4 2 jBj2 C 1 c 2 C jBj2 1
62 1 Preliminaries on (Almost-)Complex Manifolds
1.7 Solvmanifolds
63
A.4 Six-Dimensional Solvmanifolds The six-dimensional solvable non-decomposable Lie algebras were classified by G.M. Mubarakzjanov, [Mub63b, CS05], and by P. Turkowski, [Tur90]; see also [Boc09, Sect. 8]. In addition to [Boc09, Table 15], which lists the six-dimensional nilpotent non-decomposable Lie algebras, [Mor58], we refer to [Boc09, Tables 17/33], and also [Mac13, Table A.1], for a list of the six-dimensional unimodular solvable non-nilpotent non-decomposable Lie algebras; see [Boc09, Sect. 8]. The Betti numbers of six-dimensional unimodular solvable non-nilpotent Lie algebras can be found in [Mac13, Table 1] (see also [Boc09, Tables 34/36] for the first Betti numbers). See [Boc09, Tables 5–6] for a list of decomposable non-nilpotent solvmanifolds, together with their Betti numbers and results concerning formality and symplectic structures. In [CM12], six-dimensional non-completely-solvable almost-Abelian solvmanifolds for which Mostow condition does not hold are investigated; in particular, see [CM12, Table 1]. The six-dimensional unimodular solvable non-nilpotent Lie algebras admitting a symplectic structure are listed in [Mac13, Theorem 1], and their symplectic structures are listed in [Mac13, Table B.1]. In [Boc09, Sect. 9] and [Mac13], the Hard Lefschetz Condition on solvmanifolds has been investigated; in particular, [Boc09, Table 7] summarizes the relations between formality, Hard Lefschetz Condition, and even odd-degree Betti numbers, providing some known examples, see also [IRTU03, Theorem 3.1, Table 1] (see also [FMS03,dBT06,Kas13a,Kas09,Kas11,Kas12d] for examples and results on cohomologically-Kähler manifolds); furthermore, in [Mac13, Theorem 4], M. Macrì provided three solvmanifolds endowed with symplectic structures that / p;1;1 satisfy the Hard Lefschetz Condition, more precisely, G5:7 R (see also
3 [Boc09, Theorem 9.1]), and G3:4 R , and n.G3:4 G3:4 / .
Chapter 2
Cohomology of Complex Manifolds
Abstract In this chapter, we study cohomological properties of compact complex manifolds. In particular, we are concerned with studying the Bott-Chern cohomology, which, in a sense, constitutes a bridge between the de Rham cohomology and the Dolbeault cohomology of a complex manifold. In Sect. 2.1, we recall some definitions and results on the Bott-Chern and Aeppli cohomologies, see, e.g., Schweitzer (Autour de la cohomologie de Bott-Chern, arXiv:0709.3528 [math.AG], 2007), and on the @@-Lemma, referring to Deligne et al. (Invent. Math. 29(3):245–274, 1975). In Sect. 2.2, we provide an inequality à la Frölicher for the Bott-Chern cohomology, Theorem 2.13, which also allows to characterize the validity of the @@-Lemma in terms of the dimensions of the Bott-Chern cohomology groups, Theorem 2.14; the proof of such inequality is based on two exact sequences, firstly considered by J. Varouchas in (Propriétés cohomologiques d’une classe de variétés analytiques complexes compactes, Séminaire d’analyse P. Lelong-P. Dolbeault-H. Skoda, années 1983/1984, Lecture Notes in Math., vol. 1198, Springer, Berlin, 1986, pp. 233–243). Finally, in Appendix: Cohomological Properties of Generalized Complex Manifolds, we consider how to extend such results to the symplectic and generalized complex contexts.
2.1 Cohomologies of Complex Manifolds The Bott-Chern cohomology and the Aeppli cohomology provide important invariants for the study of the geometry of compact (especially, non-Kähler) complex manifolds. These cohomology groups have been introduced by R. Bott and S.S. Chern in [BC65], and by A. Aeppli in [Aep65], and hence studied by many authors, e.g., B. Bigolin [Big69, Big70] (both from the sheaf-theoretic and from the analytic viewpoints), A. Andreotti and F. Norguet [AN71] (to study cycles of algebraic manifolds), J. Varouchas [Var86] (to study the cohomological properties of a certain class of compact complex manifolds), M. Abate [Aba88] (to study annular bundles), L. Alessandrini and G. Bassanelli [AB96] (to investigate the D. Angella, Cohomological Aspects in Complex Non-Kähler Geometry, Lecture Notes in Mathematics 2095, DOI 10.1007/978-3-319-02441-7__2, © Springer International Publishing Switzerland 2014
65
66
2 Cohomology of Complex Manifolds
properties of balanced metrics), S. Ofman [Ofm85a, Ofm85b, Ofm88] (in view of applications to integration on analytic cycles), S. Boucksom [Bou04] (in order to extend divisorial Zariski decompositions to compact complex manifolds), J.-P. Demailly [Dem12] (as a tool in complex geometry), M. Schweitzer [Sch07] (in the context of cohomology theories), L. Lussardi [Lus10] (in the non-compact Kähler case), R. Kooistra [Koo11] (in the framework of cohomology theories), J.-M. Bismut [Bis11b, Bis11a] (in the context of Chern characters), L.-S. Tseng and S.-T. Yau [TY11] (in the framework of generalized geometry and type II String Theory). In this preliminary section, we recall the basic notions and classical results concerning cohomologies of complex manifolds. More precisely, we recall the definitions of the Bott-Chern and Aeppli cohomologies, and some results on Hodge theory, referring to [Sch07], see also [Dem12, Sects. VI.8.1 and VI.12.1]; then, we recall the notion of @@-Lemma, referring to [DGMS75].
2.1.1 The Bott-Chern Cohomology Let X be a complex manifold. Definition 2.1 ([BC65]). The Bott-Chern cohomology of a complex manifold X is the bi-graded algebra ; HBC .X / WD
ker @ \ ker @ im @@
:
Unlike in the case of the Dolbeault cohomology groups, for every .p; q/ 2 N2 , the conjugation induces an isomorphism p;q
'
q;p
HBC .X / ! HBC .X / : Furthermore, since ker @ \ ker @ ker d and im @@ im d, one has the natural map of graded C-vector spaces M
HBC .X / ! HdR .X I C/ ; p;q
pCqD
and, since ker @ \ ker @ ker @ and im @@ im @, one has the natural map of bi-graded C-vector spaces ; HBC .X / ! H@; .X / :
2.1 Cohomologies of Complex Manifolds
67
In general, even for compact complex manifolds, these maps are neither injective nor surjective: see, e.g., the examples in [Sch07, Sect. 1.c] or in Sect. 3.2.4. A case of special interest is when X is a compact complex manifold satisfying the @@-Lemma, namely, the property that every @-closed @-closed d-exact form ; is also @@-exact, [DGMS75], that is, the natural map HBC .X / ! HdR .X I C/ is injective (we recall that compact Kähler manifolds and, more in general, manifolds in class C of Fujiki, [Fuj78], that is, compact complex manifolds admitting a proper modification from a Kähler manifold, satisfy the @@-Lemma, [DGMS75, Lemma 5.11, Corollary 5.23]; we refer to Sect. 2.1.3 for further details). In fact, we recall the following result. Theorem 2.1 ([DGMS75, Lemma 5.15, Remarks 5.16, 5.21]). Let X be a compact complex manifold. If X satisfies the @@-Lemma, then the natural maps M
HBC .X / ! HdR .X I C/ p;q
and
; HBC .X / ! H@; .X /
pCqD
induced by the identity are isomorphisms. Proof. We provide here a different proof, which uses [AT13b, Theorem A], see Theorem 2.13. More precisely, we recall that, by [AT13b, Theorem A] and [Sch07, Sect. 2.c], we have that, for any compact X of complex dimensionn, for manifold P complex p;q np;nq .X / 2 any k 2 Z, the inequality pCqDk dimC HBC .X / C dimC HBC P p;q k dim H .X / 2 dim H .X I C/ holds. C C pCqDk dR @ now that X satisfies the @@-Lemma, namely, the natural map L Suppose p;q H pCqD BC .X / ! HdR .X I C/ induced by the identity is injective. In particular, P p;q k for any k 2 Z, it follows that pCqDk dimC HBC .X / dimC HdR .X I C/. P p;q k Hence, one gets that pCqDk dimC HBC .X / D dimC HdR .X I C/ < C1. In L p;q .X I C/ induced by the identity particular, the natural map pCqD HBC .X / ! HdR is actually an isomorphism. P p;q k Furthermore, one gets also that dimC HdR .X I C/ D pCqDk dimC H@ .X / and P P p;q p;q hence pCqDk dimC HBC .X / D pCqDk dimC H@ .X / < C1. Therefore, to ; .X / ! HdR .X I C/ finish the proof, it suffices to prove that, if the natural map HBC ; induced by the identity is injective, then also the natural map HBC .X / ! H@; .X / induced by the identity is injective. ; Suppose that the natural map HBC .X / ! HdR .X I C/ induced by the identity p;q p;q1 p;q is injective. Let ˛ D @ˇ 2 ^ X be a @-closed @-closed @-exact form. In particular, ˇ p;q1 2 ^p;q1 X is @@-closed. Consider d ˇ p;q1 2 ^pC1;q1 X ˚ ^p;q X : it is a @-closed @-closed d-exact form. Hence, by the hypothesis, the form d ˇ p;q1 is also @@-exact. Therefore there exist p;q2 2 ^p;q2 X and p1;q1 2 ^p1;q1 X such that d ˇ p;q1 D @@ p;q2 C @@ p1;q1 . In particular, it follows ; that ˛ p;q D @ˇ p;q1 D @@ p1;q1 . Then the natural map HBC .X / ! H@; .X / induced by the identity is injective. t u
68
2 Cohomology of Complex Manifolds
As for the de Rham and the Dolbeault cohomologies, a Hodge theory can be developed also for the Bott-Chern cohomology for compact complex manifolds: we recall here some results, referring to [Sch07, Sect. 2] (see also [Big69, Sect. 5], and [Lus10]). Suppose that X is a compact complex manifold. Fix a Hermitian metric on X , and define the differential operator Q BC WD
@@ C @ @ @ @ C @ @ @@ @@ C @@ @ @ C@ @C@ @ ;
see [KS60, Proposition 5] (where it is used to prove the stability of the Kähler property under small deformations of the complex structure), and also [Sch07, Sect. 2.b], [Big69, Sect. 5.1]. One has the following result. Theorem 2.2 ([KS60, Proposition 5], see also [Sch07, Sect. 2.b]). Let X be a compact complex manifold endowed with a Hermitian metric. The operator Q BC is a 4th order self-adjoint elliptic differential operator, and ker Q BC D ker @ \ ker @ \ ker @ @ :
Therefore, as a consequence of the general theory of self-adjoint elliptic differential operators, see, e.g., [Kod05, p. 450], the following result holds. Theorem 2.3 ([Sch07, Théorème 2.2], [Sch07, Corollaire 2.3]). Let X be a compact complex manifold, endowed with a Hermitian metric. Then there exist an orthogonal decomposition ^; X D ker Q BC ˚ im @@ ˚ im @ C im @ and an isomorphism ; .X / ' ker Q BC : HBC
In particular, the Bott-Chern cohomology groups of X are finite-dimensional C-vector spaces. Another consequence of general results in spectral theory, see, e.g., [KS60, Theorem 4], [Kod05, Theorem 7.3], is the semi-continuity property for the dimensions of the Bott-Chern cohomology. Theorem 2.4 ([Sch07, Lemme 3.2]). Let fXt gt 2B a complex-analytic family of compact complex manifolds. Then, for every .p; q/ 2 N2 , the function p;q
B 3 t 7! dimC HBC .Xt / 2 N is upper-semi-continuous.
2.1 Cohomologies of Complex Manifolds
69
By using the Kähler identities (in particular, the fact that D and that @ @ C @@ D 0 D @ @ C @@ ), one can prove that, on a compact Kähler manifold,
2 Q BC D C @ @ C @ @ ;
[KS60, Proposition 6], [Sch07, Proposition 2.4]. h i Indeed, recall that D and that @ @ D i ; @ @ D i @ @ D i @ h i ; @ D @@ , and hence @ @ D @@ and @ @ D @@ ; therefore 2
D D
@@ @@ C @@ @ @ C @ @@@ C @ @@ @
D @@@ @ @@ @ @ @ @@@ @ @ @@
D
@@@ @ C @@ @@ C @@ @@ C @ @ @@
D
@@@ @ C @ @@ @ C @ @@ @ C @ @ @@
D Q BC @ @ @ @ : Hence ker Q BC D ker D ker ; in particular, it follows that, on a compact Kähler manifold, the de Rham cohomology, the Dolbeault cohomology, and the Bott-Chern cohomology are isomorphic (actually, since the @@-Lemma holds on every compact Kähler manifold, one gets an isomorphism that does not depend on the choice of the Hermitian metric).
2.1.2 The Aeppli Cohomology Let X be a complex manifold. Dualizing the definition of the Bott-Chern cohomology, one can define another cohomology on X : the Aeppli cohomology. Definition 2.2 ([Aep65]). The Aeppli cohomology of a complex manifold X is the ; .X /-module bi-graded HBC HA; .X / WD
ker @@ im @ C im @
:
As for the Bott-Chern cohomology, the conjugation induces, for every .p; q/ 2 N2 , the isomorphism p;q
'
q;p
HA .X / ! HA .X / :
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2 Cohomology of Complex Manifolds
Furthermore, since ker d ker @@ and im d im @ C im @, one has the natural map of graded C-vector spaces HdR .X I C/ !
M
p;q
HA .X / ;
pCqD
and, since ker @ ker @@ and im @ im @ C im @, one has the natural map of bi-graded C-vector spaces H@; .X / ! HA; .X / I as we have noted for the Bott-Chern cohomology, such maps are, in general, neither injective nor surjective, but they are isomorphisms whenever X is compact and satisfies the @@-Lemma, [DGMS75, Lemma 5.15, Remarks 5.16, 5.21], and hence, in particular, if X is a compact complex manifold admitting a Kähler structure, [DGMS75, Lemma 5.11], or if X is a compact complex manifold in class C of Fujiki, [DGMS75, Corollary 5.23]. Remark 2.1. On a compact Kähler manifold X , the associated .1; 1/-form ! of 2 .X I R/. For general Hermitian the Kähler metric defines a non-zero class in HdR manifolds, special classes of metrics are often defined in terms of closedness of powers of !, so they define classes in the Bott-Chern or Aeppli cohomology groups (e.g., a Hermitian metric on a complex manifold of complex dimension n is said balanced if d ! n1 D 0 [Mic82], pluriclosed if @@! D 0 [Bis89], astheno-Kähler if @@! n2 D 0 [JY93, JY94], Gauduchon if @@! n1 D 0 [Gau77]). (Note that, they define possibly the zero class in the Bott-Chern or Aeppli cohomologies: for the balanced case, see [FLY12, Corollary 1.3], where it is shown that, for k 2, the complex structures on ]kj D1 S3 S3 constructed from the conifold transitions admit balanced metrics.) We refer to [Sch07, Sect. 2.c] for the following results, concerning Hodge theory for the Aeppli cohomology on compact complex manifolds. Suppose that X is a compact complex manifold. Once fixed a Hermitian metric on X , one defines the differential operator Q A WD @@ C@@ C @@ @@ C @@ @@ C @@ @@ C @@ @@ ; which turns out to be a 4th order self-adjoint elliptic differential operator such that ker Q A D ker @@ \ ker @ \ ker @ :
Hence one has an orthogonal decomposition ^; X D ker Q A ˚ im @ C im @ ˚ im @@
2.1 Cohomologies of Complex Manifolds
71
from which one gets an isomorphism HA; .X / ' ker Q A I in particular, this proves that the Aeppli cohomology groups of a compact complex manifold are finite-dimensional C-vector spaces. Furthermore, as for the Bott-Chern cohomology, if fXt gt 2B is a complex-analytic family of compact complex manifolds, with B a complex manifold, then, for every p;q .p; q/ 2 N2 , the function B 3 t 7! dimC HA .Xt / 2 N is upper-semi-continuous. Once again, whenever X is a compact Kähler manifold, by using the Kähler identities, one has 2 Q A D C @@ C @@ I h i indeed, recall that D and that @ @ D i ; @ @ D i @ @ D i @ h i ; @ D @@ , and hence @ @ D @@ ; therefore 2
D D
@@ @@ C @@ @ @ C @ @@@ C @ @@ @
D @@@ @ @@ @ @ @ @@@ @ @ @@ D
@@@ @ C @@ @@ C @@ @@ C @ @ @@
D Q A @@ @@ :
In particular, it follows that, on a compact Kähler manifold, ker Q A D ker D ker , and hence the de Rham cohomology, the Dolbeault cohomology, and the Aeppli cohomology are isomorphic (actually, since the @@-Lemma holds on every compact Kähler manifold, one gets an isomorphism that does not depend on the choice of the Hermitian metric). In fact, since ker Q BC D ker @ \ ker @ \ ker @ @ and ker Q A D ker @@ \ ker @ \ ker @ , one has the following isomorphism between the Bott-Chern cohomology and the Aeppli cohomology. Theorem 2.5 ([Sch07, Sect. 2.c]). Let X be a compact complex manifold of complex dimension n. For any .p; q/ 2 N2 , the Hodge--operator associated to a Hermitian metric on X induces an isomorphism, p;q
'
nq;np
W HBC .X / ! HA
.X / ;
between the Bott-Chern and the Aeppli cohomologies. Remark 2.2. We refer to [Dem12, Sect. VI.12], [Sch07, Sect. 4], [Koo11, Sects. 3.2 and 3.5] for more details on the sheaf-theoretic interpretation of the Bott-Chern and Aeppli cohomologies.
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2 Cohomology of Complex Manifolds
We just recall that, for any .p; q/ 2 N2 , by defining the complex LX p;q ; dLX p;q of sheaves as pr ı d L pr ı d L r;s r;s LX p;q ; dLX p;q W A0;0 rCsD1 AX ! ! rCsDpCq2 AX X ! r
@@
!
L
r;s rCsDpCq AX rp; sq
d
!
L
r
rCsDpCq rp; sq
Ar;s X ! ;
2 where Ar;s X denotes the sheaf of germs of .r; s/-forms over X for .r; s/ 2 N and pr denotes the projection onto the appropriate space, and since, for every k 2 N, the sheaves LkX p;q are fine (indeed, they are sheaves of CX1 ˝R C -modules over a para-compact space), one has, see, e.g., [Dem12, Corollary IV.4.19, (IV.12.9)],
p;q ' HBC .X / HpCq1 X I LX p;q ; dLX p;q and p1;q1 ' HA .X / : HpCq2 X I LX p;q ; dLX p;q We note also that, by the Poincaré lemma (see, e.g., [Dem12, I.1.22, Theorem I.2.24]) and by the Dolbeault and Grothendieck lemma (see, e.g., [Dem12, I.3.29]), one gets M. Schweitzer’s lemma [Sch07, Lemme 4.1], and therefore, by [Dem12, r;s Corollary IV.12.6], one gets that, for any .r; s/ 2 N2 , the natural map Ar;s X ! DX , r;s where DX denotes the sheaf of germs of .r; s/-currents over X , induces the isomorphisms, [Sch07, Sect. 4.d], [Dem12, Lemma VI.12.1], [Koo11, Proposition 3.4.1], ker @W Dp;q X ! DpC1;q X \ ker @W Dp;q X ! Dp;qC1 X p;q HBC .X / ' ; im @@W Dp1;q1 X ! Dp;q X and p1;q1
HA
.X /
ker @@W Dp1;q1 X ! Dp;q X ; ' im .@W Dp2;q1 X ! Dp1;q1 X / C im @W Dp1;q2 X ! Dp1;q1 X
that is, the Bott-Chern and Aeppli cohomologies of a compact complex manifolds can be computed both by using currents and by using just differential forms. 2 Finally, we recall that, for any .p; q/ 2 N , by defining the complex BX p;q ; dBX p;q of sheaves as (for simplicity of notation, let us assume p < q)
2.1 Cohomologies of Complex Manifolds
73
BX p;q ; dBX p;q W
C
? C ?? ?? ?? ??
OX
@
/ ˝1 X
@
@
/
/ ˝ p1 X
˚
˚
OX
/ ˝1 X
/
@
@
0
/ 0
/ 0
˚
/ ˝ q1 X @
/ 0 0
0
where OX WD ˝X0 , respectively OX WD ˝ X , denotes the sheaf of germs of holomorphic, respectively anti-holomorphic, functions over X , and ˝Xk , respectively k
˝ X , for k 2 N, denotes the sheaf of germs of holomorphic, respectively antiholomorphic, k-forms over X , one has that the natural map BX p;q ; dBX p;q ! LX p;q ; dLX p;q of complexes of sheaves is a quasi-isomorphism, [Sch07, Sect. 4.b], and then p;q HBC .X / ' HpCq X I BX p;q ; dBX p;q :
2.1.3 The @@-Lemma Let X be a compact complex manifold, and consider its complex de Rham HdR .X I C/, Dolbeault H@; .X /, conjugate Dolbeault H@; .X /, Bott-Chern ; HBC .X /, and Aeppli HA; .X / cohomologies. The identity map induces the following natural maps of (bi-)graded C-vector spaces: ; HBC .X / KK s KKK s s s KKK s s s KK s yss % ; H@; .X / HdR .X I C/ H@ .X / KKK s KKK ss ss KKK s s K% yss ; HA .X /
74
2 Cohomology of Complex Manifolds
In general, these maps are neither injective nor surjective: see, e.g., the examples in [Sch07, Sect. 1.c] or in Sect. 3.2.4. By [DGMS75, Lemma 5.15, Proposition 5.17], it turns out that, if one of the above map is an isomorphism, then all the maps are isomorphisms, [DGMS75, Remark 5.16]; this is encoded in the notion of @@-Lemma, which can be introduced in the more general setting of bounded double complexes of vector spaces. We start by recalling the following general result by P. Deligne, Ph.A. Griffiths, J. Morgan, and D.P. Sullivan, [DGMS75] (compare also [AT13a, Lemma 1.4]). Proposition 2.1 ([DGMS75, Lemma 5.15]). Let K ; ; d0 ; d00 be a bounded double complex of vector spaces (or, more in general, of objects of any Abelian category), and let .K ; d/ be the associated single complex, where d WD d0 C d00 . For each h 2 N, the following conditions are equivalent: .a/h .b/h .c/h
ker d0 \ ker d00 \ im d D im d0 d00 in K h ; 0 00 0 00 h ker d00 \ im d0 D im d0 d00 and ker d \ im0 d00 D imh d d in K ; 0 00 0 00 ker d \ ker d \ im d C im d D im d d in K ;
.a /h1 .b /h1 .c /h1
im d0 C im d00 C ker d D ker d0 d00 in K h1 ; 0 00 im d00 C ker d0 D ker d0 d00 and im d0 d00 in K h1 ; d C ker0 d00 D ker 0 00 0 00 h1 im d C im d C ker d \ ker d D ker d d in K .
The above equivalent conditions define the validity of the d0 d00 -Lemma for a double complex. Definition 2.3 ([DGMS75]). Let K ; ; d0 ; d00 be a bounded double complex of vector spaces (or, more in general, of objects of any Abelian category), and let .K ; d/ be the complex, where d WD d0 C d00 . One says that associated simple ; 0 00 0 00 K ; d ; d satisfies the d d -Lemma if, for every h 2 N, ker d0 \ ker d00 \ im d D im d0 d00
in K h ;
equivalently, if, for every h 2 N, the equivalent conditions in [DGMS75, Lemma 5.15] hold. The following result by P. Deligne, Ph.A. Griffiths, J. Morgan, and D.P. Sullivan, [DGMS75], gives a characterization for the validity of the d0 d00 -Lemma. Theorem 2.6 ([DGMS75, Proposition 5.17]). Let K ; ; d0 ; d00 be a bounded double complex of vector spaces, and let .K ; d/ be the associated simple complex, where d WD d0 C d. The following conditions are equivalent: (i) K ; ; d0 ; d00 satisfies the d0 d00 -Lemma; (ii) K ; is a sum of double complexes of the following two types (see Figure 2.1): (dots) complexes which have only a single component, with d0 D 0 and d00 D 0;
2.1 Cohomologies of Complex Manifolds
75
Fig. 2.1 Diagrams of (a) a dot and of (b) a square in a double complex
(squares) complexes which are a square of isomorphisms, K p1;q O d00
d0
d00
'
K p1;q1
/ K p;q O
d0
'
/ K p;q1
(iii) the spectral sequence defined by the filtration associated to either degree (denoted by 0 F or 00 F ) degenerates at E1 (namely, E1 D E1 ) and, for h every h 2 N, the two induced filtrations are h-opposite on HdR .X I C/, i.e., 0
'
h F p ˚ 00 F q ! HdR .X I C/ for p C q 1 D h.
In particular, we are interested in dealingwith compact complex manifolds X , for which one considers the double complex ^; X; @; @ . Definition 2.4 ([DGMS75]). A compact complex manifold X is said to satisfy the ; @@-Lemma if ^ X; @; @ satisfies the @@-Lemma, namely, if ker @ \ ker @ \ im d D im @@ ; ; .X / ! HdR .X I C/ of graded C-vector that is, in other words, if the natural map HBC spaces induced by the identity is injective.
Remark 2.3. Let X be a compact complex manifold. By considering the differential operator dc WD i @ @ ; one can say that X satisfies the d dc -Lemma, by definition, if im d \ ker dc D im d dc :
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2 Cohomology of Complex Manifolds
Since d dc D 2 i @@, and @ D
1 2
.d C i dc / and @ D
ker d \ ker dc D ker @ \ ker @
and
1 2
.d i dc /, one has im d dc D im @@ I
and hence X satisfies the d dc -Lemma if and only if X satisfies the @@-Lemma. For compact complex manifolds, P. Deligne, Ph.A. Griffiths, J. Morgan, and D.P. Sullivan’s characterization [DGMS75, Proposition 5.17] is rewritten as follows. Theorem 2.7 ([DGMS75, 5.21]). A compact complex manifold X satisfies the @@Lemma if and only if (i) the Hodge and Frölicher spectral sequence degenerates at ; the first step (that is, E1 ' E1 ), and (ii) the natural filtration on ^ X; @; @ k .X I C/ (that induces, for every L k 2 N, a Hodge structure of weight k on HdR k k p k q N is, HdR .X I C/ D pCqDk F HdR .X I C/ \ FLHdR .X I C/, where F HdR .X I C/ 1 ;2 p;q is the filtration induced by F ^ X WD X on HdR .X I C/ and p; q ^ .X I C/ is the conjugated filtration to F HdR .X I C/). FN HdR
Another characterization for the validity of the @@-Lemma, in terms of the dimensions of the Bott-Chern cohomology, will be given in Theorem 2.14. Actually, as already mentioned, if a compact complex manifold satisfies the @@Lemma, then all the natural maps between cohomologies induced by the identity turn out to be isomorphisms. Theorem 2.8 ([DGMS75, Lemma 5.15, Remarks 5.16, 5.21]). A compact complex manifold X satisfies the @@-Lemma if and only if all the natural maps ; HBC .X / KKK s s KKK s s s KKK ss s K% s ys .X I C/ H@; .X / HdR H@; .X / KKK s KKK ss ss KKK s s K% yss ; HA .X /
induced by the identity are isomorphisms. We recall that if X is a compact complex manifold endowed with a Kähler structure, then X satisfies the @@-Lemma, [DGMS75, Lemma 5.11]. Moreover, one has the following result. Theorem 2.9 ([DGMS75, Theorem 5.22]). Let X and Y be compact complex manifolds of the same dimension, and let f W X ! Y be a holomorphic birational map. If X satisfies the @@-Lemma, then also Y satisfies the @@-Lemma.
2.1 Cohomologies of Complex Manifolds
77
Indeed, one has that, if X and Y are complex manifolds of the same dimension, and W X ! Y is a proper surjective holomorphic map, then the maps
W HdR .Y I C/ ! HdR .X I C/
and
W H@; .Y / ! H@; .X /
induced by W X ! Y are injective, see, e.g., [Wel74, Theorem 3.1]; then one can use the characterization in [DGMS75, 5.21]. In particular, it follows that Moˇıšezon manifolds (that is, compact complex manifolds X such that the degree of transcendence over C of the field of meromorphic functions over X is equal to the complex dimension of X , [Moı66], equivalently, compact complex manifolds admitting a proper modification1 from a projective manifold, [Moı66, Theorem 1]), and, more in general, manifolds in class C of Fujiki (that is, compact complex manifolds admitting a proper modification from a Kähler manifold, [Fuj78]) satisfy the @@-Lemma. Corollary 2.1 ([DGMS75, Lemma 5.11, Corollary 5.23]). The @@-Lemma holds for compact Kähler manifolds, for Moˇıšezon manifolds, and for manifolds in class C of Fujiki. Remark 2.4. In [Hir62], H. Hironaka provided an example of a non-Kähler Moˇıšezon manifold of complex dimension 3 with arbitrary small deformations being projective (in fact, as proven by D. Popovici, the limit of a punctured-disk family of projective manifolds is Moˇıšezon under additional conditions, [Pop13]; see also [Pop09, Theorem 1.1], [Pop10, Theorem 1.1]); in particular, H. Hironaka’s manifold provides an example of a non-Kähler manifold satisfying the @@-Lemma. Studying twistor spaces, C. LeBrun and Y.S. Poon, and F. Campana, showed that being in class C of Fujiki is not a stable property under small deformations of the complex structures, [LP92, Theorem 1], [Cam91, Corollary 3.13]; since the property of satisfying the @@-Lemma is stable under small deformations of the complex structure, Corollary 2.2, or [Voi02, Proposition 9.21], or [Wu06, Theorem 5.12], or [Tom08, Sect. B], C. LeBrun and Y.S. Poon’s, and F. Campana’s, result yields examples of compact complex manifolds satisfying the @@-Lemma and not belonging to class C of Fujiki. Remark 2.5. For some results on Hodge decomposition and @@-Lemma for solvmanifolds, obtained by H. Kasuya, see [Kas11, Kas12b]. Finally, we recall the following obstructions to the existence of complex structures satisfying the @@-Lemma on a compact (differentiable) manifold. Theorem 2.10 ([DGMS75, Main Theorem, Corollary 1]). Let X be a compact manifold. If X admits a complex structure such that the @@-Lemma holds, then the differential graded algebra .^ X; d/ is formal. In particular, all the Massey products of any order are zero. We recall that a proper holomorphic map f W X ! Y from the complex manifold X to the complex manifold Y is called a modification if there exists a nowhere dense closed analytic subset B Y such that f bXnf 1 .B/ W X n f 1 .B/ ! Y n B is a biholomorphism.
1
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2 Cohomology of Complex Manifolds
Indeed, if X satisfies the @@-Lemma, equivalently, the d dc -Lemma, then the dc inclusion ker dc ! ^ X and the projection ker dc ! ker im dc induce the quasiisomorphisms .ker dc ; d/ KKK s s KKK ss s KKK ss s K% qis yss qis
.^ X; d/
ker dc im dc
;0
of differential graded algebras, proving that .^ X; d/ is equivalent to hence formal.
ker dc im dc
; 0 , and
2.2 Cohomological Properties of Compact Complex Manifolds and the @@-Lemma In this section, we study some cohomological properties of compact complex manifolds, especially in relation with the @@-Lemma, see [AT13b]. More precisely, we prove an inequality à la Frölicher for the Bott-Chern cohomology, Theorem 2.13, and we characterize the validity of the @@-Lemma in terms of the dimensions of the Bott-Chern cohomology groups, Theorem 2.14. Let X be a compact complex manifold of complex dimension n. 2 o of notation, for every .p; q/ 2 N , for every k 2 N, and for ] 2 n As a matter @; @; BC; A , we will denote p;q
h]
p;q
WD dimC H] .X / 2 N
X
hk] WD
and
p;q
h]
2 N;
pCqDk
while recall that the Betti numbers are denoted by k bk WD dimC HdR .X I C/ 2 N :
Recall that, for every .p; q/ 2 N2 , the conjugation induces the isomorphisms ' ' ' p;q q;p p;q q;p p;q q;p HBC .X / ! HBC .X /, and HA .X / ! HA .X /, and H@ .X / ! H@ .X /, and the Hodge--operator associated to any given Hermitian metric induces the ' ' p;q nq;np p;q nq;np .X / and H@ .X / ! H@ .X /; hence, isomorphisms HBC .X / ! HA for every .p; q/ 2 N2 , one has the equalities p;q
q;p
np;nq
D hA
np;nq
D h@
hBC D hBC D hA p;q
h@
q;p
D h@
D h@
nq;np nq;np
and ;
2.2 Cohomological Properties of Compact Complex Manifolds and the @@-Lemma
79
and therefore, for every k 2 N, one has the equalities hkBC D h2nk A
and
hk@ D hk@ D h2nk D h2nk I @ @
Recall also that the Hodge--operator (of any given Riemannian metric and volume ' k 2nk .X I R/ ! HdR .X I R/, form on X ) yields, for every k 2 N, the isomorphism HdR and hence the equality bk D b2nk :
2.2.1 J. Varouchas’ Exact Sequences In order to prove a inequality à la Frölicher for the Bott-Chern and Aeppli cohomologies and to give therefore a characterization of compact complex manifolds satisfying the @@-Lemma in terms of the dimensions of their Bott-Chern cohomology groups, we need to recall two exact sequences from [Var86]. Following J. Varouchas, one defines the (finite-dimensional) bi-graded C-vector spaces A; WD
im @ \ im @ im @@
;
B ; WD
;
E ; WD
ker @ \ im @ im @@
;
C ; WD
;
F ; WD
ker @@ ker @ C im @
and D ; WD
im @ \ ker @ im @@
ker @@ ker @ C im @
ker @@ ker @ C ker @
:
For every .p; q/ 2 N2 and k 2 N, we will denote their dimensions by ap;q WD dimC Ap;q ;
b p;q WD dimC B p;q ;
c p;q WD dimC C p;q ;
d p;q WD dimC D p;q ;
e p;q WD dimC E p;q ;
f p;q WD dimC F p;q ;
and ak WD d k WD
X
ap;q ;
b k WD
X
pCqDk
pCqDk
X
X
pCqDk
d p;q ;
e k WD
pCqDk
b p;q ;
c k WD
X
c p;q ;
pCqDk
e p;q ;
f k WD
X
f p;q :
pCqDk
The previous vector spaces give the following exact sequences, by J. Varouchas.
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2 Cohomology of Complex Manifolds
Theorem 2.11 ([Var86, Sect. 3.1]). The sequences 0 ! A; ! B ; ! H@; .X / ! HA; .X / ! C ; ! 0
(2.1)
; .X / ! H@; .X / ! E ; ! F ; ! 0 0 ! D ; ! HBC
(2.2)
and
are exact sequences of finite-dimensional bi-graded C-vector spaces. Proof. We first prove the exactness of (2.1). Since im @ ker @, the map A; ! @\im @ D B ; is injective. The kernel of the map B ; ! H@; .X / is ker @\im im @@ im @\im @ , im @@ ; H@ .X /
that is, the image of the map A; ! B ; . The kernel of the map ! HA; .X / is
The kernel of the map
ker @\im @ , that is, the image of the map B ; ! H@; .X /. im @ @\ker @@ @ D im ker , that is, the image HA; .X / ! C ; is ker im @Cim @ @Cim @ ; ! HA .X /. Finally, since im @ C im @ ker @ C im @, C ; is surjective. In particular, since HA; .X / ! C ; is
of the map H@; .X / the map HA; .X / ! surjective, then C ; has finite dimension; since the identity induces an injective ; map B ; ! HBC .X /, then B ; has finite dimension; hence, since A; ! B ; is injective, then also A; has finite dimension. We prove now the exactness of (2.2). Since im @ ker @, the map ; ; D ; ! HBC .X / is injective. The kernel of the map HBC .X / ! H@; .X / is ; im @\ker @ , that is, the image of the map D ; ! HBC .X /. The im @@ ker @\.ker @Cim @/ ; ker @\ker @ ; kernel of the map H@ .X / ! E is D , that is, the im @ im @ ; ; ; ! F ; is image of the map HBC .X / ! H@ .X /. The kernel of the map E ker @@\.ker @Cker @/ ; ker @@\ker @ ; ker @\ker @\im @ im @@
D
ker @Cim @
D
ker @Cim @
, that is, the image of the map H@ .X / ! E ;
.
;
Finally, since ker @ C im @ ker @ C ker @, the map E ! F is surjective. In ; particular, since D ; ! HBC .X / is injective, then D ; has finite dimension; since the identity induces a surjective map HA; .X / ! E ; , then E ; has finite dimension; hence, since E ; ! F ; is surjective, then also F ; has finite dimension. t u Note, [Var86, Sect. 3.1], that the conjugation yields, for every .p; q/ 2 N2 , the equalities ap;q D aq;p ;
f p;q D f q;p ; '
d p;q D b q;p ;
e p;q D c q;p ;
'
(2.3)
and the isomorphisms @W C ; ! D ;C1 and @W E ; ! B C1; yield the equalities c p;q D d p;qC1 ;
e p;q D b pC1;q I
2.2 Cohomological Properties of Compact Complex Manifolds and the @@-Lemma
81
hence, for every k 2 N, one gets the equalities d k D bk ;
ek D ck ;
and
c k D d kC1 ;
e k D b kC1 :
Remark 2.6. Following the same argument used in [Sch07] to prove the duality between Bott-Chern and Aeppli cohomology groups, we can prove the duality between A; and F ; , and, similarly, between C ; and D ; . Indeed, note that the pairing A; F ; ! C ;
Z .Œ˛ ; Œˇ/ 7!
˛^ˇ ; X
; is non-degenerate: choose a Hermitian metric g on X ; if Œ˛ 2 A; HBC .X /, then there exists a Q BC -harmonic representative ˛Q in Œ˛ 2 A; , by [Sch07, Corollaire 2.3],R that is, @˛Q D @˛Q D @@ ˛Q D 0; hence, Œ˛ Q 2 F ; , and .Œ˛ Q ; Œ˛/ Q D X ˛Q ^ ˛Q is zero if and only if ˛Q is zero if and only if Œ˛ 2 A; is zero. Analogously, the pairing
C
;
Z
D ;
!C;
.Œ˛ ; Œˇ/ 7!
˛^ˇ ; X
is non-degenerate: indeed, choose a Hermitian metric g on X ; if Œ˛ 2 D ; ; HBC .X /, then there exists a Q BC -harmonic representative ˛Q in Œ˛ 2 D ; , by [Sch07, CorollaireR2.3], that is, @˛Q D @˛Q D @@ ˛Q D 0; hence, Œ˛ Q 2 C ; , and .Œ˛ Q ; Œ˛/ Q D X ˛Q ^ ˛Q is zero if and only if ˛Q is zero if and only if Œ˛ 2 D ; is zero.
2.2.2 An Inequality à la Frölicher for the Bott-Chern Cohomology We can now state and prove an inequality à la Frölicher for the Bott-Chern and Aeppli cohomologies, Theorem 2.13. Firstly, we recall that, on a compact complex manifold X , the Frölicher inequality [Frö55, Theorem 2] relates the Hodge numbers and the Betti numbers. Theorem 2.12 ([Frö55, Theorem 2]). Let X be a compact complex manifold. Then, for every k 2 N, the following inequality holds: X pCqDk
p;q
k dimC H@ .X / dimC HdR .X I C/ :
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2 Cohomology of Complex Manifolds
Table 2.1 Dimensions of cohomologies for the small deformations of the Iwasawa manifold Classes
h1@
h1BC
(i) (ii.a) (ii.b) (iii.a) (iii.b)
5 4 4 4 4 4 4 4 4 4 b1 D 4
h1A
h2@
h2BC
6 6 6 6 6
11 10 9 8 9 8 8 6 8 6 b2 D 8
h2A
h3@
h3BC
12 11 10 11 10
14 14 12 14 12 14 10 14 10 14 b3 D 10
h3A
h4@
14 14 14 14 14
11 12 9 11 9 10 8 11 8 10 b4 D 8
h4BC
h4A
h5@
h5BC
10 8 8 6 6
5 6 4 6 4 6 4 6 4 6 b5 D 4
h5A 4 4 4 4 4
P p;q k The equality pCqDk dimC H@ .X / D dimC HdR .X I C/ holds for every k 2 N if and only if the Hodge and Frölicher spectral sequence f.Er ; dr /gr2N degenerates at the first step. It is in general not true that hkBC (respectively, hkA ) is higher than the kth Betti number of X for every k 2 N: an example is provided by the small deformations of the Iwasawa manifold I3 WD H .3I Z Œi/n H.3I C/ (see Sect. 3.2.1). In Table 2.1, we summarize the dimensions of the Bott-Chern and Aeppli cohomology groups for I3 (which have been computed in [Sch07, Proposition 1.2]) and for the small deformations of I3 (see Sect. 3.2.4). We recall that the small deformations of the Iwasawa manifold, according to I. Nakamura’s classification, [Nak75, Sect. 3], are divided into three classes, (i), (ii), and (iii), in terms of their Hodge numbers; it turns out that the Bott-Chern cohomology yields a finer classification of the Kuranishi space of I3 , allowing a further subdivision of class (ii), respectively class (iii), into subclasses (ii.a), (ii.b), respectively (iii.a), (iii.b), see Sect. 3.2.1.2. The following result gives an inequality à la Frölicher for the Bott-Chern cohomology. (We recall that, on a compact complex manifold X of complex p;q dimension n, for any .p; q/ 2 N2 , one has the equality dimC HBC .X / D P nq;np p;q dim H .X /, and, for any k 2 N, the equality pCqDk dimC HBC .X / D P C A r;s rCsD2nk dimC HA .X /, see [Sch07, Sect. 2.c].) Theorem 2.13 ([AT13b, Theorem A]). Let X be a compact complex manifold. Then, for every .p; q/ 2 N2 , the following inequality holds: p;q
p;q
p;q
p;q
dimC HBC .X / C dimC HA .X / dimC H@ .X / C dimC H@ .X / :
(2.4)
In particular, for every k 2 N, the following inequality holds: X
p;q p;q k dimC HBC .X / C dimC HA .X / 2 dimC HdR .X I C/ :
(2.5)
pCqDk
Proof. Fix .p; q/ 2 N2 . The exact sequences (2.1), respectively (2.2), yield the equality p;q
hA
p;q
D h@ C c p;q C ap;q b p;q ;
2.2 Cohomological Properties of Compact Complex Manifolds and the @@-Lemma
83
respectively p;q
p;q
hBC D h@ C d p;q C f p;q e p;q I p;q
q;p
p;q
using also the symmetries hA D hA and h@ get2 p;q
p;q
p;q
q;p
p;q
q;p
p;q
p;q
p;q
p;q
q;p
D h@ , and the equalities (2.3), we
hBC C hA D hBC C hA
D h@ C h@ C f p;q C aq;p C d p;q b q;p e p;q C c q;p D h@ C h@ C f p;q C ap;q h@ C h@
;
which proves (2.4). Now, fix k 2 N; summing over .p; q/ 2 N N such that p C q D k, we get hkBC C hkA D
X p;q p;q hBC C hA pCqDk
X p;q p;q h@ C h@ D hk@ C hk@ pCqDk
2 bk ; t u
from which we get (2.5).
Remark 2.7. Note that small deformations of the Iwasawa manifold show that both the inequalities (2.4) and (2.5) can be strict. For example, for small deformations of I3 in class (i), one has, h1BC C h1A D 10 > 8 D 2 b1 ;
h2BC C h2A D 22 > 16 D 2 b2 ;
h3BC C h3A D 28 > 20 D 2 b3 ; showing that (2.5) is strict for every k 2 f1; 2; 3; 4; 5g. On the other hand, for small deformations of I3 in class (ii) or in class (iii), one has 1;0 h1;0 BC C hA D
1 1 C h0;1 D h1;0 C h1;0 hBC C h1A D 5 > 4 D h1@ D h1;0 @ ; @ @ @ 2
showing that (2.4) is strict, for example, for .p; q/ D .1; 0/.
2
We get actually that, for every k 2 N, it holds hkBC C hkA D 2 hk@ C ak C f k .
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2 Cohomology of Complex Manifolds
(For further examples among the small deformations of the Iwasawa manifold, compare the computations in Sect. 3.2.4, which are summarized in Table 3.5.)
2.2.3 A Characterization of the @@-Lemma in Terms of the Bott-Chern Cohomology This section is devoted to give a characterization of the validity of the @@-Lemma in terms of the Bott-Chern cohomology. Note that, if a compact complex manifold X satisfies the @@-Lemma, then, for every k 2 N, it holds hkBC D hkA D hk@ D hk@ D bk , and hence (2.5) is actually an equality. In fact, we prove now that also the converse holds true: more precisely, the equality in (2.5) holds for every k 2 N if and only if the @@-Lemma holds; in particular, this gives a characterization of the validity of the @@-Lemma just in terms ˚ of hkBC k2N . Theorem 2.14 ([AT13b, Theorem B]). Let X be a compact complex manifold. The equality X
p;q p;q k dimC HBC .X / C dimC HA .X / D 2 dimC HdR .X I C/
pCqDk
holds for every k 2 N if and only if X satisfies the @@-Lemma. ; Proof. If X satisfies the @@-Lemma, then the natural maps HBC .X / ! HdR .X I C/, ; ; ; ; ; HBC .X / ! H@ .X /, and H@ .X / ! HA .X /, HdR .X I C/ ! HA .X / induced by the identity are isomorphisms, [DGMS75, Remark 5.16], and hence, for every k 2 N, one has
hkBC D hkA D hk@ D bk and hence, in particular, hkBC C hkA D 2 bk : We split the proof of the converse into the following claims. Claim 1—If hkBC ChkA D 2 bk holds for every k 2 N, then the Hodge and Frölicher spectral sequences degenerate at the first step (namely, E1 ' E1 , that is, hk@ D bk for every k 2 N) and ak D 0 D f k for every k 2 N. Since, for every k 2 N, we have 2 bk D hkBC C hkA D 2 hk@ C ak C f k 2 bk ; then hk@ D bk and ak D 0 D f k for every k 2 N.
2.2 Cohomological Properties of Compact Complex Manifolds and the @@-Lemma
Claim 2—Fix k 2 N. If akC1 WD map M
P pCqDkC1
85
dimC Ap;q D 0, then the natural
p;q
k HBC .X / ! HdR .X I C/
pCqDk
is surjective. k Let a D Œ˛ 2 HdR .X I C/. We have to prove that a admits a representative whose pure-type components are d-closed. Consider the pure-type decomposition of ˛: ˛ DW
k X
.1/j ˛ kj;j ;
j D0
where ˛ kj;j 2 ^kj;j X . Since d ˛ D 0, we get that @˛ k;0 D 0 ;
@˛ kj;j @˛ kj 1;j C1 D 0 for j 2 f0; : : : ; k 1g ;
@˛ 0:k D 0 I
by the hypothesis akC1 D 0, for every j 2 f0; : : : ; k 1g, we get that, im @ \ im @ \ ^kj;j C1 X D im @@ \ ^kj;j C1 X
@˛ kj;j D @˛ kj 1;j C1 2
and hence there exists kj 1;j 2 ^kj 1;j X such that @˛ kj;j D @@kj 1;j D @˛ kj 1;j C1 : Define WD
k1 X
.1/j kj 1;j 2 ^k1 X ˝R C :
j D0
The claim follows noting that a D Œ˛ D Œ˛ C d 2 k1 X .1/j ˛ kj;j C @kj 1;j @kj;j 1 D 4 ˛ k;0 C @k1;0 C j D1
3 C .1/k ˛ 0;k @0;k1 5
86
2 Cohomology of Complex Manifolds k1 h i X D ˛ k;0 C @k1;0 C .1/j ˛ kj;j C @kj 1;j @kj;j 1
h
j D1
i C .1/k ˛ 0;k @0;k1 ; that is, each of the pure-type components of ˛ C d is both @-closed and @-closed. Claim 3—If hkBC bk and hkBC C hkA D 2 bk for every k 2 , then hkBC D bk for every k 2 N. If n is the complex dimension of X , then, for every k 2 N, we have D 2 b2nk h2nk b2nk D bk bk hkBC D h2nk A BC and hence hkBC D bk for every k 2 N. Now, by Claim 1, we get that ak D 0 for each k 2 N; hence, by Claim 2, for every k 2 N the natural map M
p;q
k HBC .X / ! HdR .X I C/
pCqDk
induced by the identity is surjective, and hence, in particular, hkBC bk . By Claim 3 we get therefore that hkBC D bk for every k 2 N. Hence, the natural ; map HBC .X / ! HdR .X I C/ is actually an isomorphism, which is equivalent to say that X satisfies the @@-Lemma. t u Remark 2.8. We note that, using the exact sequences (2.2) and (2.1), one can prove that, on a compact complex manifold X and for every k 2 N, e k D hk@ hkBC C f k C c k1 D hk@ hkBC hk1 hk1 C f k ak1 C e k2 : A @ Remark 2.9. Note that E1 ' E1 is not sufficient to have the equality hkBC C hkA D 2 bk for every k 2 N (and hence the @@-Lemma): a counterexample is provided by small deformations of the Iwasawa manifold. Indeed, for small deformations of I3 in class (iii), since h1@ D 4 D b1 ;
h2@ D 8 D b2 ; h3@ D 10 D b3 ;
the Hodge and Frölicher spectral sequences degenerate at the first step, but h1BC C h1A D 10 > 8 D 2 b1 ;
h2BC C h2A D 16 D 2 b2 ;
h3BC C h3A D 28 > 20 D 2 b3 :
2.2 Cohomological Properties of Compact Complex Manifolds and the @@-Lemma
87
Using Theorem 2.14, we get another proof of the stability of the @@-Lemma under small deformations of the complex structure, [AT13b, Corollary 2.7]; for different proofs of the same result by means of other techniques see, e.g., [Voi02, Proposition 9.21] (by proving that the conditions in the characterization [DGMS75, 5.21] are stable under small deformations), [Wu06, Theorem 5.12] (with an inductive argument), [Tom08, Sect. B] (by an expansion in series argument). (See [AK12, AK13a] for examples on the behaviour of the @@-Lemma as for closedness.) Corollary 2.2 ([AT13b, Corollary 2.7]; [Voi02, Proposition 9.21]; [Wu06, Theorem 5.12]; [Tom08, Sect. B]). Satisfying the @@-Lemma is a stable property under small deformations of the complex structure. Proof. Let fXt gt 2B be a complex-analytic family of compact complex manifolds. Since, for every k 2 N, the dimensions hkBC .Xt / and hkA .Xt / are upper-semicontinuous functions in t, [Sch07, Lemme 3.2], while the dimensions bk .Xt / are constant in t by Ehresmann’s theorem, one gets that, if Xt0 satisfies the equality hkBC .Xt0 / C hkA .Xt0 / D 2 bk .Xt0 / for every k 2 N, the same holds true for Xt with t near t0 . t u Remark 2.10. We recall that [DGMS75, 5.21] by P. Deligne, Ph.A. Griffiths, J. Morgan, and D.P. Sullivan characterizes the validity of the @@-Lemma on a compact complex manifold in terms of the degeneracy of the Hodge and Frölicher spectral sequence and of the existence of Hodge structures in cohomology. In particular, if follows that, onP a compact complex manifold satisfying the @@-Lemma, p;q one has the equality bk D pCqDk h@ for every k 2 N (which is equivalent to the degeneracy of the Hodge and Frölicher spectral sequence) and the symmetry p;q q;p h@ D h@ for every .p; q/ 2 N2 . Note that, on a compact complex surface X , since the Hodge and Frölicher spectral sequence degenerates at the first step (see, e.g., [BHPVdV04, Theorem IV.2.8]) if h1;0 D h0;1 then b1 D 2 h1;0 is even, and hence X is Kähler, by @ @ @ [Kod64, Miy74, Siu83], or [Lam99, Corollaire 5.7], or [Buc99, Theorem 11]. As already remarked, the small deformations of I3 in class (iii) satisfy the degeneracy condition of the Hodge and Frölicher spectral sequence, but they do not satisfy either the @@-Lemma, or the symmetry of the Hodge numbers. In studying the six-dimensional nilmanifolds endowed with left-invariant complex structures, M. Ceballos, A. Otal, L. Ugarte, and R. Villacampa found an example of a compact complex manifold of complex dimension 3 such that E1 ' p;q p;q E1 and h@ D h@ for every .p; q/ 2 N2 but for which the @@-Lemma does not hold, [COUV11, Corollary 6.10]. We recall, for a compact complex manifold ; X satisfying such a property, the double complex ^ X; @; @ has the form in Fig. 2.2 (where dots denote generators of the C 1 .X I R/-module ^; X , horizontal arrows are meant as @, vertical ones as @ and zero arrows are not depicted).
88
2 Cohomology of Complex Manifolds
Fig. 2.2 An abstract example
Appendix: Cohomological Properties of Generalized Complex Manifolds The above results, concerning the characterization of @@-Lemma by means of an inequality à la Frölicher on compact complex manifolds, can be slightly generalized to a more algebraic context, allowing both to recover the results in the complex setting and to get corollaries in the symplectic and in the generalized complex case, [AT13a]. We briefly recall the results obtained in a joint work with A. Tomassini, referring to [AT13a] for more details and for the proofs. Cohomological Aspects of Bi-Differential Graded Vector Spaces. Consider a O Z-graded K-vector space A endowed with two endomorphisms ı1 2 Endı1 .A / O and ı2 2 Endı2 .A / such that ı12 D ı22 D ı1 ı2 C ı2 ı1 D 0 : Following notation in [DGMS75, Remark 5.16], consider the following cohomologies and the following natural maps between them induced by the identity: H ı ;ı Iı ı .A / WD . 1 2 1 2/
kk kkk kkk k k k u kk k H ı Iı . 1 1/
.A /
WD
ker ı1 im ı1
ker ı1 \ker ı2 im ı1 ı2
SSS SSS SSS SSS SS)
H.ı1 Cı2 Iı1 Cı2 / .Tot A / WD
SSS SSS SSS SSS SS)
H ı ı Iı ;ı .A / WD . 1 2 1 2/
ker.ı1 Cı2 / im.ı1 Cı2 /
H ı Iı .A / WD . 2 2/
k kkk kkk k k kk u kk k
ker ı1 ı2 im ı1 Cim ı2
ker ı2 im ı2
2.2 Cohomological Properties of Compact Complex Manifolds and the @@-Lemma
89
By definition, A is said to satisfy the ı1 ı2 -Lemma if and only if ker ı1 \ ker ı2 \ .im ı1 C im ı2 / D im ı1 ı2 ; namely, if and only if the natural map H.ı 1 ;ı2 Iı1 ı2 / .A / ! H.ı 1 ı2 Iı1 ;ı2 / .A / induced by the identity is injective, see [DGMS75]. (Several equivalent definitions can be given, [DGMS75, Lemma 5.15], see [AT13a, Lemma 1.4].) In order to generalize Theorems 2.13 and 2.14, the following inequality à la Frölicher can be proven, characterizing the validity of the ı1 ı2 -Lemma. (See also [AT13a, Theorem 3.3, Corollary 3.5] for other statements.) Theorem 2.15 ([AT13a, Theorem 2.4, Corollary 3.4]). Let A be a bounded ZO graded K-vector space endowed with two endomorphisms ı1 2 Endı1 .A / and ı2 2 Oı2 End .A / such that ı12 D ı22 D ı1 ı2 C ı2 ı1 D 0. Denote the greatest common divisor of ıO1 and ıO2 by GCD.ıO1 ; ıO2 /. Suppose that and dimK H.ı 2 Iı2 / A < C1 : dimK H.ı 1 Iı1 / A < C1 Then dimK H.ı1 ;ı2 Iı1 ı2 / .A / C dimK H.ı1 ı2 Iı1 ;ı2 / .A / dimK H.ı1 Iı1 / .A / C dimK H.ı2 Iı2 / .A / :
Furthermore, the following conditions are equivalent: O O
1. AGCD.ı1 ;ı2 / satisfies the ı1 ı2 -Lemma; 2. the equality X ıO pCıO q ıO pCıO q dimK H.ı11 ;ı2 Iı12ı2 / A C dimK H.ı11 ı2 Iı12;ı2 / A pCqD
D 2 dimK H.ı 1 ˝K id Cı2 ˝K ˇIı1 ˝K id Cı2 ˝K ˇ/ Tot Doub; A :
holds. Remark 2.11. We recall the following constructions. Given a Z2 -graded K-vector space A; endowed with two endomorphisms ı1 2 O O O O Endı1;1 ;ı1;2 .A; / and ı2 2 Endı2;1 ;ı2;2 .A; / such that ı12 D ı22 D ı1 ı2 C ı2 ı1 D 0, define the Z-graded K-vector space M Tot A; WD Ap;q ; pCqD
endowed with the endomorphisms O O ı1 2 Endı1;1 Cı1;2 Tot A; such that ı12 D ı22 D ı1 ı2 C ı2 ı1 D 0.
and
O O ı2 2 Endı2;1 Cı2;2 Tot A;
90
2 Cohomology of Complex Manifolds
Conversely, given a Z-graded K-vector space A endowed with two endomorO O phisms ı1 2 Endı1 .A / and ı2 2 Endı2 .A / such that ı12 D ı22 D ı1 ı2 C ı2 ı1 D 0, following [Bry88, Sect. 1.3], [Cav05, Sect. 4.2], see [Goo85, Sect. II.2], [Con85, Sect. II], take an infinite cyclic multiplicative groupLfˇ m W m 2 Zg generated by some ˇ, and consider the Z-graded K-vector space 2Z K ˇ , and define the Z2 graded K-vector space O O Doub1 ;2 A WD Aı1 1 Cı2 2 ˝K K ˇ 2 ; endowed with the endomorphisms ı1 ˝K id 2 End1;0 Doub; A
and
ı2 ˝K ˇ 2 End1;0 Doub; A ;
which satisfy .ı1 ˝K id/2 D .ı2 ˝K ˇ/2 D .ı1 ˝K id/ .ı2 ˝K ˇ/ C .ı2 ˝K ˇ/ .ı1 ˝K id/ D 0; following [Bry88, Sect. 1.3], [Cav05, Sect. 4.2], the double complex .Doub; .A / ; ı1 ˝K id; ı2 ˝K ˇ/ is called the canonical double complex associated to A . Cohomological Aspects of Complex Manifolds. Given a compact complex manifold X , by applying Theorem 2.15 to the double complex ^; X; @; @ , one straightforwardly recovers the inequality à la Frölicher for the Bott-Chern cohomology of compact complex manifolds and the corresponding characterization of the @@-Lemma by means of the Bott-Chern cohomology in [AT13b, Theorems A and B], see Theorems 2.13 and 2.14. Cohomological Aspects of Symplectic Manifolds. Given a compact manifold X endowed with a symplectic structure !, one can apply Theorem 2.15 to the Z-graded R-vector space ^ X endowed with the endomorphisms d 2 End1 .^ X / and d 2 End1 .^ X /. (We refer to Sect. 1.2 for definitions and notations concerning symplectic structures.) In fact, on a compact symplectic manifold,3 both the spectral sequences associated to the canonical double complex More in general, given a compact manifold X endowed with a Poisson bracket f; g, and denoted by G the Poisson tensor associated to f; g, by following J.-L. Koszul, [Kos85], one can define ı WD Œ G ; d 2 End1 .^ X/. One has that ı 2 D 0 and Œd; ı D 0, [Kos85, pp. 266, 265], see also [Bry88, Proposition 1.2.3, Theorem 1.3.1]. associated It holds that, on any compact Poisson manifold, the first spectral sequence 0 E ; r to the canonical double complex .Doub; ^ X; d ˝R id; ı ˝R ˇ/ degenerates at the first level, [FIdL98, Theorem 2.5]. On the other hand, an example of a compact Poisson manifold (more precisely, of a nilmanifold endowed with a co-symplectic structure) such that the second spectral sequence 00 ; .Doub; Er ^ X; d ˝R id; ı ˝R ˇ/ does not degenerate at the first level has been provided by M. Fernández, R. Ibáñez, and M. de León, [FIdL98, Theorem 5.1]. In fact, on a compact 2n-dimensional manifold X endowed with a symplectic structure !, 3
'
the symplectic-?-operator ?! W ^ X ! ^2n X induces the isomorphism ?! W 0 E r 1 ;2 ! 00 2 ;2nC1 Er , [FIdL98, Theorem 2.9].
2.2 Cohomological Properties of Compact Complex Manifolds and the @@-Lemma
91
Doub; ^ X; d ˝R id; d ˝R ˇ degenerate at the first level, [Bry88, Theorem 2.3.1], [FIdL98, Theorem 2.5]; see also [FIdL98, Theorem 2.9], [Cav06, Theorem 5.2]. Hence, one gets the following result4 . Theorem 2.16 ([AT13a, Theorem 4.4]). Let X be a compact manifold endowed with a symplectic structure !. The inequality dimR HdC d .X I R/ C dimR Hdd .X I R/ 2 dimR HdR .X I R/
(2.6)
holds. Furthermore, the equality in (2.6) holds if and only if X satisfies the Hard Lefschetz Condition. / Example 2.1 ([AT13a, Example 4.6]). Consider the Iwasawa manifold I3 WD Z Œi3 ˚ 3 C ; , and take a C3 ; -left-invariant co-frame e j j 2f1;:::;6g of T X such that d e1 D d e2 D d e3 D d e4 D 0 ;
d e 5 D e 13 C e 24 ;
d e 6 D e 14 e 23
(in order to simplify we shorten, e.g., e 12 WD e 1 ^ e 2 ). 3notation, Consider the C ; -left-invariant almost-Kähler structure .J; !; g/ on I3 defined by Je 1 WD e 6 ;
Je 2 WD e 5 ;
Je 3 WD e 4 ;
! WD e 16 C e 25 C e 34 ;
g WD ! .; J / I (it has been studied in [ATZ12, Sect. 4] as an example of an almost-Kähler structure non-inducing a decomposition in cohomology according to the almost-complex structure, [ATZ12, Proposition 4.1]). By using the theorem à la Nomizu for the symplectic cohomologies proven by M. Macrì, [Mac13, Theorem 3, Remark 4], see also [AK13b, Theorems 3.2 and 3.4], we can compute the symplectic cohomologies of the Iwasawa manifold I3 endowed with the C3 ; -left-invariant symplectic structure ! by using just C3 ; -leftinvariant forms; we summarize the dimensions of the symplectic cohomologies in Table 2.2.
4
We recall that the symplectic cohomologies, introduced by L.-S. Tseng and S.-T. Yau in [TY12a, Sect. 3], are defined as ker d C d Hd C d .XI R/ WD H.d;d Id d / .^ X/ WD im d d
and ker d d : Hdd .XI R/ WD H.d d Id;d / .^ X/ WD im d C im d
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2 Cohomology of Complex Manifolds
Table 2.2 The symplectic cohomologies of the Iwasawa manifold I3 WD Z Œi3 endowed with the symplectic structure ! WD e 1 ^ e 6 C e 2 ^ e 5 C e 3 ^ e 4 .dI d/ d; d I d d d Id dimC H] .I3 / 1 4 8 10 8 4 1
0 1 2 3 4 5 6
1 4 8 10 8 4 1
1 4 9 11 10 4 1
/ C3 ;
d d I d; d
1 4 10 11 9 4 1
Cohomological Aspects of Generalized Complex Manifolds. Finally, consider a compact differentiable manifold X endowed with an H -twisted generalized complex structure J , for a d-closed 3-form H on X . (We refer to Sect. 1.3 for definitions and notations on generalized complex structures.) As an application of Theorem 2.15, we get the following result. Theorem 2.17 ([AT13a, Theorems 4.10, 4.11]). Let X be a compact differentiable manifold endowed with an H -twisted generalized complex structure J . Then dimC GH BCJ ;H .X / C dimC GH AJ ;H .X / dimC GH @
J ;H
.X / C dimC GH @J ;H .X / : (2.7)
Furthermore, the following conditions are equivalent: • X satisfies the @J ;H @J ;H -Lemma; • the Hodge and Frölicher spectral sequences associated to the canonical double ; complex Doub UJ ; @J ;H ˝C id; @J ;H ˝C ˇ degenerate at the first level and the equality in (2.7), dimC GH BCJ ;H .X/ C dimC GH AJ ;H .X/ D dimC GH @
J ;H
.X/ C dimC GH @J ;H .X/ ;
holds. Note that, by viewing a complex structure as a generalized complex structure of maximum type, Theorem 2.17 gives the following result, to be compared with [AT13b, Theorems A and B], see Theorems 2.13 and 2.14. Corollary 2.3 ([AT13a, Corollary 4.14]). Let X be a compact complex manifold. Then the inequality X pqD
p;q
dimC HBC .X /
X pqD
p;q
dimC H@ .X /
2.2 Cohomological Properties of Compact Complex Manifolds and the @@-Lemma
93
holds. Furthermore, X satisfies the @@-Lemma if and only if (i) the Hodge and Frölicher spectral sequence of X degenerates at the first level, namely, .X I C/ D dimC Tot H@; .X / ; dimC HdR
and (ii) the equality X pqD
p;q
dimC HBC .X / D
X
p;q
dimC H@ .X /
pqD
holds. Example 2.2 ([AT13a]). As an example, in Table 2.3, we summarize the dimensions of the generalized cohomologies of the Iwasawa manifold and of its small deformations, referring to Sect. 3.2.1 for the computations.
1 1 1 1 1
A
5 4 4 3 3 b1
5 4 4 3 3 D4
@
5 4 4 3 3
BC
@
1 1 1 1 1 D1
1 1 1 1 1
(i) (ii.a) (ii.b) (iii.a) (iii.b)
BC
@
1 1 1 1 1 b0
Classes
@
dimC Tot2 H]; .X/
dimC Tot3 H]; .X/
I3 WD ZŒi3 C3 A 5 4 4 3 3
@
11 11 9 9 9 9 8 8 8 8 b2 D 8
@ 11 11 11 11 11
BC
dimC Tot1 H]; .X/ A 11 11 11 11 11
@
12 12 10 10 10 10 10 10 10 10 b3 D 10
@ 12 11 10 11 10
BC
dimC Tot0 H]; .X/ 12 11 10 11 10
A
@
11 11 9 9 9 9 8 8 8 8 b4 D 8
@ 11 11 11 11 11
BC
dimC Tot1 H]; .X/ 11 11 11 11 11
A
Table 2.3 Generalized complex cohomologies of the Iwasawa manifold and of its small deformations, [AT13a, Table 2]
5 4 4 3 3 b5
@
5 4 4 3 3 D4
@
5 4 4 3 3
BC
dimC Tot2 H]; .X/ 5 4 4 3 3
A
1 1 1 1 1 b6
@
1 1 1 1 1 D1
@
1 1 1 1 1
BC
dimC Tot3 H]; .X/ 1 1 1 1 1
A
94 2 Cohomology of Complex Manifolds
Chapter 3
Cohomology of Nilmanifolds
Abstract Nilmanifolds and solvmanifolds appear as “toy-examples” in non-Kähler geometry: indeed, on the one hand, non-tori nilmanifolds admit no Kähler structure, (Benson and Gordon, Topology 27(4):513–518, 1988; Lupton and Oprea, J. Pure Appl. Algebra 91(1–3):193–207, 1994), and, more in general, solvmanifolds admitting a Kähler structure are characterized, (Hasegawa, Proc. Am. Math. Soc. 106(1):65–71, 1989); on the other hand, the geometry and cohomology of solvmanifolds can be often reduced to study left-invariant geometry. In Sect. 3.1, it is shown that, for certain classes of complex structures on nilmanifolds (that is, compact quotients of connected simply-connected nilpotent Lie groups by co-compact discrete subgroups), the de Rham, Dolbeault, Bott-Chern, and Aeppli cohomologies are completely determined by the associated Lie algebra endowed with the induced linear complex structure, Theorem 3.6, giving a sort of result à la Nomizu for the Bott-Chern cohomology. This will allow us to explicitly study the Bott-Chern and Aeppli cohomologies of the Iwasawa manifold and of its small deformations, in Sect. 3.2, and of the complex structures on six-dimensional nilmanifolds in M. Ceballos, A. Otal, L. Ugarte, and R. Villacampa’s classification, (Ceballos et al., Classification of complex structures on 6-dimensional nilpotent Lie algebras, arXiv:1111.5873v3 [math.DG], 2011), in Sect. 3.3. Finally, in Appendix: Cohomology of Solvmanifolds, we recall some facts concerning cohomologies of solvmanifolds.
3.1 Cohomology Computations for Special Nilmanifolds We are now interested in studying the Bott-Chern and Aeppli cohomologies in the special case of left-invariant complex structures on nilmanifolds and solvmanifolds. In this section, we firstly recall some results concerning the computation of the de Rham cohomology and of the Dolbeault cohomology, for nilmanifolds and solvmanifolds, endowed with left-invariant complex structures, Sect. 3.1.2, referring to [Nom54, Hat60], respectively [Sak76, CFGU00, CF01, Rol09a, Rol11a]; then, D. Angella, Cohomological Aspects in Complex Non-Kähler Geometry, Lecture Notes in Mathematics 2095, DOI 10.1007/978-3-319-02441-7__3, © Springer International Publishing Switzerland 2014
95
96
3 Cohomology of Nilmanifolds
we state and prove the results obtained in [Ang11] about computation of the Bott-Chern and Aeppli cohomologies, Theorems 3.5 and 3.7. Using these tools, one can compute the de Rham, Dolbeault, Bott-Chern and Aeppli cohomologies for the Iwasawa manifold and for its small deformations, Sects. 3.2.2–3.2.4.
3.1.1 Left-Invariant Complex Structures on Solvmanifolds We start by recalling some facts and notations concerning left-invariant complex structures on solvmanifolds. Let X D nG be a solvmanifold, that is, a compact quotient of a connected simply-connected solvable Lie group G by a discrete and co-compact subgroup ; the Lie algebra naturally associated to G will be denoted by g and its complexification by gC WD g ˝R C. We recall that, dealing with G-left-invariant objects on X , we mean objects on X obtained by objects on G that are invariant under the action of G on itself given by left-translations; note that G-left-invariant objects on X are uniquely determined by objects on g. In particular, a G-left-invariant complex structure J on X is uniquely determined by a linear complex structure J on g satisfying the integrability condition NijJ D 0, [NN57, Theorem 1.1]; the set of G-left-invariant complex structures on X is denoted by ˚ C .g/ WD J 2 End .g/ W J 2 D idg and NijJ D 0 : Recall that the exterior differential d on X can be written using only the action of .X I TX / on C 1 .X / and the Lie bracket of the Lie algebra of vector fields on X : more precisely, recall that, if ' 2 ^k X and X0 ; : : : ; Xk 2 C 1 .X I TX /, then d ' .X0 ; : : : ; Xk / D
k X
.1/j Xj ' X0 ; : : : ; Xj 1 ; Xj C1 ; : : : ; Xk
j D0
C
X
.1/
j Ch1
'
Xj ; Xh ; X0 ; : : : ; Xj 1 ; Xj C1 ; : : : ; Xh1 ; XhC1 ; : : : ; Xk :
0 j
Hence one has a differential complex .^ g ; d/, as a differ which is isomorphic, ential complex, to the differential sub-complex ^inv X; db^inv X of .^ X; d/ given by the G-left-invariant forms on X . If a G-left-invariant complex structure on X is given, then one also has the ; double complex ^ gC ; @; @ , which is isomorphic, as a double complex, to ; the double sub-complex ^inv X; @b^; X ; @b^; X of ^; X; @; @ given by the inv inv G-left-invariant forms on X . Finally, given a G-left-invariant complex structure on G and fixed .p; q/ 2 N2 , one also has the following complexes and the following maps of complexes:
3.1 Cohomology Computations for Special Nilmanifolds
^p1;q1 gC
@@
'
p1;q1
^inv
_
X
@@
/ ^pCqC1 g C
d
/ ^p;q X inv _
/ ^pCqC1 .X I C/ ; inv _
i
^p1;q1 X
@@
'
'
i
d
/ ^p;q g C
97
d
/ ^p;q X
(3.1)
i
/ ^pCqC1 .X I C/
and ^p1;q gC ˚ ^p;q1 gC p1;q
^inv
@C@
/ ^p;q g C
' p;q1
X ˚ ^inv _
@C@
X
/ ^p;q X inv _
@@
i @C@
/ ^pC1;qC1 g C '
'
i
^p1;q X ˚ ^p;q1 X
@@
/ ^p;q X
@@
/ ^pC1;qC1 X inv _
:
(3.2)
i
/ ^pC1;qC1 X
n o For ] 2 @; @; BC; A and K 2 fR; Cg, we will write HdR .gI K/ and H]; .gC / to denote the cohomology groups of the corresponding complexes of forms on g, which are isomorphic to the cohomology groups of the corresponding complexes of G-left-invariant forms on X . The rest of this section is devoted to the problem whether these cohomologies are isomorphic to the corresponding cohomologies on X .
3.1.2 Classical Results on Computations of the de Rham and Dolbeault Cohomologies In this section, we collect some results, by K. Nomizu [Nom54], A. Hattori [Hat60], S. Console and A.M. Fino [CF01], Y. Sakane [Sak76], L.A. Cordero, M. Fernández, A. Gray, and L. Ugarte [CFGU00], S. Rollenske [Rol09a,Rol11a,Rol09b], concerning the computation of the de Rham cohomology and the Dolbeault cohomology for nilmanifolds and solvmanifolds, endowed with left-invariant complex structures. First of all, we recall the following result, concerning the de Rham cohomology: it was firstly proven by K. Nomizu for nilmanifolds, and then generalized by
98
3 Cohomology of Nilmanifolds
A. Hattori to the case of completely-solvable solvmanifolds. (See also [Mat51, Theorems 5 and 6].) Theorem 3.1 ([Nom54, Theorem 1], [Hat60, Corollary 4.2]). Let X D n G be a nilmanifold, or, more in general, a completely-solvable solvmanifold, and denote the Lie algebra naturally associated to G by g. The map of differential complexes .^ g ; d/ ! .^ X; d/ is a quasi-isomorphism: '
i W HdR .gI R/ ! HdR .X I R/ :
Proof (sketch). We just sketch the idea of the proof for nilmanifolds, as in [Rol11a, Sect. 2]. The detailed proof, for the more general case of completely-solvable solvmanifolds, can be found, for example, in [TO97, Sect. 3.2]. The idea is to view X as a torus-bundle series and to argue by induction. More precisely, suppose to have a tower of bundles Tj0
Tj1
/ X0 WD X
/ X1
:: :
Tjk1
/ Xk1
Xk D Tjk for some k 2 Nnf0g, where, for ` 2 f0; : : : ; k 1g, the fibre Tj` is a j` -dimensional torus and the base X`C1 is a nilmanifold with dim X`C1 < dim X` , and with Xk D Tjk a jk -dimensional torus. By the Leray and Serre spectral sequence, [Ler50b, Ler50a, Ser51], see, e.g., [McC01, Theorem 5.2], and by noting that the de Rham cohomology of a torus is given by left-invariant forms, one can argue by induction. The point now is to construct such a tower of bundles. Note that, for any ` 2 f0; : : : ; k 1g, the torus-bundle Tj` ,! X` D ` nG` X`C1 D `C1 nG`C1
3.1 Cohomology Computations for Special Nilmanifolds
99
induces, at the level of the naturally associated Lie algebras (respectively, Rj` , and g` , and g`C1 ), the exact sequence 0 ! Rj` ! g` ! g`C1 ! 0 : Conversely, an exact sequence at the level of Lie algebras as the above induces a bundle at the level of the associated connected simply-connected Lie groups (respectively, R`j , and G` , and G`C1 ). Such bundle induces a bundle at the level of the corresponding nilmanifolds (respectively, Tj` , and X` D ` nG` , and X`C1 D `C1 nG`C1 ) if it is compatible with the (rational structures associated to the) lattices induced by . Hence it suffices to note that the descending central series of g gives exact sequences that are compatible with the lattices induced by . t u Remark 3.1. In particular, for a nilmanifold X D n G, the dga .^ g ; d/ is the minimal model of .^ X; d/, where g denotes the Lie algebra naturally associated to G. Remark 3.2. In fact, more in general, .^ g ; d/ ! .^ X; d/ is a quasiisomorphism also for solvmanifolds X D nG satisfying the Mostow condition, namely, such that Ad and Ad G have the same Zariski closure in Aut.gC / (where g is the Lie algebra naturally associated to G), as G.D. Mostow proved, [Mos54, Theorem 8.2, Corollary 8.1], see also [Rag72, Corollary 7.29]. In [CM12], sixdimensional non-completely-solvable almost-Abelian solvmanifolds for which Mostow condition does not hold are investigated, and their de Rham cohomology is computed; in particular, examples for which the de Rham cohomology is leftinvariant even if the Mostow condition is not satisfies are provided, [CM12, Table 1]. See also [Gua07, CF11] for some results concerning the cohomology of solvmanifolds non-satisfying the Mostow condition. Remark 3.3. In general, for a (possibly non-completely-solvable) solvmanifold, the inclusion .^ g ; d/ ,! .^ X; d/ induces just an injective map in cohomology, see [Rag72, Remark 7.30], [TO97, Theorem 3.2.10], compare also [CF01, Lemma 9] and Lemma 3.2. (In particular, as a consequence, one gets that any nilmanifold satisfies b1 2, and any solvmanifold satisfies b1 1, see, e.g., [Boc09, Corollaries 2.6 and 3.12].) An example of a non-completely-solvable solvmanifold for which such a map in cohomology is not surjective was provided by P. de Bartolomeis and A. Tomassini in [dBT06, Corollary 4.2, Remark 4.3], studying the Nakamura manifold, [Nak75, Sect. 2] (see also [CF11, Example 6.2]). Remark 3.4. For further results about the de Rham cohomology of solvmanifolds, obtained by S. Console, by A.M. Fino, and by H. Kasuya, we refer to [Gua07,CF11, Kas13a, Kas12a, CFK13]. Similar results hold for the Dolbeault cohomology of nilmanifolds endowed with certain left-invariant complex structures; [Con06] and [Rol11a] are surveys on the known results.
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First of all, we recall the following lemma by S. Console and A.M. Fino, [CF01]: the argument used in the proof can be generalized to Bott-Chern and Aeppli cohomologies, see Lemma 3.2. Lemma 3.1 ([CF01, Lemma 9]). Let X D n G be a nilmanifold endowed with a G-left-invariant complex structure J , and denote the Lie algebra naturally p; associated to G by g. For any p 2 N, the map of complexes ^ gC ; @ ,! ^p; X; @ induces an injective homomorphism i in cohomology: i W H@; .gC / ,! H@; .X / : Proof. We provide here a simple proof of the result, compare [AK13b, Corollary 1.2]. For a different argument, see [CF01, Lemma 9], see also [Ang11, Lemma 3.6], Lemma 3.2, [AK12, Sect. 1.2.2]. Consider the F.A. Belgun’s symmetrization map, [Bel00, Theorem 7], Z W ^ X ˝ C ! ^ gC ; .˛/ WD ˛bm .m/ ; X
R where is a G-bi-invariant volume form on G such that X D 1, whose existence follows from J. Milnor’s lemma [Mil76, Lemma 6.2]. One has that b^ gC D idb^ gC ; and that d ı D ı d
and
J ı D ıJ ;
since J is G-left-invariant, [Bel00, Theorem 7], [FG04, Theorem 2.1]. Hence one gets two maps of double complexes, i W ^; gC ; @; @ ! ^; X; @; @ and W ^; X; @; @ ! ^; gC ; @; @ such that ı i D id^; gC : In particular, they induce the commutative diagram H@; .gC /
i
/ H ; .X / @ id
/ H ; .gC / @ 4
'
from which it follows that the map i W H@; .gC / ! H@; .X / is injective.
t u
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For an arbitrary G-left-invariant complex structure on a nilmanifold X D n G, it is not known whether i W H@; .gC / ,! H@; .X / actually is an isomorphism, but some results are known for certain classes of G-left-invariant complex structures. Theorem 3.2 ([Sak76, Theorem 1], [CFGU00, Main Theorem], [CF01, Theorem 2, Remark 4], [Rol09a, Theorem 1.10], [Rol11a, Corollary 3.10]). Let X D n G be a nilmanifold endowed with a G-left-invariant complex structure J , and denote the Lie algebra naturally associated to G by g. Then, for every p 2 N, the map of complexes ^p; gC ; @ ,! ^p; X; @
(3.3)
is a quasi-isomorphism, namely, '
i W H@; .gC / ! H@; .X / ; provided one of the following conditions holds: X is holomorphically parallelizable; J is an Abelian complex structure; J is a nilpotent complex structure; J is a rational complex structure; g admits a torus-bundle series compatible with J and with the rational structure induced by ; • dimR g D 6 and g is not isomorphic to h7 WD 03 ; 12; 13; 23 .
• • • • •
Proof (sketch). We just sketch the idea of the proof, as in [Rol11a, Sect. 3]. For more details, see, e.g., [Sak76, CF01, Rol07, Rol09a]. The idea is to argue as in the proof of the Nomizu theorem, see Theorem 3.1, thanks to the Borel and Serre spectral sequence, [Hir95, Appendix Two]. The difference now is that the bundle series has to be compatible with the holomorphic structure. In terms of the Lie algebras exact sequences, they have to be compatible both with the rational structure induced by the lattice, and with the complex structure. One can prove that, whenever the complex structure is either holomorphically parallelizable, or Abelian, or nilpotent, or rational, one can construct a series of exact sequences of Lie algebras being compatible with both the rational and complex structures. ˚For example, in the case J is Abelian, one just takes the descending central series gŒk k2N of g. When J is rational, one takes ˚ Œk g C J gŒk k2N . t u We recall (see, e.g., [Rol09a, Definition 1.5]) that, given a nilpotent Lie algebra g, a rational structure for g is a Q-vector space gQ such that gQ ˝Q R D g. A subalgebra h of g is said to be rational with respect to a rational structure gQ if the Q-vector space h \ gQ of h is a rational structure for h. If G is the connected simplyconnected Lie group associated to g, then any discrete co-compact subgroup of G induces a rational structure for g, given by Q log .
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Consider a G-left-invariant complex structure on a nilmanifold X D n G with associated Lie algebra g; we recall that: • J is called holomorphically parallelizable if the holomorphic tangent bundle is holomorphically trivial, see, e.g., [Wan54, Nak75]; • J is called Abelian if ŒJ x; Jy D Œx; y for any x; y 2 g, see, e.g., [BDMM95, ABDM11]; ˚ • J is called nilpotent if there exists a G-left-invariant co-frame ! 1 ; : : : ; ! n for 1;0 T X with respect to which the structure equations of X are of the form d !j D
X h
n o j j with Ahk ; Bhk
j
Ahk ! h ^ ! k C
X
j
Bhk ! h ^ !N k
h;k<j
C, see, e.g., [CFGU00]; • J is called rational if J gQ gQ where gQ is the rational structure for g induced by , see, e.g., [CF01]. j;h;k
We recall ˚ also the following definitions, [Rol09a, Definition 1.8]. An ascending filtration S j g j 2f0;:::;kg on g is called a torus-bundle series compatible with a linear complex structure J on g and a rational structure gQ for g if, for every j 2 f1; : : : ; kg, it holds that (i) S j g is rational ı with respect to gQ and an ideal in S j C1 g, (ii) J S j g ıD S j g, and (iii) S j C1 g S j g is Abelian. If,˚in addition, it holds that (iv) S j C1 g S j g is contained in the center of g= S j g, then S j g j 2f0;:::;kg is called a principal˚ torus-bundle series compatible with J and gQ . Finally, an ascending filtration S j g j 2f0;:::;kg on g is called a stable (principal) torus-bundle series if it is a (principal) torus-bundle series compatible with J and gQ for any complex structure J and for any rational structure gQ . By S. Rollenske’s theorem [Rol09a, Theorem 3 B], every six-dimensional nilpotent Lie algebra except h7 WD 0 ; 12; 13; 23 admits a stable torus-bundle series. Remark 3.5. For further results about the Dolbeault cohomology of solvmanifolds, obtained by H. Kasuya, we refer to [Kas13b, Kas12a, CFK13, AK13a] (see also [Kas11, Kas12b, Kas12c] for applications). The property of computing the Dolbeault cohomology using just left-invariant forms turns out to be open along curves of left-invariant complex structures: this was proven by S. Console and A.M. Fino, [CF01]. Theorem 3.3 ([CF01, Theorem 1]). Let X D n G be a nilmanifold endowed with a G-left-invariant complex structure J , and denote the Lie algebra naturally associated to G by g. Let U C.g/ be the subset containing the G-left-invariant complex structures J on X such that the inclusion i is an isomorphism: n o ' U WD J 2 C .g/ W i W H@; .gC / ,! H@; .X / C .g/ : Then U is an open set in C .g/.
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The strategy of the proof consists in proving that the dimension of the orthogonal of H@; .gC / in H@; .X / with respect to a given J -Hermitian G-left-invariant metric on X D n G is an upper-semi-continuous function in J 2 C .g/ and thus, if it is zero for a given J 2 C .g/, then it remains equal to zero in an open neighbourhood of J in C .g/. We will use the same argument in proving Theorem 3.7, which is a slight modification of the previous result in the case of the Bott-Chern cohomology. The aforementioned results suggest the following conjecture. Conjecture 3.1 ([Rol11a, Conjecture 1]; see also [CFGU00, p. 5406], [CF01, p. 112]). Let X D n G be a nilmanifold endowed with a G-left-invariant complex structure J , and denote the Lie algebra naturally associated to G by g. Then, for any p 2 N, the map of complexes (3.3) is a quasi-isomorphisms, that is, '
i W H@; .gC / ! H@; .X / : Note that, since i is always injective by [CF01, Lemma 9], this is equivalent to asking that ? dimC H@; .gC / D 0 ; where the orthogonality is meant with respect to the inner product induced by a given J -Hermitian G-left-invariant metric g on X . Finally, as an application of the previous results, we recall the following theorem by S. Rollenske, concerning the deformations of left-invariant complex structures on nilmanifolds. Theorem 3.4 ([Rol09b, Theorem 2.6]). Let X D n G be a nilmanifold endowed with a G-left-invariant complex structure J , and denote the Lie algebra naturally associated to G by g. Suppose that, for p D 1, the map of complexes (3.3) ' 1;q 1;q is a quasi-isomorphism: i W H@ .gC / ! H@ .X / for every q 2 N. Then all small deformations of the complex structure J are again G-left-invariant complex structures. More precisely, the Kuranishi family of X contains only G-left-invariant complex structures. Remark 3.6. We refer to [Kas12c] by H. Kasuya for results concerning deformations of solvmanifolds.
3.1.3 The Bott-Chern Cohomology on Solvmanifolds We recall here some results concerning the computation of the Bott-Chern cohomology for nilmanifolds and solvmanifolds, [Ang11]. Firstly, we prove a slight modification of [CF01, Lemma 9] proven by S. Console and A.M. Fino for the Dolbeault cohomology: we repeat here their argument for the case of the Bott-Chern cohomology.
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Lemma 3.2 ([Ang11, Lemma 3.6]). Let X D n G be a solvmanifold endowed with a G-left-invariant complex structure J , and denote the Lie algebra naturally associated to G by g. The map of complexes (3.1) induces an injective homomorphism ; ; i W HBC .gC / ,! HBC .X / :
Proof. Fix .p; q/ 2 N2 . Let g be a J -Hermitian G-left-invariant metric on X and consider the induced inner product h j i on ^; X . Hence, both @, @, and their adjoints @ , @ preserve the G-left-invariant forms on X and therefore also Q BC does. In such a way, we get a Hodge decomposition also at the level of G-leftinvariant forms: ^p;q gC D ker Q BC b^p;q gC ˚ im @@b^p1;q1 gC ˚ im @ b^pC1;q gC C im @ b^p;qC1 gC : p;q
p;q
Now, take Œ! 2 HBC .gC / such that i Œ! D 0 in HBC .X /, that is, ! is a G-left-invariant .p; q/-form on X and there exists a (possibly non-G-left-invariant) p;q .p 1; q 1/-form on X such that ! D @@ . Up to zero terms in HBC .gC /, p;q ? we may assume that 2 i ^ gC ^p;q X . Therefore, since @ @ @@ is a G-left-invariant form (being @@ a G-left-invariant form), we have that ˇ E 2 D ˇ 0 D @ @ @@ˇ D @@ D k!k2 and therefore ! D 0.
t u
The second general result says that, if the Dolbeault and de Rham cohomologies of a solvmanifold are computed using just left-invariant forms, then also the BottChern cohomology is computed using just left-invariant forms. The idea of the proof is inspired by [Sch07, Sect. 1.c], where M. Schweitzer used a similar argument to explicitly compute the Bott-Chern cohomology in the special case of the Iwasawa manifold. (Compare also [AK12, Theorems 1.3 and 1.6, Sect. 2.3, Corollary 2.7].) Theorem 3.5 ([Ang11, Theorem 3.7]). Let X D nG be a solvmanifold endowed with a G-left-invariant complex structure J , and denote the Lie algebra naturally associated to G by g. Suppose that '
i W HdR .gI C/ ,! HdR .X I C/
and
'
i W H@; .gC / ,! H@; .X / :
Then also '
; ; i W HBC .gC / ,! HBC .X / :
Proof. Fix .p; q/ 2 N2 . We prove the theorem as a consequence of the following claims.
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\^ X Claim 1—It suffices to prove that im dim can be computed using just G-left@@ invariant forms. \^p;q X can be computed using just G-left-invariant Indeed, in the hypothesis that im dim @@ forms, the commutative diagram p;q
0
0
im d \^p;q g C im @@
p;q HBC .gC /
/
im d \^p;q X im @@
/ H p;q .X / BC
ker d \^p;q g C im d
/ 0
/
ker d \^p;q X im d
0
/ 0
0
have exact rows and exact columns; hence, by using the Five Lemma, one gets p;q p;q that the map HBC .gC / ! HBC .X / is surjective, in fact, by Lemma 3.2, an isomorphism. Claim 2—Under the hypothesis that the Dolbeault cohomology is computed using just G-left-invariant forms, if is a G-left-invariant @-closed form, then every solution of @ D is G-left-invariant up to @-exact terms. Indeed, since Œ D 0 in H@; .X /, there is a G-left-invariant form ˛ such that D @˛. Hence, ˛ defines a class in H@; .X / and hence ˛ is G-left-
invariant up to a @-exact form, and so is. Claim 3—Under the hypothesis that the Dolbeault cohomology is computed using \^p;q X just G-left-invariant forms, the space im dim can be computed using just G-left@@ invariant forms. Consider ! p;q D d mod im @@ 2
im d \ ^p;q X im @@
:
(3.4)
P p;q Decomposing DW in pure-type components, the equality (3.4) is p;q equivalent to the system
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3 Cohomology of Nilmanifolds
8 @pCq1;0 ˆ ˆ ˆ ˆ ˆ ˆ @pCq`;`1 C @pCq`1;` ˆ ˆ < @p;q1 C @p1;q ˆ ˆ ˆ ˆ ˆ @`;pCq`1 C @`1;pCq` ˆ ˆ ˆ : @0;pCq1
D 0
mod im @@
D 0
mod im @@ for ` 2 f1; : : : ; q 1g
D ! p;q
mod im @@
D 0
mod im @@ for ` 2 f1; : : : ; p 1g
D 0
mod im @@
:
Applying several times Claim 2, we may suppose that the forms `;pCq`1 , with ` 2 f0; : : : ; p 1g, are G-left-invariant: indeed, they are G-left-invariant up to @exact terms, but @-exact terms give no contribution in the system, since it is modulo im @@. Analogously, using the conjugate version of Claim 2, we may suppose that the forms pCq`1;` , with ` 2 f0; : : : ; q 1g, are G-left-invariant. Then we may suppose that ! p;q D @p;q1 C @p1;q is G-left-invariant. t u Remark 3.7. Let X D n G be a solvmanifold endowed with a G-left-invariant complex structure J , and denotethe Lie algebra naturally associated to G by g. Note p; p; that, if the map of complexes i W ^ gC ; @ ! ^ X; @ is a quasi-isomorphism for every p 2 N, that is, '
i W H@; .gC / ,! H@; .X / ; then also the map of complexes i W .^ g ; d/ ! .^ X; d/ is a quasi-isomorphism, that is, '
i W HdR .gI C/ ,! HdR .X I C/ :
Indeed, the map of double complexes i W ^; gC ; @; @ ! ^; X; @; @ induces a map between the corresponding Hodge and Frölicher spectral sequences: iW
˚ ; ˚ Er .gC / ; dr r2N ! Er; .X / ; dr r2N :
Since, see, e.g., [McC01, Theorem 2.15], .gC / E1; .gC / ' H@; .g/ ) HdR
and
E1; .X / ' H@; .X / ) HdR .X I C/ ; one gets that, if i W E1; .gC / ! E1; .X / is an isomorphism, then also i W HdR .gC / ! HdR .X I C/ is an isomorphism, see, e.g., [McC01, Theorem 3.5].
Remark 3.8. The results in Lemma 3.2 and Theorem 3.5 can be generalized to an algebraic and more general context, see [AK12, Theorems 1.6 and 1.3]: as corollaries of such a generalization, one gets both results on the cohomologies of solvmanifolds endowed with left-invariant complex structures, see [AK12, Sect. 2.4], and
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107
another proof of a result concerning the symplectic cohomologies of solvmanifolds endowed with left-invariant symplectic structures, originally proven by M. Macrì, [Mac13, Theorem 3], see [AK13b, Theorems 3.2 and 3.4]. As a corollary of [Nom54, Theorem 1], [Sak76, Theorem 1], [CFGU00, Main Theorem], [CF01, Theorem 2, Remark 4], [Rol09a, Theorem 1.10], [Rol11a, Corollary 3.10], and Theorem 3.5, we get the following result. Theorem 3.6 ([Ang11, Theorem 3.8]). Let X D n G be a nilmanifold endowed with a G-left-invariant complex structure J , and denote the Lie algebra naturally associated to G by g. Suppose that one of the following conditions holds: X is holomorphically parallelizable; J is an Abelian complex structure; J is a nilpotent complex structure; J is a rational complex structure; g admits a torus-bundle series compatible with J and with the rational structure induced by ; • dimR g D 6 and g is not isomorphic to h7 WD 03 ; 12; 13; 23 .
• • • • •
Then the de Rham, Dolbeault, Bott-Chern and Aeppli cohomologies can be computed as the cohomologies of the corresponding sub-complexes given by the space of G-left-invariant forms on X ; in other words, the inclusions of the several subcomplexes of G-left-invariant forms on X into the corresponding complexes of forms on X are quasi-isomorphisms: '
i W HdR .gI R/ ,! HdR .X I R/
and
'
i W H]; .gC / ,! H]; .X / ;
for ] 2 f@; @; BC; Ag. Remark 3.9. Note that Theorem 3.6, and [Hat60, Corollary 4.2], allow to straightforwardly compute the de Rham, Dolbeault, Bott-Chern, and Aeppli cohomologies of nilmanifolds, endowed with certain left-invariant complex structures, respectively the de Rham cohomology of completely-solvable solvmanifolds, just by computing the space of left-invariant (, or , or Q BC , or Q A -)harmonic forms with respect to a left-invariant Riemannian, or Hermitian, metric. Indeed, suppose that X is a nilmanifold, endowed with a left-invariant com'
.gI R/ ,! plex structure, or a completely-solvable solvmanifold, satisfying i W HdR '
.X I R/, or i W H]; .gC / ,! H]; .X /, for some ] 2 f@; @; BC; Ag. Let g be HdR a left-invariant Riemannian, or Hermitian, metric on X . Hence, the operators , , Q BC , Q A send the subspace of left-invariant forms to the subspace of left-invariant forms, and induce the self-adjoint operators
2 End ^ g ;
2 End ^; gC ; Q A 2 End ^; gC ;
Q BC 2 End ^; gC ;
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3 Cohomology of Nilmanifolds
with respect to the inner products h; i induced by g on the space ^ g and on the space ^; gC . Hence, one gets the orthogonal decompositions ^ g D ker ˚ im ;
^; gC D ker ˚ im ;
^; gC D ker Q BC ˚ im Q BC ;
^; gC D ker Q A ˚ im Q A
(one could argue also by using the F.A. Belgun symmetrization trick [Bel00, Theorem 7]). It follows that ; HBC .gC / ' ker Q BC ;
H@; .gC / ' ker ;
.gI R/ ' ker ; HdR
HA; .gC / ' ker Q A : Remark 3.10. Let X D n G be a 2n-dimensional solvmanifold endowed with a G-left-invariant complex structure J , and denote the Lie algebra naturally associated to G by g. The map of complexes (3.2) induces an injective homomorphism i W HA; .gC / ,! HA; .X / : '
; ; Furthermore, if i W HBC .gC / ,! HBC .X /, then the map of complexes (3.2) is a quasi-isomorphism, that is, '
i W HA; .gC / ,! HA; .X / : Indeed, fix a G-left-invariant Hermitian metric g on X . Recall that '
n2 ;n1 W HA1 ;2 .X / ! HBC .X /
is an isomorphism, [Sch07, Sect. 2.c]. Analogously, note that, by Remark 3.9 ' and since g is G-left-invariant, the map W ^1 ;2 gC ! ^n2 ;n1 gC induces an isomorphism '
n2 ;n1 .gC / : W HA1 ;2 .gC / ! HBC
Note also that the diagram HA1 ;2 .gC /
i
'
'
n2 ;n1 HBC .gC /
/ H 1 ;2 .X / A
i
/ H n2 ;n1 .X / BC
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109
n2 ;n1 commutes, since g is G-left-invariant. Since the map i W HBC .gC / ,! n2 ;n1 HBC .X / is injective by Lemma 3.2, then also the map HA1 ;2 .gC / ! n2 ;n1 n2 ;n1 HA1 ;2 .X / is injective. If i W HBC .gC / ,! HBC .X / is actually an 1 ;2 1 ;2 isomorphism, then also i W HA .gC / ! HA .X / is an isomorphism.
Remark 3.11. In order to generalize Theorem 3.6 to solvmanifolds, in [AK12, AK13a], finite-dimensional complexes allowing to compute the Bott-Chern cohomology of certain classes of solvmanifolds endowed with left-invariant complex structures are constructed, yielding explicit computations for the completelysolvable Nakamura manifold and for the holomorphically parallelizable Nakamura manifold. See Appendix: Cohomology of Solvmanifolds for further details, for which we refer to [AK12, AK13a]. A slight modification of [CF01, Theorem 1] by S. Console and A.M. Fino gives the following result, which says that the property of computing the Bott-Chern cohomology using just left-invariant forms is open in the space of left-invariant complex structures on solvmanifolds. Theorem 3.7 ([Ang11, Theorem 3.9]). Let X D nG be a solvmanifold endowed with a G-left-invariant complex structure J , and denote the Lie algebra naturally associated to G by g. Let ] 2 f@; @; BC; Ag. Suppose that '
.gC / ,! H]; .X / : i W H]; J J Then there exists an open neighbourhood U of J in C .g/ such that any JQ 2 U still satisfies '
i W H]; .gC / ,! H]; .X / : Q Q J
J
In other words, the set U WD
o n ' ; .g / ,! H .X / J 2 C .g/ W i W H]; C ] J J
is open in C .g/. Proof. As a matter of notation, for " > 0 small enough, we consider f.X; Jt / W t 2 .0; "/g .0; "/ a complex-analytic family of G-left-invariant complex structures on X , where .0; "/ WD ft 2 Cm W jtj < "g for some m 2 N n f0g; moreover, let fgt gt 2.0;"/ be a family of Jt -Hermitian G-left-invariant metrics on X depending smoothly on t. We will denote by @t WD @Jt and @t WD gt @Jt gt the @ operator and its gt -adjoint respectively for the Hermitian structure .Jt ; gt / and we set t WD ]Jt one of the differential operators involved in the definition of the Dolbeault, conjugate Dolbeault, Bott-Chern or Aeppli cohomologies with respect to .Jt ; gt /;
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3 Cohomology of Nilmanifolds
we remark that t is a self-adjoint elliptic differential operator for all the considered cohomologies. ? By hypothesis, we have that H]; .g / D f0g, where the orthogonality is C J0 meant with respect to the inner product induced by g0 , and we have to prove the same replacing 0 with t 2 .0; "/. Therefore, it will suffice to prove that ? .0; "/ 3 t 7! dimC H]; .g / 2N C J t
is an upper-semi-continuous function at 0. For any t 2 .0; "/, being t a self-adjoint elliptic differential operator, there exists a complete orthonormal basis ? fei .t/gi 2I of eigen-forms for t spanning ^J; gC , the orthogonal complement t of the space of G-left-invariant forms, see [KS60, Theorem 1]. For any i 2 I and t 2 .0; "/, let ai .t/ be the eigen-value corresponding to ei .t/; t depending differentiably on t 2 .0; "/, for any i 2 I , the function .0; "/ 3 t 7! ai .t/ 2 C is continuous, see [KS60, Theorem 2]. Therefore, for any t0 2 .0; "/, choosing a constant c > 0 such that c 62 fai .t0 / W i 2 I g, the function c W .0; "/ ! N ;
t 7! dim span fei .t/ W ai .t/ < cg
is locally constant at t0 ; moreover, for any t 2 .0; "/ and for any c > 0, we have ? c .t/ dimC H]; .g / : C J t
Since the spectrum of t0 has no accumulation point for any t0 2 .0; "/, see [KS60, Theorem 1], the theorem follows choosing c > 0 small enough so that ? c .0/ D dimC H]; .g / . t u C J 0
In particular, the left-invariant complex structures on nilmanifolds belonging to the classes of Theorem 3.6, and their small deformations satisfy the following conjecture, which generalizes Conjecture 3.1. Conjecture 3.2 ([Rol11a, Conjecture 1], [Ang11, Conjecture 3.10]; see also [CFGU00, p. 5406], [CF01, p. 112]). Let X D n G be a nilmanifold endowed with a G-left-invariant complex structure J , and denote the Lie algebra naturally associated to G by g. Then the de Rham, Dolbeault, Bott-Chern and Aeppli cohomologies can be computed as the cohomologies of the corresponding subcomplexes given by the space of G-left-invariant forms on X , that is, ? dimR HdR .gI R/ D 0
and
? dimC H]; .gC / D 0;
where ] 2 f@; @; BC; Ag, and the orthogonality is meant with respect to the inner product induced by a given J -Hermitian G-left-invariant metric g on X .
3.2 The Cohomologies of the Iwasawa Manifold and of Its Small Deformations
111
3.2 The Cohomologies of the Iwasawa Manifold and of Its Small Deformations The Iwasawa manifold is one of the simplest example of non-Kähler complex manifold: as such, it has been studied by several authors, and it has turned out to be a fruitful source of interesting behaviours, see, e.g., [FG86,Nak75,AB90,Bas99, AGS97, KS04, Ye08, Sch07, AT11, Ang11, Fra11]. In this section, we recall the construction of the Iwasawa manifold Sect. 3.2.1.1, see, e.g., [FG86], [Nak75, Sect. 2], and of its Kuranishi space, Sect. 3.2.1.2, see [Nak75, Sect. 3]; then we write down the de Rham cohomology, Sect. 3.2.2, and the Dolbeault cohomology, Sect. 3.2.3 (using [Nom54, Theorem 1], and [Sak76, Theorem 1] and [CF01, Theorem 1]), and we compute the Bott-Chern and Aeppli cohomologies, Sect. 3.2.4 (using Theorems 3.6 and 3.7), of the Iwasawa manifold and of its small deformations.
3.2.1 The Iwasawa Manifold and Its Small Deformations 3.2.1.1 The Iwasawa Manifold In order to answer to a question by S. Iitaka in [Iit72], I. Nakamura classified in [Nak75, Sect. 2] the three-dimensional holomorphically parallelizable solvmanifolds into four classes by numerical invariants, giving the Iwasawa manifold I3 as an example in the second class. Let H.3I C/ be the three-dimensional Heisenberg group over C defined by 80 1 3 1 9 < 1z z = H.3I C/ WD @ 0 1 z2 A 2 GL.3I C/ W z1 ; z2 ; z3 2 C ; : ; 0 0 1 where the product is the one induced by multiplication. (Equivalently, one matrix can consider H.3I C/ as isomorphic to C3 ; , where the group structure on C3 is defined as .z1 ; z2 ; z3 / .w1 ; w2 ; w3 / WD .z1 C w1 ; z2 C w2 ; z3 C z1 w2 C w3 / :/ It is straightforward to prove that H.3I C/ is a connected simply-connected complex 2-step nilpotent Lie group, that is, the Lie algebra .h3 ; Œ; / naturally associated to H.3I C/ satisfies Œh3 ; h3 ¤ 0 and Œh3 ; Œh3 ; h3 D 0. One finds that 8 ' 1 WD d z1 ˆ ˆ < ' 2 WD d z2 ˆ ˆ : 3 ' WD d z3 z1 d z2
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complex Fig.;3.1 The double ^ .h3 ˝R C/ ; @; @
is a H.3I C/-left-invariant co-frame for the space of .1; 0/-forms on H.3I C/, and that the structure equations with respect to this co-frame are 8 d '1 D 0 ˆ ˆ < : d '2 D 0 ˆ ˆ : 3 d ' D ' 1 ^ ' 2 Consider the action on the left of H .3I Z Œi/ WD H.3I C/ \ GL .3I Z Œi/ on H .3I C/ and take the compact quotient I3 WD H .3I Z Œi/n H.3I C/ : One gets that I3 is a three-dimensional complex nilmanifold, whose (H.3I C/left-invariant) complex structure J0 is the one inherited by the standard complex structure on C3 ; I3 is called the Iwasawa manifold. (See also [KS04].) 1 2 3 The forms ' , ' and ' , being H.3I C/-left-invariant, define a co-frame also 1;0 for T I3 . Note that I3 is a holomorphically parallelizable manifold, that is, its holomorphic tangent bundle is holomorphically trivial. Since, for example, ' 3 is a non-closed holomorphic form, it follows that I3 admits no Kähler metric. In fact, one can show that I3 is not formal, having a non-zero Massey triple product, see [FG86, p. 158]; therefore the underlying differentiable manifold of I3 has no complex structure admitting Kähler metrics, see [DGMS75, Main Theorem], even though all the topological obstructions concerningPthe Betti numbers are satisfied. Nevertheless, I3 admits the balanced metric ! WD 3j D1 ' j ^ 'N j . We sketch in Fig. 3.1 the structure of the finite-dimensional double complex ^; .h3 ˝R C/ ; @; @ : the dots denote a basis of ^; .h3 ˝R C/ , horizontal arrows are meant as @, vertical ones as @ and zero arrows are not depicted.
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113
Table 3.1 Dimensions of the cohomologies of the Iwasawa manifold ]
h1;0 ]
h0;1 ]
@ @ BC A dR
3 2 2 3 2 2 3 3 b1 D 4
h2;0 ]
h1;1 ]
3 6 2 6 3 4 2 8 b2 D 8
h0;2 ]
h3;0 ]
h2;1 ]
2 3 3 2
1 6 1 6 1 6 1 6 b3 D 10
h1;2 ]
h0;3 ]
h3;1 ]
h2;2 ]
6 6 6 6
1 1 1 1
2 6 3 6 2 8 3 4 b4 D 8
h1;3 ]
h3;2 ]
h2;3 ]
3 2 2 3
2 3 3 2 3 3 2 2 b5 D 4
In Fig. 3.2, we highlight what generators of the double complex ^; .h3 ˝R C/ ; @; @ give a non-trivial contribution in Dolbeault, respectively conjugate Dolbeault, respectively Bott-Chern, respectively Aeppli cohomology, in such a way to get immediately the corresponding cohomological diamonds. As a matter of notation, a filled point yields a non-trivial cohomology class, while empty points do not contribute to cohomology. We summarize the dimensions of the cohomology groups for the Iwasawa manifold in Table 3.1. Remark 3.12. The pictures in Fig. 3.1, as well as in Fig. 3.2, have been inspired by an answer by G. Kuperberg on “Dolbeault cohomology of Hopf manifolds” on MathOverflow, see http://mathoverflow.net/questions/25723/dolbeaultcohomology-of-hopf-manifolds. Such pictures have inspired and motivate Theorems 2.13 and 2.14. 3.2.1.2 Small Deformations of the Iwasawa Manifold Still in [Nak75], I. Nakamura explicitly constructed the Kuranishi family of deformations of I3 , showing that it is smooth and depends on six effective parameters, [Nak75, pp. 94–95], compare also [Rol11b, Corollary 4.9]. In particular, he computed the Hodge numbers of the small deformations of I3 proving that they have not to remain invariant along a complex-analytic family of complex structures, [Nak75, Theorem 2], compare also [Ye08, Sect. 4]; moreover, he proved in this way that the property of being holomorphically parallelizable is not stable under small deformations, [Nak75, p. 86], compare also [Rol11b, Theorem 5.1, Corollary 5.2]. Firstly, we recall in the following theorem the results by I. Nakamura concerning the Kuranishi space of the Iwasawa manifold. Theorem 3.8 ([Nak75, pp. 94–96]). Consider the Iwasawa manifold I3 WD H .3I Z Œi/n H.3I C/. There exists a locally complete complex-analytic family of complex structures fXt D .I3 ; Jt /gt2.0;"/ , deformations of I3 , depending on six parameters t D .t11 ; t12 ; t21 ; t22 ; t31 ; t32 / 2 .0; "/ C6 ; ˚ where " > 0 is small enough, .0; "/ WD s 2 C6 W jsj < " , and X0 D I3 .
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3 Cohomology of Nilmanifolds
b
a
c
d
Fig. 3.2 Cohomologies of the Iwasawa manifold. (a) The diagram for the Dolbeault cohomology H@; .I3 / of the Iwasawa manifold I3 . (b) The diagram for the conjugate Dolbeault cohomology ; .I3 / H@; .I3 / of the Iwasawa manifold I3 . (c) The diagram for the Bott-Chern cohomology HBC ; of the Iwasawa manifold I3 . (d) The diagram for the Aeppli cohomology HA .I3 / of the Iwasawa manifold I3
A set of holomorphic coordinates for Xt is given by 8 1 P2 1 1 k ˆ < WD .t/ WD z C PkD1 t1k zN 2 2 WD 2 .t/ WD z2 C kD1 t2k zNk ˆ P : 3 WD 3 .t/ WD z3 C 2kD1 t3k C t2k z1 zNk C A zN1 ; zN2 D .t/ zN3 where D .t/ WD det
t11 t12 t21 t22
3.2 The Cohomologies of the Iwasawa Manifold and of Its Small Deformations
115
and 2 2 1 : t11 t21 zN1 C 2 t11 t22 zN1 zN2 C t12 t22 zN2 A zN1 ; zN2 WD 2 For every t 2 .0; "/, the universal covering of Xt is C3 ; more precisely, X t D t n C3 ; where t is the subgroup generated by the transformations 1 2 3 .! 1 ; ! 2 ; ! 3 / 1 2 3 7! ; ; Q ; Q ; Q ; varying ! 1 ; ! 2 ; ! 3 2 .Z Œi/3 , where 8 Q 1 WD 1 C ! 1 C t11 !N 1 C t12 !N 2 ˆ ˆ ˆ ˆ ˆ ˆ < Q 2 WD 2 C ! 2 C t21 !N 1 C t22 !N 2 : ˆ ˆ Q 3 WD 3 C ! 3 C t31 !N 1 C t32 !N 2 C ! 1 2 ˆ ˆ ˆ ˆ : C t21 !N 1 C t22 !N 2 1 C ! 1 C A !N 1 ; !N 2 D .t/ !N 3 Proof (sketch). We just briefly sketch the idea of the proof, referring to [Nak75, pp. 94–96], see also [Uen75, pp. 217–219], for further details. notations, consider the H.3I C/-left-invariant co-frame above ˚ 1With2 the ' ; ' ; ' 3 of T 1;0 I3 , where ' 1 WD d z1 ;
' 2 WD d z2 ;
' 3 WD d z3 z1 d z2 ;
and its dual frame f#1 ; #2 ; #3 g of T 1;0 I3 , where #1 WD
@ ; @z1
#2 WD
@ @ C z1 3 ; @z2 @z
#3 WD
@ : @z3
One has ˛ ˝ H 0;1 .I3 I I3 / D C #1 ˝ 'N 1 ; #2 ˝ 'N 1 ; #3 ˝ 'N 1 ; #1 ˝ 'N 2 ; #2 ˝ 'N 2 ; #3 ˝ 'N 2 : The vector-valued .0; 1/-forms solving the Maurer and Cartan equation, 8 @ .t/ C 12 Œ .t/; .t/ D 0 ˆ ˆ < .0/ D 0 ; j ˆ ˆ : @ D #h ˝ 'N for h 2 f1; 2; 3g and 2 f1; 2g @th tD0
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3 Cohomology of Nilmanifolds
are .t/ D
3 X 2 X
th #h ˝ 'N .t11 t22 t21 t12 / #3 ˝ 'N 3
hD1 D1
Finally, a set of holomorphic coordinates for Xt is obtained by solving, for 2 f1; 2; 3g, (
@ .t/
.t/ .t/ D 0
.0/ D z
:
For t small enough, the map ˚t W 1 .t/; 2 .t/; 3 .t/ 7! z1 ; z2 ; z3 is a diffeomorphism of C3 . Hence one gets the commutative diagram C3 I3
˚t '
'
/ C3 / Xt
which induces a covering map C3 ! Xt . Hence Xt D t C3 for a group t of analytic automorphisms of C3 . u t Remark 3.13. Note that, by [Rol11b, Theorem 4.5], if X D n G is a holomorphically parallelizable nilmanifold and G is -step nilpotent, then Kur.X / is cut out by polynomial equations of degree at most ; furthermore, by [Rol11b, Corollary 4.9], the Kuranishi space of X is smooth if and only if the associated Lie algebra g to G is a free 2-step nilpotent Lie algebra, i.e., g ' bm with m D dimC H@0;1 .X /, where bm WD Cm ˚ ^2 Cm with Lie bracket Œa1 C b1 ^ c1 ; a2 C b2 ^ c2 WD a1 ^ a2 for a1 ; b1 ; c1 ; a2 ; b2 ; c2 2 Cm . According to the classification by I. Nakamura, the small deformations of I3 are divided into three classes, (i)–(iii), in terms of their Hodge numbers: such classes are explicitly described by means of polynomial relations in the parameters, see [Nak75, Sect. 3]. As we will see in Sect. 3.2.4, it turns out that the Bott-Chern cohomology yields a finer classification of the Kuranishi space of I3 ; more precisely, h2;2 BC assumes different values within class (ii), respectively class (iii), according to the rank of a certain matrix whose entries are related to the complex structure equations with respect to a suitable co-frame, whereas the numbers corresponding to class (i) coincide with those for I3 : this allows a further subdivision of classes (ii) and (iii) into subclasses (ii.a), (ii.b), and (iii.a), (iii.b). More precisely, the classes and subclasses of this classification are characterized by the following values of the parameters:
3.2 The Cohomologies of the Iwasawa Manifold and of Its Small Deformations
117
class (i) t11 D t12 D t21 D t22 D 0; class (ii) D .t/ D 0 and .t11 ; t12 ; t21 ; t22 / ¤ .0; 0; 0; 0/: subclass (ii.a) D .t/ D 0 and rk S D 1; subclass (ii.b) D .t/ D 0 and rk S D 2; class (iii) D .t/ ¤ 0: subclass (iii.a) D .t/ ¤ 0 and rk S D 1; subclass (iii.b) D .t/ ¤ 0 and rk S D 2. The matrix S is defined by S WD
11N 22N 12N 21N 11N 22N 21N 12N
where 11N ; 12N ; 21N ; 22N 2 C and 12 2 C are complex numbers depending only on t such that d 't3 DW 12 't1 ^ 't2 C 11N 't1 ^ 'Nt1 C 12N 't1 ^ 'Nt2 C 21N 't2 ^ 'Nt1 C 22N 't2 ^ 'Nt2 ; being 't1 WD d t1 ;
't3 WD d t3 z1 d t2 t21 zN1 C t22 zN2 d t1 ;
't2 WD d t2 ;
see Sect. 3.2.1.3. As we will show, see Sect. 3.2.1.3, the first order asymptotic behaviour of 12 ; 11N ; 12N ; 21N ; 22N for t near 0 is the following: 8 12 ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ < 11N 12N ˆ ˆ ˆ ˆ 21N ˆ ˆ ˆ ˆ : 22N
D 1 C o .jtj/ D t21 C o .jtj/ D t22 C o .jtj/
for
t 2 classes (i), (ii) and (iii) ;
(3.5)
D t11 C o .jtj/ D t12 C o .jtj/
and, more precisely, for deformations in class (ii) we actually have that 8 12 ˆ ˆ ˆ ˆ ˆ ˆ ˆ 11N ˆ < 12N ˆ ˆ ˆ ˆ ˆ 21N ˆ ˆ ˆ : 22N
D 1 C o .jtj/ D t21 .1 C o .1// D t22 .1 C o .1// D t11 .1 C o .1// D t12 .1 C o .1//
for
t 2 class (ii) :
(3.6)
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3 Cohomology of Nilmanifolds
The complex manifold Xt is endowed with the Jt -Hermitian H.3I C/-leftinvariant metric gt , which is defined as follows: gt WD
3 X
j
j
't ˇ 'Nt :
j D1
3.2.1.3 Structure Equations for Small Deformations of the Iwasawa Manifold In this section, we give the structure equations for the small deformations of the Iwasawa manifold; we will use these computations in Sects. 3.2.3 and 3.2.4 to write the Bott-Chern cohomology of Xt , and in Theorem 4.8 to prove that the cohomological property of being C 1 -pure-and-full is not stable under small deformations of the complex structure. Fix t 2 .0; "/ C6 , and consider the small deformation Xt of the Iwasawa manifold I3 . Consider the system of complex coordinates on Xt given by 8 P ˆ t1 WD z1 C 2D1 t1 zN ˆ ˆ ˆ < P : t2 WD z2 C 2D1 t2 zN ˆ ˆ ˆ ˆ P : 3 t WD z3 C 2D1 .t3 C t2 z1 /Nz C A .Nz/ Consider 8 ˆ 't1 WD d t1 ˆ ˆ ˆ < 't2 WD d t2 ˆ ˆ ˆ ˆ : ' 3 WD d 3 z d 2 t zN1 C t zN2 d 1 1 21 22 t t t t as a co-frame of .1; 0/-forms on Xt (that is, as a t -invariant co-frame of .1; 0/forms on C3 ). We want to write the structure equations for Xt with respect to this co-frame. A straightforward computation gives 8 < z1 D t1 C 1 Nt1 C 2 t2 C 3 Nt2 ; : 2 z D ˛ 0 t1 C 1 Nt1 C 2 t2 C 3 Nt2 where ˛, ˇ, , i (for i 2 f1; 2; 3g), j (for j 2 f0; 1; 2; 3g) are complex numbers depending just on t, and defined as follows:
3.2 The Cohomologies of the Iwasawa Manifold and of Its Small Deformations
8 ˆ ˆ ˛ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˇ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ 1 ˆ ˆ ˆ ˆ ˆ ˆ < 2 ˆ ˆ ˆ ˆ 3 ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ 0 ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ 1 ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ 2 ˆ ˆ ˆ ˆ ˆ : 3
WD
119
1
1 jt22 j t21 tN12 WD t21 tN11 C t22 tN21 2
1 1 jt11 j ˛ ˇ .t11 tN12 C t12 tN22 / t12 tN21 WD t11 1 C ˛ tN12 t21 C ˛ jt22 j2 WD
2
WD ˛ .t11 tN12 C t12 tN22 / WD t12 1 C ˛ tN12 t21 C ˛ jt22 j2
:
WD ˇ WD 1 ˇ t21 WD 1 C 2 ˇ WD 3 ˇ t22
For the complex structures˚in the class (i), one checks that the structure equations (with respect to the co-frame 't1 ; 't2 ; 't3 ) are the same as the ones for I3 , that is, 8 ˆ d 't1 D 0 ˆ ˆ ˆ < d 't2 D 0 ˆ ˆ ˆ ˆ : d ' 3 D ' 1 ^ ' 2 t t t
for
t 2 class (i) :
For small deformations in classes (ii) and (iii), we have that 8 ˆ d 't1 D 0 ˆ ˆ ˆ ˆ ˆ ˆ ˆ d '2 D 0 ˆ ˆ < t for t 2 classes (ii) and (iii) ; d 't3 D 12 't1 ^ 't2 ˆ ˆ ˆ ˆ ˆ ˆ C11N 't1 ^ 'Nt1 C 12N 't1 ^ 'Nt2 ˆ ˆ ˆ ˆ : C21N 't2 ^ 'Nt1 C 22N 't2 ^ 'Nt2 where 12 ; 11N ; 12N ; 21N ; 22N 2 C are complex numbers depending just on t. The asymptotic behaviour of 12 ; 11N ; 12N ; 21N ; 22N 2 C is the following:
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3 Cohomology of Nilmanifolds
8 12 ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ 11N < 12N ˆ ˆ ˆ ˆ ˆ 21N ˆ ˆ ˆ : 22N
D 1 C o .jtj/ D t21 C o .jtj/ D t22 C o .jtj/
for
t 2 classes (i), (ii) and (iii) ;
(3.7)
D t11 C o .jtj/ D t12 C o .jtj/
more precisely, for deformations in class (ii) we actually have that 8 12 ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ 11N < 12N ˆ ˆ ˆ ˆ ˆ 21N ˆ ˆ ˆ : 22N
D 1 C o .jtj/ D t21 .1 C o .1// D t22 .1 C o .1//
for
t 2 class (ii) :
(3.8)
D t11 .1 C o .1// D t12 .1 C o .1//
The explicit values of 12 ; 11N ; 12N ; 21N ; 22N 2 C in the case of class (ii) are the following, [AT11, p. 416]: 8 12 ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ 11N ˆ ˆ ˆ ˆ < 12N ˆ ˆ ˆ ˆ ˆ ˆ ˆ 21N ˆ ˆ ˆ ˆ ˆ ˆ ˆ : N 22
WD C t21 N 3 N C t22 ˛N N 3 WD t21 1 C t21 tN12 ˛ C jt22 j2 ˛ WD t22 1 C t21 tN12 ˛ C jt22 j2 ˛ WD t11 1 C t21 tN12 ˛ C jt22 j2 ˛ WD t12 1 C t21 tN12 ˛ C jt22 j2 ˛
for
t 2 class (ii) :
Note that, for small deformations in class (ii), one has 12 ¤ 0 and 11N ; 12N ; 21N ; 22N ¤ .0; 0; 0; 0/.
3.2.2 The de Rham Cohomology of the Iwasawa Manifold and of Its Small Deformations Recall that, by Ch. Ehresmann’s theorem, every complex-analytic family of compact complex manifolds is locally trivial as a differentiable family of compact differentiable manifolds, see, e.g., [MK06, Theorem 4.1]. Therefore the de Rham cohomology of small deformations of the Iwasawa manifold is the same as the de
3.2 The Cohomologies of the Iwasawa Manifold and of Its Small Deformations
121
Table 3.2 A basis of harmonic representatives for the de Rham cohomology of the Iwasawa P3 j manifold with respect to the metric g0 WD Nj j D1 ' ˇ ' k .I HdR 3 I C/
g0 -Harmonic representatives
kD1
' 1 , ' 2 , 'N 1 , 'N 2
kD2
13
11N
23
k .I dimC HdR 3 I C/
4 12N
21N
22N
1N3N
' ,' ,' ,' ,' ,' ,' ,' 131N
132N
231N
232N
11N3N
2N3N 12N3N
8 21N3N
22N3N
kD3
'
kD4
' 1231N , ' 1232N , ' 131N3N , ' 132N3N , ' 231N3N , ' 232N3N , ' 11N2N3N , ' 21N2N3N
kD5
'
123
,'
1231N3N
,'
,'
1232N3N
,'
,'
131N2N 3N
,'
,'
,'
231N2N 3N
,'
,'
,'
,'
1N 2N 3N
10 8 4
Fig. 3.3 Graph associated to the Iwasawa manifold
Rham cohomology of I3 , which can be computed by using K. Nomizu’s theorem [Nom54, Theorem 1]. In Table 3.2, we list the harmonic representatives with respect to the metric g0 instead of their classes and, as usually, we shorten the notation writing, for example, ' ABN WD ' A ^ 'N B . .I3 I R/ are of Remark 3.14. Note that all the g0 -harmonic representatives of HdR p;q q;p pure type with respect to J0 , that is, they are in .^ I3 ˚ ^ I3 / \ ^pCq I3 for some p; q 2 f0; 1; 2; 3g; this is no more true for Jt with t ¤ 0 small enough, see Theorem 4.8.
Remark 3.15. In [PT09], H. Pouseele and P. Tirao studied nilmanifolds associated with graphs, [DM05], providing a combinatorial way for computing the second de Rham cohomology group, [PT09, Theorem 1], and characterizing the graphs giving rise to a symplectic or contact nilmanifold, [PT09, Theorem 3, Proposition 4]. The Iwasawa manifold can be seen as associated to the graph in Fig. 3.3.
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3 Cohomology of Nilmanifolds
3.2.3 The Dolbeault Cohomology of the Iwasawa Manifold and of Its Small Deformations The Hodge numbers of the Iwasawa manifold and of its small deformations have been computed by I. Nakamura in [Nak75, p. 96]. The gt -harmonic representatives for H@; .Xt /, for t small enough, can be computed using the considerations in Sect. 3.1.2 and the structure equations given in Sect. 3.2.1.3. We collect here the results of the computations. In order to reduce the number of cases under consideration, recall that, on a compact complex Hermitian manifold X of complex dimension n, for any .p; q/ 2 N2 , the Hodge--operator and the conjugation induce an isomorphism p;q
'
nq;np
H@ .X / ! H@
'
np;nq
.X / ! H@
.X / :
• 1-forms. It is straightforward to check that ˛ ˝ H@1;0 .Xt / D C 't1 ; 't2 ; 't3
for
t 2 class (i)
and ˛ ˝ H@0;1 .Xt / D C 'Nt1 ; 'Nt2
t 2 classes (i), (ii), and (iii) :
for
Since @ 't3 ¤ 0 for Xt in class (ii) or in class (iii), one has ˛ ˝ H@1;0 .Xt / D C 't1 ; 't2
for
t 2 classes (ii) and (iii) W
this means in particular that Xt is not holomorphically parallelizable for t in classes (ii) and (iii), [Nak75, pp. 86, 96]. Summarizing, ( dimC H@1;0 .Xt /
D
3
for t 2 class (i)
2
for t 2 classes (ii) and (iii)
;
and dimC H@0;1 .Xt / D 2
for
t 2 classes (i), (ii), and (iii) :
• 2-forms. A straightforward computation yields ˛ ˝ H@2;0 .Xt / D C 't12 ; 't13 ; 't23 for t 2 class (i) ; E D N N N N N N for t 2 class (i) ; H@1;1 .Xt / D C 't11 ; 't12 ; 't21 ; 't22 ; 't31 ; 't32
3.2 The Cohomologies of the Iwasawa Manifold and of Its Small Deformations
123
and E D NN NN H@0;2 .Xt / D C 't13 ; 't23
for
t 2 classes (i), (ii), and (iii) :
We now compute H@2;0 .Xt / for t 2 classes (ii) and (iii). The H.3I C/-leftinvariant .2; 0/-forms are of the type A 't12 C B 't13 C C 't23 with A; B; C 2 C, so one has to solve the linear system 0
1 0 1 0 1 0 0 0 A 0 @ 0 N N A @ B A D @ 0 A I 21 11 C 0 0 22N 12N since the associated matrix to the system has rank 0 for t 2 class (i), rank 1 for t 2 class (ii), and rank 2 for t 2 class (iii), one concludes that dimC H@2;0 .Xt / D 2
t 2 class (ii)
for
(the generators being 't12 and a linear combination of 't13 and 't23 ) and dimC H@2;0 .Xt / D 1
for
t 2 class (iii)
(the generator being 't12 ). It remains to compute H@1;1 .Xt / for t 2 classes (ii) and (iii). For such t, one has that: three independent Jt -harmonic .1; 1/-forms are of the type 1 D N N N N A 't11 C B 't12 C C 't21 C D 't22 where A; B; C; D 2 C satisfy the equation 0
11N 12N 21N
1 A BB C C 22N B @C A D 0 ; D
whose matrix has rank 1 for t 2 classes (ii) and (iii) (while its rank is 0 for t 2 class (i)); two other independent Jt -harmonic .1; 1/-forms are of the type 13N 23N 31N 32N 2 D E 't C F 't C G 't C H 't where E; F; G; H 2 C are solution of the system
12 0 12N 11N 0 12 21N 22N
0
1 E
BF C 0 B C @ A D ; G 0 H
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3 Cohomology of Nilmanifolds
whose matrix has rank 2 for t 2 classes (i), (ii) and (iii); note also that no .1; 1/form with a non-zero component in 't33N can be Jt -harmonic. Hence, one can conclude that dimC H@1;1 .Xt / D 5
for
t 2 classes (ii) and (iii) :
Summarizing, 8 3 ˆ ˆ < 2;0 dimC H@ .Xt / D 2 ˆ ˆ : 1
for t 2 class (i) for t 2 class (ii) ; for t 2 class (iii)
and ( dimC H@1;1 .Xt /
D
6
for t 2 class (i)
5
for t 2 classes (ii) and (iii)
;
and dimC H@0;2 .Xt / D 2
for
t 2 classes (i), (ii), and (iii) :
• 3-forms. Finally, we have to compute H@3;0 .Xt / and H@2;1 .Xt /. A straightforward linear algebra computation yields to ˛ ˝ H@3;0 .Xt / D C 't123
for
t 2 classes (i), (ii), and (iii)
and E D N N N N N N H@2;1 .Xt / D C 't121 ; 't122 ; 't131 ; 't132 ; 't231 ; 't232
for
t 2 class (i) :
It remains to compute H@2;1 .Xt / for t 2 classes (ii) and (iii). Firstly, one notes that four of the six generators of the space of H.3I C/-left-invariant .2; 1/forms that are Jt -harmonic for t 2 class (i) can be slightly modified to get four @Jt -holomorphic .2; 1/-forms for t 2 class (ii) or class (iii): more precisely, one has H@2;1 .Xt / 22N 123N 132N 21N 123N 231N 12N 123N 232N 11N 123N 131N I C 't ' ; 't ' ; 't ' ; 't ' 12 t 12 t 12 t 12 t
3.2 The Cohomologies of the Iwasawa Manifold and of Its Small Deformations
125
in other words, four independent Jt -harmonic .2; 1/-forms are of the type 2 D C 't123N C D 't131N C E 't132N C F 't231N C G 't232N , where C; D; E; F; G 2 C are solution of the linear system 1 C BD C C B B C 11N B E C D 0 ; B C @F A G 0
12 22N 21N 12N
whose matrix has rank 1 for every t 2 classes (i), (ii), and (iii). Note that one can reduce to study the -harmonicity of the .2; 1/-forms of the type 1 D A 't121N C B 't122N : indeed, a .2; 1/-form D 1 C 2 C H 't133N C L 't233N , where H; L 2 C, is -harmonic if and only if H D 0 D L and both 1 and 2 are -harmonic. A .2; 1/-form of the type 1 is -harmonic if and only if A; B 2 C solve the linear system
11N 12N 21N 22N
A 0 D ; B 0
whose matrix has rank 0 for t 2 class (i), rank 1 for t 2 class (ii) and rank 2 for t 2 class (iii). In particular, one gets that dimC H@2;1 .Xt / D 5
for
t 2 class (ii)
and dimC H@2;1 .Xt / D 4
for
t 2 class (iii) :
Summarizing, dimC H@3;0 .Xt / D 1
for
t 2 classes (i), (ii), and (iii) ;
and 8 6 ˆ ˆ < 2;1 dimC H@ .Xt / D 5 ˆ ˆ : 4
for t 2 class (i) for t 2 class (ii) : for t 2 class (iii)
We summarize the Hodge numbers of the Iwasawa manifold and of its small deformations in Table 3.3, see also [Nak75, p. 96], and the dimensions of the conjugate Dolbeault cohomology of the Iwasawa manifold and of its small deformations in Table 3.4.
126
3 Cohomology of Nilmanifolds
Table 3.3 Dimensions of the Dolbeault cohomology of the Iwasawa manifold and of its small deformations, see also [Nak75, p. 96] H; @
h1;0 @
h0;1 @
h2;0 @
h1;1 @
h0;2 @
h3;0 @
h2;1 @
h1;2 @
h0;3 @
h3;1 @
h2;2 @
h1;3 @
h3;2 @
h2;3 @
I3 and (i) (ii) (iii)
3 2 2
2 2 2
3 2 1
6 5 5
2 2 2
1 1 1
6 5 4
6 5 4
1 1 1
2 2 2
6 5 5
3 2 1
2 2 2
3 2 2
Table 3.4 Dimensions of the conjugate Dolbeault cohomology of the Iwasawa manifold and of its small deformations H; @
h1;0 @
h0;1 @
h2;0 @
h1;1 @
h0;2 @
h3;0 @
h2;1 @
h1;2 @
h0;3 @
h3;1 @
h2;2 @
h1;3 @
h3;2 @
h2;3 @
I3 and (i)
2
3
2
6
3
1
6
6
1
3
6
2
3
2
(ii)
2
2
2
5
2
1
5
5
1
2
5
2
2
2
(iii)
2
2
2
5
1
1
4
4
1
1
5
2
2
2
3.2.4 The Bott-Chern and Aeppli Cohomologies of the Iwasawa Manifold and of Its Small Deformations In this section, using Theorems 3.6 and 3.7, we explicitly compute the dimensions ; of HBC .Xt /, for t small enough, [Ang11, Sect. 5.3]: such numbers are summarized in Table 3.5. In order to reduce the number of cases under consideration, recall that, on a compact complex Hermitian manifold X of complex dimension n, for every .p; q/ 2 N2 , ' p;q q;p the conjugation induces an isomorphism HBC .X / ! HBC .X /, and the Hodge-p;q
'
nq;np
operator induces an isomorphism HBC .X / ! HA that
.X /; furthermore, note
p;0 HBC .X / ' ker dW ^p;0 X ! ^pC1 .X I C/ and that n;0 HBC .X / ' H@n;0 .X / :
• 1-forms It is straightforward to check that ˛ ˝ 1;0 .Xt / D C 't1 ; 't2 HBC
for
t 2 classes (i), (ii), and (iii) :
• 2-forms It is straightforward to compute ˛ ˝ 2;0 .Xt / D C 't12 ; 't13 ; 't23 HBC
for
t 2 class (i) :
3.2 The Cohomologies of the Iwasawa Manifold and of Its Small Deformations
127
Table 3.5 Dimensions of the de Rham, Dolbeault, Bott-Chern, and Aeppli cohomologies of the Iwasawa manifold and of its small deformations HdR
b1
b2
b3
b4
b5
I3 and (i), (ii), (iii)
4
8
10
8
4
H; @
h1;0 @
h0;1 @
h2;0 @
h1;1 @
h0;2 @
h3;0 @
h2;1 @
h1;2 @
h0;3 @
h3;1 @
h2;2 @
h1;3 @
h3;2 @
h2;3 @
I3 and (i) (ii) (iii)
3 2 2
2 2 2
3 2 1
6 5 5
2 2 2
1 1 1
6 5 4
6 5 4
1 1 1
2 2 2
6 5 5
3 2 1
2 2 2
3 2 2
H; BC
h1;0 BC
h0;1 BC
h2;0 BC
h1;1 BC
h0;2 BC
h3;0 BC
h2;1 BC
h1;2 BC
h0;3 BC
h3;1 BC
h2;2 BC
h1;3 BC
h3;2 BC
h2;3 BC
I3 and (i) (ii.a) (ii.b) (iii.a) (iii.b)
2 2 2 2 2
2 2 2 2 2
3 2 2 1 1
4 4 4 4 4
3 2 2 1 1
1 1 1 1 1
6 6 6 6 6
6 6 6 6 6
1 1 1 1 1
2 2 2 2 2
8 7 6 7 6
2 2 2 2 2
3 3 3 3 3
3 3 3 3 3
H; A
h1;0 A
h0;1 A
h2;0 A
h1;1 A
h0;2 A
h3;0 A
h2;1 A
h1;2 A
h0;3 A
h3;1 A
h2;2 A
h1;3 A
h3;2 A
h2;3 A
I3 and (i) (ii.a) (ii.b) (iii.a) (iii.b)
3 3 3 3 3
3 3 3 3 3
2 2 2 2 2
8 7 6 7 6
2 2 2 2 2
1 1 1 1 1
6 6 6 6 6
6 6 6 6 6
1 1 1 1 1
3 2 2 1 1
4 4 4 4 4
3 2 2 1 1
2 2 2 2 2
2 2 2 2 2
2;0 The computations for HBC .Xt / reduce to find A; B; C 2 C satisfy the linear system
D A 't12 CB 't13 CC 't23 where
0
1 0 1 0 1 0 0 0 A 0 @ 0 N N A @ B A D @ 0 A ; 21 11 C 0 0 22N 12N whose matrix has rank 0 for t 2 class (i), rank 1 for t 2 class (ii), and rank 2 for t 2 class (iii); so, in particular, we get that 2;0 dimC HBC .Xt / D 2
for
2;0 dimC HBC .Xt / D 1
for
t 2 class (ii)
and t 2 class (iii) ˝ ˛ 2;0 .Xt / D C 't12 ). (more precisely, for t 2 class (iii) we have HBC 1;1 .Xt / for t 2 classes (i), (ii), and (iii). First of all, it It remains to compute HBC is easy to check that E D N N N N 1;1 for t 2 classes (i), (ii), and (iii) ; HBC .Xt / C 't11 ; 't12 ; 't21 ; 't22
128
3 Cohomology of Nilmanifolds
and equality holds if t 2 class (i), hence, in particular, if t D 0. This immediately implies that E D N N N N 1;1 HBC .Xt / D C 't11 ; 't12 ; 't21 ; 't22
for
t 2 classes (i), (ii), and (iii) I
1;1 indeed, the function t 7! dimC HBC .Xt / is upper-semi-continuous at 0, since 1;1 HBC .Xt / is isomorphic to the kernel of the self-adjoint elliptic differential operator Q BCJt b^1;1 Xt . (One can explain this argument saying that the new parts appearing in the computations for t ¤ 0 are “too small” to balance out the lack for the @-closure or the @-closure.) From another point of view, we can note that .1; 1/-forms of the type D A 't13N C B 't23N C C 't31N C D 't32N C E 't33N are Q BCJt -harmonic if and only if E D 0 and A; B; C; D 2 C satisfy the linear system 1 0 1 0 0 1 12 0 12N 11N A 0 B 0 12 22N 21N C B B C B0C C B C B B C @ N N 12 0 A @ C A D @ 0 A ; 12 11 D 0 22N 21N 0 12
whose matrix has rank 4 for every t 2 classes (i), (ii), and (iii). • 3-forms It is straightforward to compute ˛ ˝ 3;0 for t 2 classes (i), (ii), and (iii) : .Xt / D C 't123 HBC Moreover, N N N N N N N N N 2;1 .Xt / D C 't121 ; 't122 ; 't131 22 't123 ; 't132 C 21 't123 ; 't231 HBC 12 12 N N N N N C 12 't123 ; 't232 11 't123 12 12 for
t 2 classes (i), (ii), and (iii) I
in particular, D E N N N N N N 2;1 HBC .Xt / D C 't121 ; 't122 ; 't131 ; 't132 ; 't231 ; 't232
for
t 2 class (i) :
From another point of view, one can easily check that E D N N 2;1 HBC .Xt / C 't121 ; 't122
for
t 2 classes (i), (ii), and (iii) ;
and that the .2; 1/-forms of the type D A 't123N C B 't131N C C 't132N C D 't231N C E 't232N C F 't133N C G 't233N are Q BCJt -harmonic if and only if F D 0 D G and A; B; C; D; E 2 C satisfy the equation
3.2 The Cohomologies of the Iwasawa Manifold and of Its Small Deformations
129
1 A BB C C B B C 11N B C C D 0 ; B C @D A E 0
12 22N 21N 12N
whose matrix has rank 1 for every t 2 classes (i), (ii), and (iii). Note in particular 3;0 2;1 that the dimensions of HBC .Xt / and of HBC .Xt / do not depend on t. • 4-forms It is straightforward to compute D E N N 3;1 HBC .Xt / D C 't1231 ; 't1232
for
t 2 classes (i), (ii), and (iii)
and E D NN NN NN NN NN NN NN NN 2;2 HBC .Xt / D C 't1213 ; 't1223 ; 't1312 ; 't1313 ; 't1323 ; 't2312 ; 't2313 ; 't2323 t 2 class (i) :
for
Moreover, one can check that E D NN NN NN NN 2;2 HBC .Xt / C 't1213 ; 't1223 ; 't1312 ; 't2312
for
t 2 classes (i), (ii), and (iii) ;
NN and that no .2; 2/-form with a non-zero component in 't1212 can be Q BCJt 2;2 harmonic. For HBC .Xt / with t 2 classes (ii) and (iii), we get a new behaviour: there are subclasses in both class (ii) and class (iii), which can be distinguished 2;2 by the dimension of HBC .Xt /. Indeed, consider .2; 2/-forms of the type D N N N N 1313 1323 231N 3N 232N 3N C B 't C C 't C D 't ; a straightforward computation shows A 't that such a is Q BCJt -harmonic if and only if A; B; C; D 2 C satisfy the linear system 0 1 A
BB C 22N 12N 21N 11N 0 B C @ AD : 22N 21N 12N 11N C 0 D
As one can straightforwardly note, the rank of the matrix involved is 0 for t 2 class (i), while it is 1 or 2 depending on the values of the parameters within class (ii), or within class (iii). Therefore 2;2 .Xt / D 7 dimC HBC
for
t 2 subclasses (ii.a) and (iii.a)
for
t 2 subclasses (ii.b) and (iii.b) :
and 2;2 .Xt / D 6 dimC HBC
130
3 Cohomology of Nilmanifolds
3;2 • 5-forms Finally, let us compute HBC .Xt /. It is straightforward to check that
E D NN NN NN 3;2 .Xt / D C 't12312 ; 't12313 ; 't12323 HBC
for
t 2 classes (i), (ii), and (iii) W
in particular, it does not depend on t 2 .0; "/. We summarize the results of the computations above in the following theorem, and the dimensions of the Bott-Chern cohomology and of the Aeppli cohomology of the Iwasawa manifold and of its small deformations in Table 3.5. Theorem 3.9 ([Ang11, Theorem 5.1]). Consider the Iwasawa manifold I3 WD H .3I Z Œi/n H.3I C/ and the family fXt D .I3 ; Jt /gt2.0;"/ of its small deformap;q tions, where " > 0 is small enough and X0 D I3 . Then the dimensions hBC WD p;q p;q 3p;3q .Xt / does not depend on t 2 .0; "/ hBC .Xt / WD dimC HBC .Xt / D dimC HA whenever p C q is odd or .p; q/ 2 f.1; 1/; .3; 1/; .1; 3/g, and they are equal to 0;1 h1;0 BC D hBC D 2 ; 0;2 h2;0 BC D hBC 2 f1; 2; 3g ; 0;3 h3;0 BC D hBC D 1 ; 1;3 h3;1 BC D hBC D 2 ;
h1;1 BC D 4 ; 1;2 h2;1 BC D hBC D 6 ;
h2;2 BC 2 f6; 7; 8g ;
2;3 h3;2 BC D hBC D 3 :
Remark 3.16 ([Ang11, Remark 5.2]). As a consequence of the computations above, we notice that the Bott-Chern cohomology yields a finer classification of the small 2;2 deformations of I3 than the Dolbeault cohomology: indeed, note that dimC HBC .Xt / assumes different values according to different parameters in class (ii), respectively in class (iii); in a sense, this says that the Bott-Chern cohomology “carries more informations” about the complex structure that the Dolbeault one. Note also that most of the dimensions of Bott-Chern cohomology groups are invariant under small deformations: this happens for example for the odd-degree Bott-Chern cohomology groups. In order to have a complete view, we summarize in Table 3.5 the dimensions of the de Rham, Dolbeault, Bott-Chern, and Aeppli cohomologies of the Iwasawa manifold and of its small deformations, see [Ang11, Appendix].
3.3 Cohomologies of Six-Dimensional Nilmanifolds In this section, we provide the results of the computations, obtained jointly with M.G. Franzini and F.A. Rossi in [AFR12], of the Bott-Chern cohomology for each of the complex structures on six-dimensional nilmanifolds in M. Ceballos,
3.3 Cohomologies of Six-Dimensional Nilmanifolds
131
A. Otal, L. Ugarte, and R. Villacampa’s classification, [COUV11], see Table 1.4. In view of [Rol11a, Corollary 3.10], and noting that the unique (up to equivalence) left-invariant complex structure on the nilmanifold corresponding to h7 is a rational complex structure, one is reduced to study left-invariant Q BC -harmonic forms. In Figs. 3.4 and 3.5, we provide a graphical visualization of the non-Kählerianity of such six-dimensional nilmanifolds by means of the degrees 1 , 2 , and 3 , where X p;q p;q dimC HBC .X / C dimC HA .X / 2 bk 2 N : k WD pCqDk
Remark 3.17. Note that the Dolbeault, respectively Bott-Chern, cohomology being left-invariant is not an up-to-equivalence property: that is, the Bott-Chern numbers in Table 3.6 provide a complete picture except for nilmanifolds with Lie algebra isomorphic to h7 . Theorem 3.10 ([AFR12, Theorem 2.1], [LUV12, §3]). Consider a six-dimensional nilpotent Lie algebra endowed with a linear integrable complex structure. Equivalently, consider a six-dimensional nilmanifold with Lie algebra nonisomorphic to h7 D .0; 0; 0; 12; 13; 23/ and endowed with any left-invariant complex structure, or a nilmanifold with Lie algebra isomorphic to h7 and endowed with the left-invariant complex structure in M. Ceballos, A. Otal, L. Ugarte, and R. Villacampa’s classification, [COUV11]. Then the dimensions of the Bott-Chern cohomology are provided in Table 3.6. Remark 3.18. Similar computations have been performed by A. Latorre, L. Ugarte, and R. Villacampa, [LUV12]: more precisely, they computed the dimensions of the Bott-Chern cohomology for any complex structure on a six-dimensional nilpotent Lie algebra using the classification of [COUV11], with the aim to study variations of the cohomology under deformations of the complex structure, and the behaviour of the cohomology in relation to the existence of balanced Hermitian metrics or strongly-Gauduchon metrics. Balanced metrics (that is, Hermitian metrics whose fundamental form is co-closed, [Mic82]) on six-dimensional nilmanifolds have been studied by A. Latorre, L. Ugarte, and R. Villacampa in [LUV12]; see also [Uga07]; see also [Fra11] by M.G. Franzini for some results concerning balanced metrics on nilmanifolds and solvmanifolds. On the other hand,1 as regards pluriclosed (or strong Kähler with torsion, shortly SKT) metrics, namely, Hermitian metrics whose fundamental form ! satisfies the condition @@! D 0, [Bis89], and the
1
We recall that, on six-dimensional nilmanifolds endowed with left-invariant complex structures, the existence of pluriclosed metrics and the existence of balanced metrics are complementary properties: indeed, by [FPS04, Proposition 1.4], see also [AI01, Remark 1], any Hermitian metric being both pluriclosed and balanced is in fact Kähler, and by [FPS04, Theorem 1.2] the pluriclosed property is satisfied by either all left-invariant Hermitian metrics or by none.
132
3 Cohomology of Nilmanifolds Δ3 :=
p+q=3
p,q (hp,q BC + hA ) − 2 b3
12
11
18c
21b
21a
21d
21c 17a
21e
22
18b
18a
10
20a 17c
17b
19 20b
17d
9
04 8 13 03
7
06c
09b
09a
09c
15a 07a
06b 6
06a 09d 02a
02b
08
16b
14
09e
16a 05
02c
09f
01a
15b
01b
07b
5
24 23
4 12
3
26 2 10
11 25
1
0 00
1
2
3
4
5
6
7
8
9 Δ2 :=
p+q=2
p,q (hp,q BC + hA ) − 2 b2
1
2
Δ1 :=
p+q=1
p,q (hp,q BC + hA ) − 2 b1
Fig. 3.4 Non-Kählerianity of six-dimensional nilmanifolds endowed with the left-invariant complex structures in M. Ceballos, A. Otal, L. Ugarte, and R. Villacampa’s classification, [COUV11], from [AFR12]
possible connection between the existence of such metrics and cohomological properties, we recall here some results and examples obtained in [AFR12]. See also [Uga07].
3.3 Cohomologies of Six-Dimensional Nilmanifolds
133
h3BC + h3A − ·b3 h3BC + h3A − 2 · b3
18b 18c
18a 17d 17c 17b 18a
17a
19
12
22
21e 20b 21d 21a 21b 21c
21a
06a 05 02c 02b 02a 06b 01b 06c 01a 07a 16a
08 09a 09b 09c
11
11
10
10
9
9
03 04
8 03
16b
07b
13
15b 15a
06a 8 09a
11
2
02a
2
05 02b 09d
06c 09b
7
6
6
5
5
4 12
4
10
24
26
06a
02c
04 06b
09f 01a
07a
01b
14
15b
17c
20a 17b
21b
21c
17d
20b
19 21e
21d 22
07b
13 08
09c
09e
16a
15a
16b
12
23
10
7
25
h1BC + h1A − 2 · b1
02a
18b 18c
14
09d 09e 09f
h1BC + h1A − 2 · b1
17a
12
20a
11
25 26
23 24
3
3
2
2
1
1
0 00
0 00 0
1
0
1
0 00 0
1
2
3
4
5
6
7
8
2 h2BC + hA − 2 · b2
09a
02c
06b
02b
06c
10
09b
03 1
04
09d
05
07a
01a
23
12 2
09c
11
09e
24
09f
14
01b
15a
3 26
16a
25
15b 16b
18a
13 4
17a 5
21a
17b
08
17c
07b
18b
21c
6
18c 21b
20a 17d 7
21d 22
19
8
20b 21e
h2BC + h2A − 2 · b2
Fig. 3.5 Non-Kählerianity of six-dimensional nilmanifolds endowed with the left-invariant complex structures in M. Ceballos, A. Otal, L. Ugarte, and R. Villacampa’s classification, [COUV11]
J2D
J1 J2 J1 J2D
J2D J2D J1D J1D J2
h3 h3 h4 h4
h4 h4 h5 h5 h5
h5
03 04 05 06a
06b 06c 07a 07b 08
09a
J3
.; D/
J2D
h2
h2
02b
02c
J J1D J1D J2D
h1 h2 h2 h2
00 01a 01b 02a
2 0 < 4 .ImD/ < 1
D 0; ReD ¤ 12 ;
D1 D 2 2 0; 14 D0
0 < ImD <
2 2
C 4 ReD; ImD > 0 n [ 0 < 2 < 12 ; ReD D 0; o
n
D D D D
D 2 R n f2; 0; 1g
o
ReD ¤ 1; 2 jDj C 2 ReD D 0; ImD > 0
D ¤ i; ImD D 1 DDi ReD ¤ 1; 2 jDj C 2 ReD ¤ 0; ImD > 0 ReD D 1; ImD > 0
0; c 0; B 2 C; D2C S .B; c/ WD c 4 2 jBj2 C 1 c 2 C 2 2 jBj 1
2 2 2 2
2
2 2 2 2 2
2
X
2
3 2 2 2
1;0
hBC
X
Balanced [LUV12, Uga07]
X
X X
[AFR12, Uga07]
SKT
2
2 2 2 2 2
2 2 2 2
2
2
3 2 2 2
0;1
hBC
1
1 1 1 2 3
1 1 1 1
1
1
3 1 1 1
2;0
hBC
4
4 5 4 6 4
4 4 4 4
5
4
9 4 4 4
1;1
hBC
1
1 1 1 2 3
1 1 1 1
1
1
3 1 1 1
0;2
hBC
1
1 1 1 1 1
1 1 1 1
1
1
1 1 1 1
3;0
hBC
6
6 6 6 6 6
6 6 6 6
6
6
9 6 6 6
2;1
hBC
6
6 6 6 6 6
6 6 6 6
6
6
9 6 6 6
1;2
hBC
1
1 1 1 1 1
1 1 1 1
1
1
1 1 1 1
0;3
hBC
2
2 2 3 3 2
3 3 3 2
2
2
3 3 3 2
3;1
hBC
6
7 6 6 6 8
7 7 6 6
6
7
9 6 7 6
2;2
hBC
2
2 2 3 3 2
3 3 3 2
2
2
3 3 3 2
1;3
hBC
3
3 3 3 3 3
3 3 3 3
3
3
3 3 3 3
3;2
hBC
3
3 3 3 3 3
3 3 3 3
3
3
3 3 3 3
2;3
hBC
4
4 4 4 4 4
5 5 4 4
4
4
6 4 4 4
b1
8
8 8 8 8 8
9 9 8 8
8
8
15 8 8 8
b2
10
10 10 10 10 10
10 10 10 10
10
10
20 10 10 10
b3
2
2 2 2 2 2
0 0 2 2
2
2
0 2 2 2
1
0
1 1 2 6 6
1 1 2 0
1
1
0 2 3 0
2
8
8 8 8 8 8
8 8 8 8
8
8
0 8 8 8
3
Table 3.6 Dimensions of the Bott-Chern cohomology of the six-dimensional nilpotent Lie algebras endowed with the linear integrable complex structure classified by M. Ceballos, A. Otal, L. Ugarte, and R. Villacampa in [COUV11], see [AFR12, Table 2], see also [LUV12]
134 3 Cohomology of Nilmanifolds
.; D/
2 1 ; 2 o 2
J JB
JB
h11 h12
h12
16b
ReB D 12 ; ImB ¤ 0
1 2
X
X
X
BD ReB ¤ 12 ; ImB ¤ 0
15b 16a
1
B
J3 J J J J J JB
1 ; 2
.; D/
h5 h6 h7 h8 h9 h10 h11
09f 10 11 12 13 14 15a
J3
˚ B 2 R n 0;
D 0; D D 0
.; D/
h5
09e
1 2
2 > 1; ReD D 0; 2 0 < ImD < 21 ; 2 .ImD/ D 2 1 ˚ 0 < 2 < 12 ; D D 0 ˚1 2 [ 2 < 1; D D 0 ˚ [ 2 > 1; D D 0
.; D/
h5
09d
J3
D 0; D D
.; D/
ImD D 0
˚ ReD 62 0; 12 ; 1 C 4 ReDo > 0;
n [ D 0;
2 < 3; 0 < 4 .ImD/ o ImD > 0
.ImD/ ¤ 1 n D 0; ReD D 12 ;
2
0 < ImD <
h5
J3
J3
1 2 < 1; 2 ReD D 0;
09c
09b”
09b’ h5
n
o 2 0 < ImD < 1 2 n [ 2 > 1; ReD D 0;
[
X
1
1 1
2 2 1 2 1 1 1
2
2
2
2
1
1 1
2 2 1 2 1 1 1
2
2
2
2
1
1 1
2 2 2 2 1 1 1
2
1
1
1
4
4 4
4 5 5 6 4 4 4
4
5
4
4
1
1 1
2 2 2 2 1 1 1
2
1
1
1
1
1 1
1 1 1 1 1 1 1
1
1
1
1
5
5 5
6 6 6 7 5 5 5
6
6
6
6
5
5 5
6 6 6 7 5 5 5
6
6
6
6
1
1 1
1 1 1 1 1 1 1
1
1
1
1
2
2 2
2 2 2 3 3 2 2
2
2
2
2
6
6 5
7 6 5 8 6 5 5
6
6
8
7
2
2 2
2 2 2 3 3 2 2
2
2
2
2
3
3 3
3 3 3 3 3 3 3
3
3
3
3
3
3 3
3 3 3 3 3 3 3
3
3
3
3
3
3 3
4 4 3 5 4 3 3
4
4
4
4
6
6 6
8 9 8 11 7 6 6
8
8
8
8
8
8 8
10 12 12 14 8 8 8
10
10
10
10
4
4 3
3 1 2 2 4 3 3
2
1
2
1
8
8 8
8 4 4 4 8 8 8
8
8
8
8
(continued)
2
2 2
2 2 2 0 0 2 2
2
2
2
2
3.3 Cohomologies of Six-Dimensional Nilmanifolds 135
c 62 f0; jB 1j ; jBjg ; B ¤ 1; S .B; c/ > 0 0 < c D jBj < 12
.B; c/
c > 2; B D 1
c D 0; B ¤ 0; jBj ¤ 1
c D 0; B D 0 jBj D 1; B ¤ 1
.B; c/
.B; c/
.B; c/
.B; c/
h15
h15 h16 h 19 h 19 C h26 C h26
21d
21e 22 23 24 25 26
J3 JB J1 J2 J1 J2
J3
J3
J3
h15
h15
21b
21c
J3
h15
21a
c2 6 f0; 1g cD0
X X
1
J .B; c/ J1 J2c J2c
B D 1; c D 2
J .B; c/
h14
h14 h15 h15 h15
1 1
c D 1; B D 1 B ¤ 1; c 62 f0; jBj ; jB 1jg ; S .B; c/ D 0 c D jBj D 12 ; ReB ¤ 12
1 1 1 1 1 1
1
1
1
1
1 1 1 1
1
1
1
1;0
hBC
Balanced [LUV12, Uga07]
18b
J .B; c/ J .B; c/
SKT
[AFR12, Uga07]
c 62 fjB 1j ; jBjg ; .c; jBj/ ¤ .0; 1/ ; S .B; c/ < 0; B ¤ 1 c D jBj > 12 ; ReB ¤ 1 ; B ¤1 2 c 62 f0; 1g; c < 2; B D 1
18c 19 20a 20b
J
h13
h13 h14
17c
17d 18a
J .B; c/
h13
17b
.B; c/
J
h13
17a
.B; c/
0; c 0; B 2 C; D2C S .B; c/ WD c 4 2 jBj2 C 1 c 2 C 2 jBj2 1
Table 3.6 (continued)
1 1 1 1 1 1
1
1
1
1
1 1 1 1
1
1 1
1
1
1
0;1
hBC
2 2 1 1 1 1
2
1
1
1
1 1 1 2
1
1 1
1
1
1
2;0
hBC
4 4 2 2 2 2
4
5
4
4
5 5 4 4
4
5 4
5
4
4
1;1
hBC
2 2 1 1 1 1
2
1
1
1
1 1 1 2
1
1 1
1
1
1
0;2
hBC
1 1 1 1 1 1
1
1
1
1
1 1 1 1
1
1 1
1
1
1
3;0
hBC
5 5 3 3 3 3
5
5
5
5
5 5 5 5
5
5 5
5
5
5
2;1
hBC
5 5 3 3 3 3
5
5
5
5
5 5 5 5
5
5 5
5
5
5
1;2
hBC
1 1 1 1 1 1
1
1
1
1
1 1 1 1
1
1 1
1
1
1
0;3
hBC
2 2 2 2 2 2
2
2
2
2
2 3 3 3
2
2 2
2
2
2
3;1
hBC
7 5 4 4 3 3
5
5
6
5
5 5 5 5
6
6 5
5
6
5
2;2
hBC
2 2 2 2 2 2
2
2
2
2
2 3 3 3
2
2 2
2
2
2
1;3
hBC
3 3 2 2 2 2
3
3
3
3
3 3 3 3
3
3 3
3
3
3
3;2
hBC
3 3 2 2 2 2
3
3
3
3
3 3 3 3
3
3 3
3
3
3
2;3
hBC
3 3 3 3 2 2
3
3
3
3
3 3 3 3
3
3 3
3
3
3
b1
5 5 5 5 4 4
5
5
5
5
5 5 5 5
5
5 5
5
5
5
b2
6 6 6 6 6 6
6
6
6
6
6 6 6 6
6
6 6
6
6
6
b3
2 2 0 0 2 2
2
2
2
2
2 2 2 2
2
2 2
2
2
2
1
9 7 2 2 3 3
7
6
6
5
6 8 7 9
6
7 5
6
6
5
2
12 12 4 4 4 4
12
12
12
12
12 12 12 12
12
12 12
12
12
12
3
136 3 Cohomology of Nilmanifolds
3.3 Cohomologies of Six-Dimensional Nilmanifolds
137
More precisely, the following result points out which of the complex manifolds in Table 3.6 admit pluriclosed metrics. Theorem 3.11 ([AFR12, Theorem 4.1]). Let X be a six-dimensional nilmanifold and J be a left-invariant complex structure on X . Then .X; J / admits a leftinvariant pluriclosed metric if and only if [00] [01b] [02b] [06b]
.X; .X; .X; .X;
J / D .h 1 ; J /; J / D h2 ; J1D with D D i; J / D h2 ; J2D with D 2 C such that ReD D 1 and ImD > 0; J / D h4 ; J2D with D D 1; .;
[09b’] .X; J / D h5 ; J3 D/ with D 0 and D D 12 Ci y such that 0 < 4y 2 < 3; .; [09c] .X; J / D h5 ; J3 D/ with D 0 and D D 12 ; [12] .X; J / D .h8 ; J /, where we use the notation in Tables 1.4 and 3.6, and in [COUV11]. The following example shows a curve of complex structures admitting pluriclosed metrics and with jumping Bott-Chern cohomology numbers. Example 3.1 (A curve of complex structures on a six-dimensional nilmanifold such that each complex structure admits a pluriclosed metric and the dimensions of the Bott-Chern cohomology groups are not constant, [AFR12, Example 4.3]). Consider the nilmanifold X with associated Lie algebra h2 WD 04 ; 12; 34 ; and consider the family fJt gt 2R of left-invariant complex structures on X such that 1 a left-invariant for the C .X /-module of the .1; 0/-forms on .X; Jt / is ˚ 1 co-frame 2 3 given by t ; t ; t satisfying 8 d ˆ ˆ < d ˆ ˆ : d
1 t
D0
2 t
D0
3 t
D t t1 ^ t2 C t1 ^ N t1 C t t1 ^ N t2 C t 2 C i
: 2 t
^ N t2
One has three different cases (we use the notation in Tables 1.4 and 3.6, and [COUV11]): t D 0: for t D 0, the left-invariant complex structure Jt is equivalent to the leftinvariant Abelian complex structure J1D with D D i (case 01b in Table 3.6); t ¤ 0: for t ¤ 0, the left-invariant complex structure Jt is equivalent to the leftinvariant non-Abelian complex structure J2D with D D 1 C t12 i (case 02b in Table 3.6).
138
3 Cohomology of Nilmanifolds
In particular, by Theorem 3.11, .X; Jt / admits a left-invariant pluriclosed 3;1 1;3 metric for every t 2 R. Notwithstanding, note that dimC HBC .X; J0 / D dimC HBC 3;1 1;3 .X; J0 / D 3, while dimC HBC .X; Jt / D dimC HBC .X; Jt / D 2 for t ¤ 0. For the sake of completeness, we provide the following example, compare also [COUV11, Theorem 7.9]: it shows a curve of complex structures on a sixdimensional nilmanifold admitting both pluriclosed and balanced metrics. Example 3.2 (A curve of compact complex manifolds endowed with special metrics, [AFR12, Example 4.6], see also [COUV11, Theorem 7.9]). Let X be a nilmanifold with associated Lie algebra h4 WD 04 ; 12; 14 C 23 ; ˚ and consider on it the family J2D D2Rnf0g of left-invariant non-Abelian complex ˚ 1 2 3 as a C 1 .X I C/; !D ; !D structures whose .1; 0/-forms are generated by !D module and such that 8 d ! 1 WD 0 ˆ ˆ < D 2 : WD 0 d !D ˆ ˆ : N N N 3 12 11 12 22 d !D WD !D C !D C !D C D !D By [FPS04, Theorem 3.2] and [Uga07, Theorem 26], the Lie algebra h4 admits both pluriclosed and balanced left-invariant Hermitian structures. Let r;s;t;u;v;z 11N 22N 33N 12N 21N 23N 32N C u !D ˝D WD i r 2 !D C s 2 !D C t 2 !D uN !D C v !D vN !D N
N
13 31 Cz !D zN !D
the generic J2D -Hermitian structure, where r; s; t 2 R n f0g
and u; v; z 2 C
satisfy 8 r 2 s 2 > juj2 ˆ ˆ ˆ ˆ ˆ < s 2 t 2 > jvj2 ; ˆ r 2 t 2 > jzj2 ˆ ˆ ˆ ˆ : 2 2 2 r s t C 2 Re .i uN vN z/ > t 2 juj2 C r 2 jvj2 C s 2 jzj2 see, e.g., [UV09, pp. 3–4].
3.3 Cohomologies of Six-Dimensional Nilmanifolds
139
By computing NN
r;s;t;u;v;z 1212 D i t 2 .1 D/ !D ; @@ ˝D r;s;t;u;v;z is pluriclosed if and only if D D 1, compare also [FPS04, we have that ˝D Theorem 1.2]. Computing
r;s;t;u;v;z 2 1 r;s;t;u;v;z 1231N 2N 1 r;s;t;u;v;z 121N 2N 3N d ˝D D D !D D !D 2 2 where r;s;t;u;v;z D WD D t 2 r 2 D jvj2 C t 2 s 2 jzj2 C i t 2 u C v zN ; r;s;t;u;v;z we have that ˝D is balanced if and only if D, r, s, t, u, v, z are such that r;s;t;u;v;z D D 0. 2 Firstly, suppose that 0 < D < 14 . Let s; t 2 R n f0g be such that s 2 > D C s 2 and consider the J2D -Hermitian metric 1;s;t;i .DCs 2 /;0;0
s;t WD ˝D ˝D
N N N N N D i ! 11 C s 2 ! 22 C t 2 ! 33 Ci D C s 2 ! 12 C ! 21 I
s;t a straightforward computation shows that ˝D is balanced, see also [COUV11, Sect. 7]. In the case D D 14 , it turns out that J2D admits no balanced metric, see also [COUV11, Theorem 7.9]. Indeed, for the sake of completeness, we note that, if ˝Q were a balanced metric, then one should have
Z
Q2
0D
3
˝ ^ d !1 X
4
!3 1 4
Z
Q2
˝ ^
D X
p
p
2 2 2! 1 C !1 2 4 4 1
! ^
p
2! 1 1 4
! p 2 2 C ! > 0; 2 14
which yields an absurd. Summarizing, we have a curve such that:
• for 0 < D < 14 , there exists a balanced metric on X; J2D ;
1 • for D D 14 , the complex manifold X; J24 admits no balanced metric and no pluriclosed metric; • for D D 1, there exists a pluriclosed metric on X; J21 . Compare also [COUV11, Theorem 7.9], where the same example is studied in order to prove that the existence of balanced metrics and the existence of stronglyGauduchon metrics are not closed under deformations of the complex structure.
140
3 Cohomology of Nilmanifolds
Remark 3.19. For the study of the behaviour of the cohomology in relation to the existence of Hermitian balanced metrics or strongly-Gauduchon metrics, we refer to [LUV12] by A. Latorre, L. Ugarte, and R. Villacampa, where, for any balanced structure, they also determine L.-S. Tseng and S.-T. Yau’s spaces parametrizing the deformations in type IIB supergravity, see [TY11].
Appendix: Cohomology of Solvmanifolds Enlarging the class of nilmanifolds to solvmanifolds, it turns out that the leftinvariant forms are usually not enough to recover the whole de Rham cohomology: see, for example, the non-completely-solvable solvmanifold provided in [dBT06, Sects. 3 and 4, Corollary 4.2]. The de Rham cohomology of solvmanifolds has been studied by several author, e.g., A. Hattori [Hat60], G.D. Mostow [Mos54], D. Guan [Gua07], S. Console and A.M. Fino [CF11], and by H. Kasuya [Kas13a, Kas12a, CFK13]; several results concerning the Dolbeault cohomology (and Hodge symmetry, Hodge decomposition, formality, and the Hodge and Frölicher spectral sequence on solvmanifolds) have been proven by H. Kasuya [Kas13b, Kas12a, Kas11, Kas12d, Kas12b]. In this appendix, we briefly summarize some results concerning the Dolbeault and Bott-Chern cohomologies of a class of solvmanifolds, [Kas13a,Kas13b,AK12], recalling the results on the computations of the cohomologies of the Nakamura manifold as an example. As regards the de Rham cohomology of solvmanifolds, H. Kasuya proved the following result in [Kas13a], see also [Kas12a]. Theorem 3.12 ([Kas13a, Corollary 7.6], [Kas12a, Theorem 1.3]). Let nG be a solvmanifold. Then there exists a finite-dimensional sub-complex A of the de Rham complex .^ X ˝R C; d/ such that the inclusion A ; d ,! ^ nG ˝R C; d is a quasi-isomorphism. Remark 3.20 ([Kas13a, Kas12a]). The sub-complex A in the previous theorem is constructed as follows, see [Kas13a, Kas12a]. Consider a connected simply-connected n-dimensional solvable Lie group G admitting a discrete co-compact subgroup . Denote by g the solvable Lie algebra of G and by gC WD g ˝R C its complexification, and let n be the nilradical2 of g.
2
Recall that the nilradical of a solvable Lie algebra is the maximal nilpotent ideal in g.
3.3 Cohomologies of Six-Dimensional Nilmanifolds
141
Consider the adjoint representation adW g 3 X 7! ŒX; 2 gl.g/ and, for every X 2 g, take the semi-simple part .adX /s 2 gl.g/ of the Jordan decomposition3 of adX . By considering a vector sub-space V of g such that g D V ˚ n as the direct sum of vector spaces and such that, for any A; B 2 V , it holds that .adA /s .B/ D 0, see, e.g., [DtER03, Proposition III.1.1], one can define the representation ads W g D V ˚ n 3 .A; X / 7! .ads /ACX WD .adA /s 2 gl.g/. Denote by Ads W G ! Aut.gC / the unique representation which lifts ads W g ! gl.gC /, see, e.g., [War83, Theorem 3.27]. Set C WD fˇ ı Ads 2 Hom .GI C / W ˇ 2 Hom.T I C / s.t. .ˇ ı Ads / b D 1g, where T is the Zariski-closure of Ads .G/ in L Aut.gC /. Hence, define A as the space consisting of Ads .G/-invariant elements of ˛2C ˛ ^ gC , namely, A WD
8 < :
'2
M ˛2C
˛ ^ gC W .Ads /g .'/ D ' for every g 2 G
9 = ;
^ nG ˝R C :
By considering a basis fX1 ; : : : ; Xn g of gC with respect to which Ads D diag .˛1 ; : : : ; ˛n / W G ! Aut.gC / for some characters ˛1 2 Hom.GI C /, . . . , ˛n 2 Hom.GI C /, and by considering its dual basis fx1 ; : : : ; xn g of gC , then one gets, for p 2 Z, ˇ D E ˇ p A D C ˛i1 ˛ip xi1 ^ ^ xip ˇ 1 i1 < < ip n such that ˛i1 ˛ip D 1 :
As regards the Dolbeault cohomology of solvmanifolds, one can focus on two special classes of solvmanifolds: namely, holomorphically parallelizable solvmanifolds, [Wan54, Nak75], and solvmanifolds of splitting type, [Kas13b]. Definition 3.1 ([Kas13b, Assumption 1.1]). A solvmanifold X D nG endowed with a G-left-invariant complex structure J is said to be of splitting type if G is a semi-direct product Cn Ë N satisfying the following assumptions: 1. N is a connected simply-connected 2m-dimensional nilpotent Lie group endowed with an N -left-invariant complex structure JN ; 2. for any t 2 Cn , one has that .t/ 2 GL.N / is a holomorphic automorphism of N with respect to JN ; 3. induces a semi-simple action on the Lie algebra of N .
Recall that, given a vector space V , any linear map A 2 gl.V / admits a unique Jordan decomposition A D As C An , where As 2 gl.V / is semi-simple (that is, each As -invariant subspace of V admits an As -invariant complementary sub-space in V ), and An 2 gl.V / is nilpotent (that is, there exists N 2 N such that AN n D 0), and As and An commute, see, e.g., [DtER03, II.1.10], [Kir08, Theorem 5.59].
3
142
3 Cohomology of Nilmanifolds
For such classes of solvmanifolds, H. Kasuya proved the following result in [Kas13b], see also [Kas12a]. Theorem 3.13 ([Kas13b, Corollary 4.2], [Kas12a, Corollary 6.2]). Let nG be a solvmanifold endowed with a G-left-invariant complex structure. Suppose that • either nG is ofsplitting type, with the nilpotent factor N such that the inclusion ^; n ; @ ,! ^; N; @ is a quasi-isomorphism, where n is the Lie algebra of N , • or nG is holomorphically parallelizable. ; Then there exists a finite-dimensional sub-complex B of the Dolbeault complex ^; X; @; @ such that the inclusion
B; ; @ ,! ^; nG ; @ is a quasi-isomorphism. Remark 3.21 ([Kas13b]). In the case of solvmanifolds of splitting type, the subcomplex B; in the previous theorem is constructed as follows, see [Kas13b]. Consider the standard basis fX1 ; : : : ; Xn g of Cn , and consider a basis fY1 ; : : : ; Ym g of the i-eigen-space n1;0 of JN 2 Aut.n/ such that the induced action on n1;0 is represented by D diag .˛1˚; : : : ; ˛m / for ˛1 2 Hom.Cn I C /, . . . , 1 ˛m 2 Hom.Cn I C / characters of Cn . Let x1 ; : : : ; xn ; ˛11 y1 ; : : : ; ˛m ym be the 1;0 basis of ^ gC which is dual to fX1 ; : : : ; Xn ; ˛1 Y1 ; : : : ; ˛m Ym g. By [Kas13b, Lemma 2.2], for any j 2 f1; : : : ; mg, there exist unique unitary characters ˇj and j of Cn such that ˛j ˇj1 and ˛N j j1 are holomorphic. (By shortening, e.g., ˛I WD ˛i1 ˛ik for a multi-index I D .i1 ; : : : ; ik /) define B; ^; nG , for .p; q/ 2 Z2 , as p;q
B
ˇ ˝ WD C xI ^ ˛J1 ˇJ yJ ^ xN K ^ ˛N L1 L yNL ˇ jI j C jJ j D p and jKj C jLj D q such that .ˇJ L / b D 1i :
Remark 3.22 ([Kas11]). In [Kas11], H. Kasuya studied conditions assuring that a splitting type solvmanifold satisfies Hodge symmetry and Hodge decomposition. More precisely, let G D Cn Ë Cm be such that X WD nG is a solvmanifold of splitting type. Consider a set fz1 ; : : : ; zn g of coordinates on Cn and a set fw1 ; : : : ; wm g of coordinates on Cm . With the same notations as in Remark 3.21, let ˛1 2 Hom.Cn I C /, . . . , ˛m 2 Hom.Cn I C / be characters such that D diag.˛1 ; : : : ; ˛m /, and let ˇj 2 Hom.Cn I C / and j 2 Hom.Cn I C / be the unique unitary characters such that ˛j ˇj1 and ˛N j j1 are holomorphic, for j 2 f1; : : :P ; mg, [Kas13b, Lemma the G-left-invariant Hermitian metric P 2.2].1Consider g WD nhD1 d zh ˇ d zNh C m N k1 d wk ˇ d wN k . H. Kasuya proved in [Kas11, kD1 ˛k ˛ Theorem 1.4] that (see also [Has06, Kas11, Remark 2]):
3.3 Cohomologies of Six-Dimensional Nilmanifolds
143
• if the characters ˛1 , . . . , ˛m are real-valued (namely, G is completely-solvable), then ker g D ker g , that is, X is a non-Kähler manifold satisfying the Hodge symmetry; • if, for any multi-indices K; L f1; : : : ; mg, it holds that .ˇK L /b D 1 if and only if ˛K ˛N L D 1, then ker g D ker g , that is, X satisfies the Hodge decomposition. In particular, by [DGMS75, 5.21], it follows that, if both the characters ˛1 , . . . , ˛m are real-valued and, for any multi-indices K; L f1; : : : ; mg, it holds that .ˇK L /b D 1 if and only if ˛K ˛N L D 1, then X is a non-Kähler manifold satisfying the @@-Lemma, [Kas12b, Corollary 1.5]. Remark 3.23 ([Kas12a]). In the case of holomorphically parallelizable solvmanifolds, the sub-complex B; in the previous theorem is constructed as follows, see [Kas12a]. Let G be a connected simply-connected complex solvable Lie group of complex dimension n and admitting a lattice , and denote the Lie algebra naturally associated to G by g. Let N be the nilradical of G. There exists a connected simply-connected complex nilpotent subgroup C G such that G D C N , see, e.g., [Dek00, Proposition 3.3]. The diagonalizable representation Ads constructed in Remark 3.20 can be identified with the map G D C N 3 c n 7! .Adc /s 2 Aut.g/, where .Adc /s is the semi-simple part of the Jordan decomposition of Adc , for c 2 C , see [Kas12a, Remark 4]. Consider a basis fX1 ; : : : ; Xn g of the Lie algebra g1;0 of the G-left-invariant holomorphic vector fields on G with respect to which .Adc /s D diag .˛1 .c/; : : : ; ˛n .c//, for c 2 C , for some characters ˛1 2 Hom.C I C /, . . . , ˛n 2 Hom.C I C / of C . Let 1;0 which is dual to fX1 ; : : : ; Xn g. fx1 ; : : : ; xn g be the basis of g (By regarding ˛1 , . . . , ˛n as characters of G, and by shortening, e.g., ˛I WD ˛i1 ˛ik for a multi-index I D .i1 ; : : : ; ik /,) define B; ^; nG , for .p; q/ 2 Z2 , as p;q
B
ˇ ˇ ˛N I ˛N I WD ^p g1;0 ˝C C xN I ˇˇ jI j D q such that D1 : ˛I ˛I
Finally, as regards Bott-Chern cohomology, in a joint work with H. Kasuya, we proved the following result in [AK12]. Theorem 3.14 ([AK12, Theorems 2.16 and 2.25]). Let nG be a solvmanifold endowed with a G-left-invariant complex structure. Suppose that • either nG is ofsplitting type, with the nilpotent factor N such that the inclusion ^; n ; @ ,! ^; N; @ is a quasi-isomorphism, where n is the Lie algebra of N , • or nG is holomorphically parallelizable.
144
3 Cohomology of Nilmanifolds
; Then there exists a finite-dimensional sub-complex C of the Dolbeault complex ^; X; @; @ such that the inclusion
C; ; @; @ ,! ^; nG ; @; @
induces the isomorphism ker @W C ; ! C C1; \ ker @W C ; ! C ;C1 ; . nG / ' HBC : im @@W C 1;1 ! C ; Remark 3.24. The sub-complex C; in the previous theorem is constructed as C; WD B; C BN ; ; see [AK12]. Remark 3.25. We refer to [AK13a] for further results extending the class of solvmanifolds whose Dolbeault and Bott-Chern cohomologies are computable by means of a finite-dimensional sub-complex of the double complex of forms. More precisely, [AK13a, Theorems 1.1 and 1.2] provide conditions under which suitable deformations of the above finite-dimensional sub-complex still allow to compute the cohomologies of small deformations of a solvmanifold. As an application, we computed the Dolbeault and Bott-Chern cohomologies of the holomorphically parallelizable Nakamura manifold and of some of its deformations in [AK13a, Sect. 4]. These examples show that the property of degeneration of the Hodge and Frölicher spectral sequence at the first level (as shown also by M.G. Eastwood and M.A. Singer in [ES93, Theorem 5.4]) and the property of satisfying the @@-Lemma are not closed under deformations of the complex structure, [AK13a, Corollary 6.1]. The previous results allow to compute explicitly the de Rham, Dolbeault, and Bott-Chern cohomologies of the completely-solvable Nakamura manifold and of the holomorphically parallelizable Nakamura manifold, [Kas13b, AK12]. Example 3.3 (The completely-solvable Nakamura manifold, [Kas13b, Example 1], [AK12, Example 2.17]). The completely-solvable Nakamura manifold, firstly studied by I. Nakamura in [Nak75, p. 90], is an example of a cohomologically Kähler non-Kähler solvmanifold, [dAFdLM92, FMS03, Example 3.1], [dBT06, Sect. 3]. Consider the group G WD C Ë C2 ;
where
x p e 0 2 GL C2 : x C 1 y WD x 0 e
3.3 Cohomologies of Six-Dimensional Nilmanifolds
145
Table 3.7 A basis of harmonic representatives for the completely-solvable Nakamura manifold with respect to the metric g WD d z1 ˇ d zN1 C ez1 Nz1 d z2 ˇ d zN2 C ez1 CNz1 d z3 ˇ d zN3 k .XI C/ HdR
k k k k k
D1 D2 D3 D4 D5
g-Harmonic representatives
k .XI C/ dimC HdR
d z1 , d zN1 d z23 , d z11N , d z23N , d z32N , d z2N3N d z123 , d z231N , d z123N , d z132N , d z12N3N , d z21N3N , d z31N2N , d z1N2N3N d z1231N , d z121N3N , d z232N3N , d z131N2N , d z11N2N3N d z1232N3N , d z231N2N3N
2 5 8 5 2
ex 0 is conjugate to an element of SL.2I Z/. For some a 2 R, the matrix 0 ex Hence there exists a discrete co-compact subgroup
WD
aZCb
p 1 Z Ë C2
of G, where C2 is a lattice of C2 . The completely-solvable Nakamura manifold is the completely-solvable solvmanifold X WD nG . o n p Consider holomorphic coordinates fz1 ; z2 ; z3 g on X , where z1 WD x C 1 y is the holomorphic coordinate on C (as a matter of notations, we shorten, for example, ez1 d z121N WD ez1 d z1 ^ d z2 ^ d zN1 ). By A. Hattori’s theorem, [Hat60, Corollary 4.2], the de Rham cohomology of nG does not depend on and can be computed using just G-left-invariant forms on nG ; in Table 3.7, we list the harmonic representatives of the de Rham cohomology classes with respect to the G-left-invariant Hermitian metric g WD d z1 ˇ d zN1 C ez1 Nz1 d z2 ˇ d zN2 C ez1 CNz1 d z3 ˇ d zN3 . As regards Dolbeault cohomology and Bott-Chern cohomology, they depend on the lattice, see [Kas13b, AK12]. In particular, following [Kas13b, Example 1], one has to distinguish three different cases: (i) b 2 .2 Z/ ; (ii) b 2 .2 Z C 1/ ; (iii) b 62 Z . The Dolbeault cohomology of the completely-solvable Nakamura manifold was computed in [Kas13b, Example 5.1]. The Bott-Chern cohomology of the completely-solvable Nakamura manifold was computed in [AK12, Example 2.17], and is summarized in Table 3.8, see [AK12, Tables 4, 5, and 3]. Finally, Table 3.9 summarizes the dimensions of the de Rham, Dolbeault, and Bott-Chern cohomologies of the completely-solvable Nakamura manifold, [Kas13b, Example 1], [AK12, Example 2.17], see [AK12, Table 6]. In particular, note that the completely-solvable Nakamura manifold in case (iii) satisfies the @@-Lemma (compare also [Kas13b, Kas11]).
˛
C hd z231N ; d z123N ; d z132N i C hd z12N3N ; d z21N3N ; d z31N2N i
C hŒd z2N3N i C hŒd z123 i ˝ C Œd z231N ; Œe2z1 d z122N ; Œe2z1 d z133N ; Œd z123N ; Œd z132N i ˝ C Œd z12N3N ; Œe2Nz1 d z21N2N ; Œe2Nz1 d z31N3N ;
.3; 3/
.2; 3/
.3; 2/
.1; 3/
C hŒd z1232N3N i C hŒd z231N2N3N i C hŒd z1231N2N3N i
C hŒez1 d z121N2N3N ; Œez1 d z131N2N3N ; Œd z231N2N3N ; ˛ ŒeNz1 d z121N2N3N ; ŒeNz1 d z131N2N3N
C hŒd z1231N2N3N i
C hŒd z11N2N3N i
Œe2z1 d z131N3N ; Œe2Nz1 d z131N3N ; Œd z232N3N ; Œd z131N2N
C hŒd z1231N i ˝ C Œd z121N3N ; Œe2z1 d z121N2N ; Œe2Nz1 d z121N2N ;
C hŒd z1N2N3N i
C hŒez1 d z1231N2N ; Œez1 d z1231N3N ; Œd z1232N3N ; ˛ ŒeNz1 d z1231N2N ; ŒeNz1 d z1231N3N
Œe2z1 d z131N3N ; Œez1 d z132N3N ; Œd z232N3N ; Œe2Nz1 d z121N2N ; ˛ ŒeNz1 d z231N2N ; Œe2Nz1 d z131N3N ; ŒeNz1 d z231N3N ˝ ˛ C Œd z11N2N3N ; ŒeNz1 d z21N2N3N ; ŒeNz1 d z31N2N3N
C hŒd z1231N ; Œe d z1232N ; Œe d z1233N i ˝ C Œe2z1 d z121N2N ; Œd z121N3N ; Œez1 d z122N3N ; Œd z131N2N ;
z1
.3; 1/
.2; 2/
z1
C hŒd z1N2N3N i
.0; 3/
.1; 2/
.2; 1/
.3; 0/
.0; 2/
Œd z21N3N ; Œd z31N2N i
C hd z2N 3N i C hd z123 i
C hŒd z11N ; Œd z23N ; Œd z32N i
C hŒd z11N ; Œez1 d z12N ; Œez1 d z13N ; Œd z23N ; ˛ Œd z32N ; ŒeNz1 d z21N ; ŒeNz1 d z31N ˛ ˝ C Œd z2N 3N ; ŒeNz1 d z1N 2N ; ŒeNz1 d z1N3N
C hŒd z123 i ˝ C Œez1 d z121N ; Œe2z1 d z122N ; Œd z123N ; Œez1 d z131N ; Œd z132N ; ˛ Œe2z1 d z133N ; Œd z231N ; ŒeNz1 d z121N ; ŒeNz1 d z131N ˝ Nz C Œe 1 d z11N2N ; Œe2Nz1 d z21N2N ; Œd z31N2N ; ŒeNz1 d z11N3N ; Œd z21N3N ; ˛ Œe2Nz1 d z31N3N ; Œd z12N3N ; Œez1 d z11N2N ; Œez1 d z11N3N
C hd z11N ; d z23N ; d z32N i
C hŒd z23 i
C hŒez1 d z12 ; Œez1 d z13 ; Œd z23 i
.2; 0/
.1; 1/
C hŒd z1N i
C hŒd z1N i
.0; 1/
C hd z1231N2N3N i
C hd z231N2N3N i
C hd z1232N3N i
C hd z11N2N3N i
C hd z121N3N ; d z232N3N ; d z131N2N i
C hd z1231N i
C hd z1N 2N3N i
C hd z23 i
C hd z1N i
C hd z1 i
C hŒd z1 i
C h1i
C h1i
C h1i
C hŒd z1 i
Case (iii)
Case (ii)
.0; 0/
Case (i)
.1; 0/
; HBC .`nG/
Table 3.8 The Bott-Chern cohomology of the completely-solvable Nakamura manifold, [AK12, Example 2.17, Tables 4, 5, and 3]
146 3 Cohomology of Nilmanifolds
3.3 Cohomologies of Six-Dimensional Nilmanifolds Table 3.9 The dimensions of the de Rham, Dolbeault, and Bott-Chern cohomologies of the completely-solvable Nakamura manifold, [Kas13b, Example 1], [AK12, Example 2.17], see [AK12, Table 6]
147
Case (i)
Case (ii)
Case (iii)
H]; .X/
dR
@
BC
@
BC
@
BC
.0; 0/
1
1
1
1
1
1
1
.1; 0/ .0; 1/
2
3 3
1 1
1 1
1 1
1 1
1 1
.2; 0/ .1; 1/ .0; 2/
5
3 9 3
3 7 3
1 5 1
1 3 1
1 3 1
1 3 1
.3; 0/ .2; 1/ .1; 2/ .0; 3/
8
1 9 9 1
1 9 9 1
1 5 5 1
1 5 5 1
1 3 3 1
1 3 3 1
.3; 1/ .2; 2/ .1; 3/
5
3 9 3
3 11 3
1 5 1
1 7 1
1 3 1
1 3 1
.3; 2/ .2; 3/
2
3 3
5 5
1 1
1 1
1 1
1 1
.3; 3/
1
1
1
1
1
1
1
Table 3.10 The de Rham cohomology for the holomorphically parallelizable Nakamura manifold, [Kas12a, Sect. 7] k .XI C/ HdR
Case (a)
Case (b)
kD1 kD2 kD3
C hd z1 ; d z1N i C hd z11N ; d z23 ; d z23N ; d z32N ; d z2N3N i Chd z123 ; d z123N ; d z132N ; d z31N2N ; d z21N3N ; d z1N2N3N ; d z1N 23 ; d z12N3N i C hd z1231N ; d z131N2N ; d z232N3N ; d z121N3N ; d z11N2N3N i C hd z231N2N3N ; d z1232N3N i
C hd z1 ; d z1N i C hd z11N ; d z23 ; d z2N3N i C hd z123 ; d z1N 2N3N ; d z1N23 ; d z12N3N i
kD4 kD5
C hd z1231N ; d z232N3N ; d z11N2N3N i C hd z231N2N3N ; d z1232N3N i
Example 3.4 (The holomorphically parallelizable Nakamura manifold, [Kas12a, Sect. 7], [Kas13b, Example 1], [AK12, Example 2.26]). The holomorphically parallelizable Nakamura manifold was firstly considered by I. Nakamura in [Nak75, p. 90], where it is provided as an example of a three-dimensional holomorphically parallelizable solvmanifold in the third class of Nakamura classification, [Nak75, Sect. 2]. Consider the group G D C Ë C2
where
.z/ D
ez 0 0 ez
::
C hŒez1 d z12 ; Œez1 d z13 ; Œd z23 i
C hŒez1 d z12 ; Œez1 d z13 ; Œd z23 i ˝ ˛ C Œd z11N ; Œez1 d z12N ; Œez1 d z13N ; Œd z23N ; Œd z32N ; ŒeNz1 d z21N ; ŒeNz1 d z31N ˝ ˛ C Œd z2N3N ; ŒeNz1 d z1N2N ; ŒeNz1 d z1N3N
.1; 1/
.2; 3/
.3; 2/
.1; 3/
.2; 2/
C hŒd z1231N ; Œe d z1232N ; Œe d z1233N i ˝ C Œe2z1 d z121N2N ; Œd z121N3N ; Œez1 d z122N3N ; Œd z131N2N ; Œe2z1 d z131N3N ; Œez1 d z132N3N ; ˛ Œd z232N3N ; Œe2Nz1 d z121N2N ; ŒeNz1 d z231N2N ; Œe2Nz1 d z131N3N ; ŒeNz1 d z231N3N ˝ ˛ C Œd z11N2N 3N ; ŒeNz1 d z21N2N3N ; ŒeNz1 d z31N2N3N ˝ ˛ C Œez1 d z1231N2N ; Œez1 d z1231N3N ; Œd z1232N3N ; ŒeNz1 d z1231N2N ; ŒeNz1 d z1231N3N ˝ z ˛ C Œe 1 d z121N2N3N ; Œez1 d z131N2N3N ; Œd z231N2N3N ; ŒeNz1 d z121N2N3N ; ŒeNz1 d z131N2N3N
z1
.3; 1/
z1
C hŒd z1N2N3N i
Œd z12N3N ; Œez1 d z11N2N ; Œez1 d z11N3N i
.0; 3/
.1; 2/
.2; 1/
.3; 0/
.0; 2/
C hŒd z123 i ˝ C Œez1 d z121N ; Œe2z1 d z122N ; Œd z123N ; Œez1 d z131N ; Œd z132N ; Œe2z1 d z133N ; ˛ Œd z231N ; ŒeNz1 d z121N ; ŒeNz1 d z131N ˝ C ŒeNz1 d z11N2N ; Œe2Nz1 d z21N2N ; Œd z31N2N ; ŒeNz1 d z11N3N ; Œd z21N3N ; Œe2Nz1 d z31N3N ;
C hŒd z1N i
C hŒd z1N i
.0; 1/
C hŒez1 d z121N2N3N ; Œez1 d z131N2N3N ; Œd z231N2N 3N i
C hŒd z11N2N 3N i ˝ ˛ C ŒeNz1 d z1231N2N ; ŒeNz1 d z1231N3N ; Œd z1232N3N
C hŒd z1231N i ˝ ˛ C Œez1 d z122N3N ; Œez1 d z132N3N ; Œd z232N3N ; ŒeNz1 d z231N2N ; ŒeNz1 d z231N3N
C hŒd z1N2N3N i
˝ ˛ C ŒeNz1 d z11N2N ; ŒeNz1 d z11N3N ; Œd z12N3N ;
C hŒez1 d z121N ; Œez1 d z131N ; Œd z231N i
C hŒd z123 i
C hŒd z11N i ˝ ˛ C ŒeNz1 d z1N2N ; ŒeNz1 d z1N 3N ; Œd z2N 3N
C hŒd z1 i
C hŒd z1 i
.1; 0/
.2; 0/
Case (b)
Case (a)
; HBC .X/
Table 3.11 The Bott-Chern cohomology of the holomorphically parallelizable Nakamura manifold, [AK12, Example 2.26, Tables 8 and 11]
148 3 Cohomology of Nilmanifolds
3.3 Cohomologies of Six-Dimensional Nilmanifolds Table 3.12 The dimensions of the de Rham, Dolbeault, and Bott-Chern cohomologies of the holomorphically parallelizable Nakamura manifold, [Kas12a, Sect. 7], [Kas13b, Example 2], [AK12, Example 2.26], see [AK12, Table 13]
149
Case (a)
Case (b)
dimC H; ] .X/
dR
@
BC
.0; 0/
1
1
.1; 0/ .0; 1/
2
.2; 0/ .1; 1/ .0; 2/
dR
@
BC
1
1
1
1
3 3
1 1
2
3 1
1 1
5
3 9 3
3 7 3
3
3 3 1
3 1 3
.3; 0/ .2; 1/ .1; 2/ .0; 3/
8
1 9 9 1
1 9 9 1
4
1 3 3 1
1 3 3 1
.3; 1/ .2; 2/ .1; 3/
5
3 9 3
3 11 3
3
1 3 3
1 5 1
.3; 2/ .2; 3/
2
3 3
5 5
2
1 3
3 3
.3; 3/
1
1
1
1
1
1
p p p There exist a C 1 b 2 C and c C p1 d 2 C such thatpZ.a C 1 b/ C Z.c C p 1 d / is a lattice in C and .a C 1 b/ and .c C 1 d / are conjugate to elements of SL.4I Z/, where we regard SL.2I C/ SL.4I R/, see [Has10]. Hence there exists a lattice p p WD Z a C 1 b C Z c C 1 d Ë C2 of G, where C2 is a lattice of C2 . The holomorphically parallelizable Nakamura manifold is the holomorphically parallelizable solvmanifold X WD nG , [Nak75, Sect. 2]. Consider holomorphic coordinates fz1 ; z2 ; z3 g on X , where fz1 g is the holomorphic coordinate on C (as a matter of notations, we shorten, for example, e2z1 d z122N WD e2z1 d z1 ^ d z2 ^ d zN2 ). According to [Kas12a], the de Rham, Dolbeault, and Bott-Chern cohomologies depend on the lattice, see [Kas12a, Theorems 1.3 and 1.4], [AK12, Theorem 2.25]. In particular, following [Kas12a, § 7.2], one has to distinguish two different cases4 : (a) (b)
b 2 Z and d 2 Z ; either b 62 Z or d 62 Z .
4 Note that the case b 2 .2 Z/ and d 2 .2 Z/ can be identified with the case (i) of the completely-solvable Nakamura manifold, see [Yam05, Sect. 3].
150
3 Cohomology of Nilmanifolds
The de Rham cohomology and the Dolbeault cohomology of the holomorphically parallelizable Nakamura manifold were computed in [Kas12a, Sect. 7] and [Kas13b, Example 2] (compare also [Nak75, pp. 96/100]); in Table 3.10 we recall the de Rham cohomology for the holomorphically parallelizable Nakamura manifold as computed in [Kas12a, Sect. 7]. The Bott-Chern cohomology of the completelysolvable Nakamura manifold was computed in [AK12, Example 2.26], and is summarized in Table 3.11, see [AK12, Tables 8 and 11]. Finally, Table 3.12 summarizes the dimensions of the de Rham, Dolbeault, and Bott-Chern cohomologies of the holomorphically parallelizable Nakamura manifold, [Kas12a, §], [Kas13b, Example 2], [AK12, Example 2.26], see [AK12, Table 13].
Chapter 4
Cohomology of Almost-Complex Manifolds
Abstract Let X be a differentiable manifold endowed with an almost-complex structure J . Note that if J is not integrable, then the Dolbeault cohomology is not defined. In this chapter, we are concerned with studying some subgroups of the de Rham cohomology related to the almost-complex structure: these subgroups have been introduced by T.-J. Li and W. Zhang in (Comm. Anal. Geom. 17(4):651– 683, 2009), in order to study the relation between the compatible and the tamed symplectic cones on a compact almost-complex manifold, with the aim to throw light on a question by S.K. Donaldson, (Two-forms on four-manifolds and elliptic equations, Inspired by S. S. Chern, Nankai Tracts Math., vol. 11, World Sci. Publ., Hackensack, NJ, 2006, pp. 153–172, Question 2) (see Sect. 4.4.2), and it would be interesting to consider them as a sort of counterpart of the Dolbeault cohomology groups in the non-integrable (or at least in the non-Kähler) case, see Drˇaghici et al. (Int. Math. Res. Not. IMRN 1:1–17, 2010, Lemma 2.15, Theorem 2.16). In particular, we are interested in studying when they let a splitting of the de Rham cohomology, and their relations with cones of metric structures.
4.1 Subgroups of the de Rham (Co)Homology of an Almost-Complex Manifold Let X be a 2n-dimensional (differentiable) manifold endowed with an almostcomplex structure J . In this section, we set the notation concerning C 1 -pure-andfull and pure-and-full almost-complex structures, as introduced in [LZ09]; then we briefly review some results to motivate the study of these topics, see Remark 4.3, and we study the relations between C 1 -pure-and-fullness and pure-and-fullness.
D. Angella, Cohomological Aspects in Complex Non-Kähler Geometry, Lecture Notes in Mathematics 2095, DOI 10.1007/978-3-319-02441-7__4, © Springer International Publishing Switzerland 2014
151
152
4 Cohomology of Almost-Complex Manifolds
4.1.1 C 1 -Pure-and-Full and Pure-and-Full Almost-Complex Structures Let S N N and define 8 9 0 1 < = M HJS .X I R/ WD Œ˛ 2 HdR .X I R/ W ˛ 2 @ ^p;q X A \ ^ X I : ; .p;q/2S
note that a real differential form ˛ with a component of type .p; q/ has also a component of type .q; p/, and hence we are interested in studying the sets S such that whenever .p; q/ 2 S , also .q; p/ 2 S . As a matter of notation, we will usually list the elements of S instead of writing S itself. Note that, for every k 2 N, one has X
.p;q/;.q;p/
HJ
k .X I R/ HdR .X I R/ ;
pCqDk p q
but, in general, the sum is neither direct nor the equality holds: several examples of these facts will be provided in the sequel. .2;0/;.0;2/ .1;1/ 2 The subgroups HJ .X I R/ and HJ .X I R/ of HdR .X I R/ are of special interest for their interpretation as the J -anti-invariant, respectively, J -invariant part of the second de Rham cohomology group. Indeed, note that the endomorphism J b^2 X 2 End .^ X / naturally extending J 2 End.TX/ (that is, J˛ WD ˛ .J ; J / for every ˛ 2 ^2 X ) satisfies .J b^2 X /2 D id^2 X ; hence, one has the splitting ^2 X D ^C J X ˚ ^J X ;
where, for ˙ 2 fC; g, ^˙ J X WD
˚
˛ 2 ^2 X W J˛ D ˙˛ :
2 Since HdR .X I R/ contains, in particular, the classes represented by the symplectic .1;1/ forms, and HJ .X I R/ contains, in particular, the classes represented by the .1; 1/forms associated to the Hermitian metrics on X , in [LZ09], T.-J. Li and W. Zhang 2 were interested in studying the J -invariant subgroupof HdR .X I R/, namely,
HJC .X / WD HJ
.1;1/
.X I R/ D
˚ 2 Œ˛ 2 HdR .X I R/ W J˛ D ˛ ;
2 .X I R/, namely, and the J -anti-invariant subgroup of HdR
HJ .X / WD HJ
.2;0/;.0;2/
.X I R/ D
˚ 2 Œ˛ 2 HdR .X I R/ W J˛ D ˛ :
4.1 Subgroups of the de Rham (Co)Homology of an Almost-Complex Manifold
153
Note that, as in the general case, one has that 2 HJC .X / C HJ .X / HdR .X I R/ 2 .X I R/. The following but, in general, the sum is neither direct nor equal to HdR definition, by T.-J. Li and W. Zhang, singles out the almost-complex structures 2 whose subgroups HJC .X / and HJ .X / provide a decomposition of HdR .X I R/.
Definition 4.1 ([LZ09, Definitions 2.2, 2.3, Lemma 2.2]). An almost-complex structure J on a manifold X is said to be • C 1 -pure if HJ .X / \ HJC .X / D f0g; 2 • C 1 -full if HJ .X / C HJC .X / D HdR .X I R/; 1 1 • C -pure-and-full if it is both C -pure and C 1 -full, i.e., if the following cohomology decomposition holds: 2 HdR .X I R/ D HJ .X / ˚ HJC .X / :
We will also use the following definition, which is a natural generalization of the notion of C 1 -pure-and-fullness to higher degree cohomology groups. Definition 4.2. Let X be a manifold endowed with an almost-complex structure J , P .p;q/;.q;p/ k .X I R/ pCqDk HJ .X I R/: and fix k 2 N. Consider HdR p q
• if M
.p;q/;.q;p/
HJ
k .X I R/ HdR .X I R/
pCqDk p q
(namely, the sum is direct), then J is called C 1 -pure at the kth stage; • if X .p;q/;.q;p/ k .X I R/ D HJ .X I R/ ; HdR pCqDk p q
then J is called C 1 -full at the kth stage; • if J is both C 1 -pure at the kth stage and C 1 -full at the kth stage, that is, k .X I R/ D HdR
M
.p;q/;.q;p/
HJ
.X I R/ I
pCqDk p q
then J is called C 1 -pure-and-full at the kth stage. Analogous definitions can be given for the de Rham cohomology with complex coefficients. More precisely, let S N N and define
154
4 Cohomology of Almost-Complex Manifolds
HJS .X I C/ WD
8 < :
.X I C/ W ˛ 2 Œ˛ 2 HdR
M
^p;q X
.p;q/2S
9 = ;
(as previously, we will usually list the elements of S instead of writing S itself); with .X I R/. such notation, one has in particular that HJS .X I R/ D HJS .X I C/ \ HdR Remark 4.1. Note that, when X is a compact manifold endowed with an integrable almost-complex structure J , then, for any .p; q/ 2 N N, .p;q/
HJ
p;q pCq .X I C/ D im HBC .X / ! HdR .X I C/ ;
p;q
pCq
where the map HBC .X / ! HdR .X I C/ is the one induced by the identity (note that ker @ \ ker @ ker d and im @@ im d). Indeed, any d-closed .p; q/-form is both @-closed and @-closed. Note that, for every k 2 N, one has X
.p;q/
HJ
k .X I C/ HdR .X I C/ ;
pCqDk
but, in general, the sum is neither direct nor the equality holds. We can then give the following definition. Definition 4.3. Let X be a manifold endowed with an almost-complex structure J , P .p;q/ k .X I C/ pCqDk HJ .X I C/: and fix k 2 N. Consider HdR • if M
.p;q/
HJ
k .X I C/ HdR .X I C/
pCqDk
(namely, the sum is direct), then J is called complex-C 1 -pure at the kth stage; • if X .p;q/ k .X I C/ D HJ .X I C/ ; HdR pCqDk
then J is called complex-C 1 -full at the kth stage; • if J is both complex-C 1-pure at the kth stage and complex-C 1-full at the kth stage, that is, k HdR .X I C/ D
M
.p;q/
HJ
.X I C/ I
pCqDk
then J is called complex-C 1 -pure-and-full at the kth stage.
4.1 Subgroups of the de Rham (Co)Homology of an Almost-Complex Manifold
155
Remark 4.2. In general, being complex-C 1-full at the 2nd stage is a stronger condition that being C 1 -full. Furthermore, if J is integrable, then being complexC 1 -pure-and-full at the 2nd stage is stronger than being C 1 -pure-and-full. More precisely, for any (possibly non-integrable) almost-complex structure J , it holds, [DLZ10, Lemma 2.11], (
HJC .X / D HJ
.1;1/
.1;1/
HJ
2 .X I C/ \ HdR .X I R/
.X I C/ D HJC .X / ˝R C
;
and .2;0/
HJ
.0;2/
.X I C/ C HJ
.X I C/ HJ .X / ˝R C ;
and, if J is integrable, it holds 8 < H .X / D H .2;0/ .X I C/ C H .0;2/ .X I C/ \ H 2 .X I R/ J dR J J I : H .2;0/ .X I C/ C H .0;2/ .X I C/ D H .X / ˝ C J
J
R
J
indeed, d ^2;0 X ^3;0 X ˚ ^2;1 X and d ^0;2 X ^1;2 X ˚ ^0;3 X . (Compare also [DLZ10, Lemma 2.12] for further results in the case of four-dimensional manifolds.) Note also that, if J is C 1 -pure, then .1;1/
HJ
.2;0/ .0;2/ .X I R/ \ HJ .X I R/ C HJ .X I R/ D f0g :
The construction of the subgroups HJS .X I R/ HdR .X I R/ and the notion of C -pure-and-full almost-complex structures can be repeated using the complex of currents D X WD D2n X; d instead of the complex of differential forms .^ X; d/ and the de Rham homology HdR .X I R/ instead of the de Rham coho mology HdR .X I R/. (We refer to Sect. 1.6 for notations and references concerning currents and de Rham homology.) As in the smooth case, accordingly to T.-J. Li and W. Zhang, [LZ09], given S N N, let 8 9 0 1 < = M Dp;q X A \ D X : HSJ .X I R/ WD Œ˛ 2 HdR .X I C/ W ˛ 2 @ : ; 1
.p;q/2S
J In particular, the almost-complex structures on X for which H.2;0/;.0;2/ .X I R/ dR J and H.1;1/ .X I R/ provide a decomposition of H2 .X I R/ are emphasized by the following definition by T.-J. Li and W. Zhang.
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Definition 4.4 ([LZ09, Definition 2.15, Lemma 2.16]). An almost-complex structure J on a manifold X is said to be: • pure if J J H.2;0/;.0;2/ .X I R/ \ H.1;1/ .X I R/ D f0g I
• full if J J H.2;0/;.0;2/ .X I R/ C H.1;1/ .X I R/ D H2dR .X I R/ I
• pure-and-full if it is both pure and full, i.e., if the following decomposition holds: J J H.2;0/;.0;2/ .X I R/ ˚ H.1;1/ .X I R/ D H2dR .X I R/ :
The following are natural generalizations of the notion of pure-and-fullness. Definition 4.5. Let X be a manifold endowed with an almost-complex structure J , P k J .X I R/ pCqDk H.p;q/;.q;p/ .X I R/: and fix k 2 N. Consider HdR p q
• if M
J H.p;q/;.q;p/ .X I R/ HkdR .X I R/
pCqDk p q
(namely, the sum is direct), then J is called pure at the kth stage; • if X J HkdR .X I R/ D H.p;q/;.q;p/ .X I R/ ; pCqDk p q
then J is called full at the kth stage; • if J is both pure at the kth stage and full at the kth stage, that is, HkdR .X I R/ D
M
J H.p;q/;.q;p/ .X I R/ I
pCqDk p q
then J is called pure-and-full at the kth stage. As regards de Rham homology with complex coefficients, given S N N, let 8 9 < = M Dp;q X ; HSJ .X I C/ WD Œ˛ 2 HdR .X I C/ W ˛ 2 : ; .p;q/2S
so that HSJ .X I R/ D HSJ .X I C/ \ HdR .X I R/.
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157
Definition 4.6. Let X be a manifold endowed with an almost-complex structure J , P J and fix k 2 N. Consider HkdR .X I C/ pCqDk H.p;q/ .X I C/: • if M
J H.p;q/ .X I C/ HkdR .X I C/
pCqDk
(namely, the sum is direct), then J is called complex-pure at the kth stage; • if X J HkdR .X I C/ D H.p;q/ .X I C/ ; pCqDk
then J is called complex-full at the kth stage; • if J is both complex-pure at the kth stage and complex-full at the kth stage, that is, M J HkdR .X I C/ D H.p;q/ .X I C/ I pCqDk
then J is called complex-pure-and-full at the kth stage. .p;q/;.q;p/
.X I R/ and the notion of Remark 4.3. The study of the subgroups HJ C 1 -pure-and-full almost-complex structure have been introduced by T.-J. Li and W. Zhang in [LZ09], in order to study the relations between the compatible and the tamed symplectic cones on a compact almost-complex manifold, and inspired by a question by S.K. Donaldson, [Don06, Question 2]: whether, on a compact four-dimensional manifold endowed with an almost-complex structure J tamed by a symplectic form, there exists also a symplectic form compatible with J , see Sect. 4.4.2. In [DLZ10], T. Drˇaghici, T.-J. Li, and W. Zhang investigated the four-dimensional case, proving, in particular, that every almost-complex structure on a compact four-dimensional manifold is C 1 -pure-and-full; they also obtained further results for four-dimensional almost-complex manifolds in [DLZ11], where they studied the dimensions of the subgroups HJC .X / and HJ .X /. In [FT10], A.M. Fino and A. Tomassini studied the C 1 -pure-and-fullness in connection with other properties on almost-complex manifolds: in particular, by studying almost-complex solvmanifolds, they provided the first explicit example of a nonC 1 -pure-and-full almost-complex structure. Jointly with A. Tomassini, we studied in [AT11] the behaviour of C 1 -pure-and-fullness under small deformations of the complex structure or along curves of almost-complex structures, proving in particular its instability. In [AT12a] we continued the study of the cohomological properties related to the existence of an almost-complex structure, focusing, in particular, on the study of the cone of semi-Kähler structures on a compact semi-Kähler manifold. In [ATZ12], jointly with A. Tomassini and W. Zhang, we further studied cohomological properties of almost-Kähler manifolds, especially
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4 Cohomology of Almost-Complex Manifolds
in relation with W. Zhang’s Lefschetz-type property; in particular, an example of a non-C 1 -full almost-Kähler structure on a compact manifold is provided. In [DZ12], T. Drˇaghici and W. Zhang reformulated the S.K. Donaldson “tamed to compatible” question in terms of spaces of exact forms, proving, in particular, that an almost-complex structure J on a compact four-dimensional manifold admits a compatible symplectic form if and only if it admits tamed symplectic forms with any arbitrarily given J -anti-invariant component. For further results on the study of J -anti-invariant forms and J -anti-invariant de Rham cohomology classes on a (possibly non-compact) manifold endowed with an almost-complex structure J , see [HMT11] by R.K. Hind, C. Medori, and A. Tomassini, where a result concerning analytic continuation for J -anti-invariant forms is proven. In [LT12], T.-J. Li and A. Tomassini studied the analogue of the above problems for linear (possibly nonintegrable) complex structures on four-dimensional unimodular Lie algebras; in particular, they proved that an analogue of the decomposition in [DLZ10, Theorem 2.3] holds for every four-dimensional unimodular Lie algebra endowed with a linear (possibly non-integrable) complex structure; furthermore, they considered the linear counterpart of Donaldson’s “tamed to compatible” question, and of the tamed and compatible symplectic cones, studying, in particular, a sufficient condition on a four-dimensional Lie algebra g (which holds, for example, for four-dimensional unimodular Lie algebras) in order that a linear (possibly non-integrable) complex structure admits a taming linear symplectic form if and only if it admits a compatible linear symplectic form. The case of non-unimodular Lie algebras was studied in [DL13] by T. Drˇaghici and H. Leon. The paper [DLZ12] by T. Drˇaghici, T.-J. Li, and W. Zhang furnishes a survey on the known results concerning the subgroups HJC .X / and HJ .X /, especially in dimension 4, and their application to S.K. Donaldson’s “tamed to compatible” question. Remark 4.4. The notion of C 1 -pure-and-fullness can be restated also in the symplectic context. By adapting T.-J. Li and W. Zhang theory, [LZ09], to the symplectic case, one can define subgroups of the de Rham cohomology connected with the symplectic structure ! on X as, for r; s 2 N, H!.r;s/ .X I R/ WD
˚
2rCs 2rCs Lr ˇ .s/ 2 HdR .X I R/ W ˇ .s/ 2 P^s X HdR .X I R/ ;
and ask when the Lefschetz decomposition on the space of forms moves to cohomology, that is, when, in X
k H!.r;s/ .X I R/ HdR .X I R/ ;
2rCsDk
the inclusion is actually an equality and the sum is actually a direct sum. We note the relations between the above subgroups and the primitive cohomologies introduced by L.-S. Tseng and S.-T. Yau (see Sect. 1.2.2.2). As regards L.-S. Tseng in [TY12a] and S.-T. Yau’s primitive d C d -cohomology PHdC d .X I R/, note that, for every r; s 2 N,
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159
im Lr PHds C d .X I R/ ! HdR .X I R/ D Lr H!.0;s/ .X I R/ H!.r;s/ .X I R/ : In [TY12a, Sect. 4.1], L.-S. Tseng and S.-T. Yau have introduced also the primitive cohomology groups PHds .X I R/ WD
ker d \ ker d \P^s X ; im dbP^s1 X \ker d
where s 2 N, proving that the homology on co-isotropic chains is naturally dual to PHd2n .X I R/, see [TY12a, pp. 40–41]; in [Lin13, Proposition A.5], Y. Lin proved that, if the Hard Lefschetz Condition holds on X , then H!.0;/ .X I R/ D PHd .X I R/ : For results on C 1 -pure-and-fullness for symplectic structures, see [AT12b]. In particular, we note that a symplectic counterpart of [DLZ10, Theorem 2.3] holds: more precisely, for every compact symplectic manifold (non-necessarily of .1;0/ .0;2/ 2 dimension 4), the decomposition HdR .X I R/ D H! .X I R/ ˚ H! .X I R/ holds, [AT12b, Theorem 2.6]. Remark 4.5. The notion of C 1 -pure-and-fullness can be restated also in the D-complex context, where it assumes further significance due to the fact that the D-complex counterpart of Dolbeault cohomology1 is, in general, non-finite
Recall that, given K 2 End.TX/ such that K 2 D idTX , one can define, by duality, an endomorphism K 2 End.T X/ such that K 2 D idT X , and hence one gets a natural decomposition T X D T C X ˚ .T X/ into eigen-bundles. Extending K 2 End.T X/ to K 2 End .^ X/, one gets a decomposition of the bundle of differential `-forms, for ` 2 N: more precisely, M p; q p; q ^C X where ^C X WD ^p T C X ˝ ^q .T X/ I ^` X D 1
pCqD`
If the almost-D-complex structure K is integrable, then the exterior differential splits as d D @C C @ where p; q
pC1; q
@C WD ^pC1; q X ıdW ^C X ! ^C X C
and
p; q
p; qC1
@ WD ^p; qC1 X ıdW ^C X ! ^C X C
r; s r; s (where ^r;Cs X W ^; C X ! ^C X denotes the natural projection onto ^C X, for every r;
s 2 N). In particular, the condition d2 D 0 is rewritten as
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4 Cohomology of Almost-Complex Manifolds
dimensional,2 [Ros13, Proposition 3.1.11]. In fact, D-complex geometry is, in a sense, the “hyperbolic analogue” of complex geometry: in particular, there is no Hodge theory available in the non-elliptic context of D-complex geometry. For results on C 1 -pure-and-fullness for (almost-)D-complex structures, see [AR12]; see also [Ros13]. In particular, we note that an analogue of [DLZ10, Theorem 2.3] holds for left-invariant D-complex structures on four-dimensional nilmanifolds, [AR12, Theorem 3.17], while counterexamples exist removing either the assumption on the dimension, [AR12, Examples 3.1 and 3.2], or on the nilpotency, [AR12, Example 4.1], or on the integrability, [AR12, Example 3.4], see [AR12, Remark 3.18]. Furthermore, as a difference between the complex and the D-complex cases (compare [DLZ10, Lemma 2.15, Theorem 2.16], or [LZ09, Proposition 2.1]), we note that admitting a D-Kähler structure does not yield decomposition in cohomology with respect to the D-complex structure, [AR12, Examples 3.1, 3.2, Proposition 3.3]. Finally, in [AR12], explicit examples of small deformations of the D-complex structure on nilmanifolds and solvmanifolds have been studied,3 proving in particular that being D-Kähler is not a stable property under small deformations of the D-complex structure, [AR12, Example 4.1, Theorem 4.2] (compare with [KS60, Theorem 15] in the complex case), and studying the behaviour of C 1 pure-and-fullness under small deformations of the D-complex structure, [AR12, Example 4.1, Proposition 4.3] (see Sect. 4.3.1.1 for the almost-complex case), and the semi-continuity properties of the dimensions of the K-(anti-)invariant subgroups of cohomology, [AR12, Examples 4.1, 4.4, 4.5, Proposition 4.6] (see Sect. 4.3.2 for the almost-complex case).
8 ˆ ˆ < ˆ ˆ :
@2C D 0 @C @ C @ @C D 0 ; @2 D 0
and hence one can define a D-complex counterpart of the Dolbeault cohomology by considering ; q the cohomology of the differential complex ^C X; @C for every q 2 N, that is, H@; .XI R/ WD C
ker @C : im @C
For example, [AR12, p. 533], consider two compact manifolds X C and X having the same C dimension and consider the natural D-complex structure on X X , i.e., the D-complex structure given by the decomposition T X C X D TX C ˚ TX , where KbTX˙ D ˙ idT .X C X / , for ˙ 2 fC; g (recall that every D-complex manifold is locally of this form, C see, e.g., [CMMS04, Proposition 2]); one has that the vector space H@0;0 X X ' C 1 .X / C does not have finite dimension. 3 See [MT11, Ros12a] for more results concerning deformations of (almost-)D-complex structures. 2
4.1 Subgroups of the de Rham (Co)Homology of an Almost-Complex Manifold
161
4.1.2 Relations Between C 1 -Pure-and-Fullness and Pure-and-Fullness The following result4 summarizes the relations between C 1 -pure-and-fullness and pure-and-fullness, and between complex-C 1-pure-and-fullness and complex-pureand-fullness. Theorem 4.1 ([AT11, Theorem 2.1], see [LZ09, Proposition 2.5]). Let J be an almost-complex structure on a compact 2n-dimensional manifold X . The following relations between (complex-)C 1-pure-and-full and (complex-)pure-and-full notions hold: for any k 2 N, C 1 -full at the kth stage
+3 pure at the kth stage
full at the .2n k/ th stage
+3 C 1 -pure at the .2n k/ th stage ;
and complex-C 1 -full at the kth stage
+3 complex-pure at the kth stage
+3 complex-C 1 -pure at the .2n k/ th stage :
complex-full at the .2n k/ th stage
Proof. The horizontal implications follow by considering the non-degenerate duality pairing .XI R/ HdR .XI R/ ! R; respectively h; i W HdR .XI C/ HdR .XI C/ ! C; h; i W HdR
and noting that, for any .p; q/ 2 N2 , D E .p;q/;.p;q/ ker HJ .X I R/;
X
J H.r;s/;.s;r/ .X I R/
f.r;s/;.s;r/g¤f.p;q/;.q;p/g
and
D E J ker ; H.p;q/;.q;p/ .X I R/
X
.r;s/;.s;r/
HJ
.X I R/ ;
f.r;s/;.s;r/g¤f.p;q/;.q;p/g
4
For an analogous result in the setting of almost-D-complex structures in the sense of F. R. Harvey and H. B. Lawson, see [AR12, Proposition 1.4]; for an analogous result in the setting of symplectic structures, see [AT12b, Proposition 2.4].
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4 Cohomology of Almost-Complex Manifolds
respectively D E .p;q/ ker HJ .X I C/; D E J ker ; H.p;q/ .X I C/
X
J H.r;s/ .X I C/
and
.r;s/¤.p;q/
X
.r;s/
HJ
.X I C/ :
.r;s/¤.p;q/
As an example, we give the details to prove that if J is C 1 -full at the kth stage then it is also pure at the kth stage, when k D 2. Let J J .X I R/ \ H.1;1/ .X I R/ ; c 2 H.2;0/;.0;2/
with c ¤ Œ0. Hence, hc; i bH .2;0/;.0;2/.X IR/ D 0 J
and
hc; i bH .1;1/ .X IR/ D 0 I J
since J is C 1 -full, it follows that hc; i bHdR 2 .X IR/ D 0, and hence c D Œ0. To prove the vertical implications, it isR enough to note that the quasi-isomorphism T W ^ X ! D2n X defined as T' WD X ' ^ (see Sect. 1.6) induces an injective map .p;q/;.q;p/
HJ
J .X I R/ ! H.np;nq/;.nq;np/ .X I R/ ; .p;q/
HJ for any .p; q/ 2 N2 .
respectively
J .X I C/ ! H.np;nq/ .X I C/ ;
t u
Remark 4.6. On a compact 2n-dimensional manifold X endowed with an almost2 2n2 complex structure J , further linkings between HdR .X I R/ and HdR .X I R/ could 1 provide further relations between C -pure-and-full and pure-and-full notions: for example, A.M. Fino and A. Tomassini proved in [FT10, Theorem 3.7] that, given a J -Hermitian metric g on X , if there exists a basis of g-harmonic representatives 2 for HdR .X I R/ being of pure type with respect to J , then J is both C 1 -pure-andfull and pure-and-full. Furthermore, A.M. Fino and A. Tomassini proved in [FT10, Theorem 4.1] that, given a J -compatible symplectic form ! on X satisfying the ' nCk nk .X I R/ ! HdR .X I R/ Hard Lefschetz Condition (that is, the map ! k ^W HdR is an isomorphism for every k 2 N), if J is C 1 -pure-and-full, then J is also pureand-full (compare Proposition 4.4 for a similar result). Setting 2n D 4 and k D 2 in Theorem 4.1, it follows that, on compact fourdimensional almost-complex manifolds, C 1 -fullness implies C 1 -pureness. The following result states that, for higher dimensional manifolds, C 1 -pureness and C 1 -fullness are not, in general, related properties.
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163
Proposition 4.1 ([AT12a, Proposition 1.4]). There exist both examples of compact manifolds endowed with almost-complex structures being C 1 -full and non-C 1pure, and examples of compact manifolds endowed with almost-complex structures being C 1 -pure and non-C 1-full. Proof. The proof follows from the following examples, [AT12a, Examples 1.2 and 1.3]. Step 1—Being C 1 -full does not imply being C 1 -pure. Take a nilmanifold N1 with associated Lie algebra h16 WD 03 ; 12; 14; 24 : Consider the left-invariant complex structure on N1 whose space of .1; 0/-forms is generated, as a C 1 .N1 I C/-module, by 8 1 1 2 ˆ ˆ ' WD e C i e < ' 2 WD e 3 C i e 4 : ˆ ˆ : 3 ' WD e 5 C i e 6 ˚ Writing the structure equations in terms of ' 1 ; ' 2 ; ' 3 , 8 1 ˆ ˆ2 d' D 0 < ; 2 d ' 2 D ' 11N ˆ ˆ : 2 d ' 3 D i ' 12 C i ' 12N the integrability condition is easily verified. K. Nomizu’s theorem [Nom54, Theorem 1] makes the computation of the cohomology straightforward: in fact, listing the harmonic with P j representatives respect to the left-invariant Hermitian metric g WD N j instead of their j ' ˇ ' classes, one finds E D E D E D N N N N NN NN 2 HdR .N1 I C/ D C ' 13 ; ' 13 ˚ C ' 13 ' 31 ˚ C ' 12 C ' 12 ; ' 21 ' 12 ; where .2;0/;.0;2/
HJ
E D E D N N NN NN .N1 I C/ D C ' 13 ; ' 13 ˚ C ' 12 C ' 12 ; ' 21 ' 12
and .1;1/
HJ
E D E D N N N N NN .N1 I C/ D C ' 13 ' 31 ˚ C ' 12 C ' 12 ; ' 21 ' 12 :
In particular, J is a C 1 -full, non-C 1-pure complex structure.
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4 Cohomology of Almost-Complex Manifolds
Step 2—Being C 1 -pure does not imply being C 1 -full. Take a nilmanifold N2 with associated Lie algebra h2 WD 04 ; 12; 34 : and consider on it the left-invariant complex structure given requiring that the forms 8 ' 1 WD e 1 C i e 2 ˆ ˆ < ' 2 WD e 3 C i e 4 ˆ ˆ : 3 ' WD e 5 C i e 6 are of type .1; 0/. The integrability condition follows from the structure equations 8 2 d '1 D 0 ˆ ˆ < : 2 d '2 D 0 ˆ ˆ : N N 2 d ' 3 D i ' 11 i ' 22 K. Nomizu’s theorem [Nom54, Theorem 1] gives E D E D N N NN 2 HdR .N2 I C/ D C ' 12 ; ' 12 ˚ C ' 12 ; ' 21 E D N N N N NN NN ˚C ' 13 C ' 13 ; ' 31 ' 13 ; ' 32 ' 23 ; ' 23 ' 23 ; where .2;0/;.0;2/
HJ
E D NN .N2 I C/ D C ' 12 ; ' 12
and .1;1/
HJ
E D N N .N2 I C/ D C ' 12 ; ' 21 I
this can be proven arguing as follows: with respect to the left-invariant Hermitian P metric g WD j ' j ˇ 'N j , one computes @ ' 13 D @ ' 23 D @ ' 12 D 0 ; that is, ' 13 , ' 12 and ' 23 are g-orthogonal to the space @ ^1;0 N2 ; in the same way, one computes N
N
N
N
@ ' 12 D @ ' 12 D @ ' 13 D @ ' 13 D 0
4.2 C 1 -Pure-and-Fullness for Special Manifolds
165
(compare also Proposition 4.2). In particular, J is a C 1 -pure, non-C 1 -full complex structure. t u
4.2 C 1 -Pure-and-Fullness for Special Manifolds In this section, we study the property of being C 1 -pure-and-full on special classes of (almost-)complex manifolds. After recalling some motivating results by T. Drˇaghici, T.-J. Li, and W. Zhang, we study C 1 -pure-and-fullness for left-invariant complexstructures on solvmanifolds, providing some examples in dimension 4 or higher; furthermore, we consider almost-complex manifolds endowed with special metric structures, namely, semi-Kähler, and almost-Kähler structures.
4.2.1 Special Classes of C 1 -Pure-and-Full (Almost-)Complex Manifolds In this section, we recall some results by T. Drˇaghici, T.-J. Li, and W. Zhang, providing classes of C 1 -pure-and-full and pure-and-full (almost-)complex manifolds. They could be considered as motivations to study C 1 -pure-and-fullness: in fact, .;/ [DLZ10, Lemma 2.15, Theorem 2.16] suggests that the subgroups HJ .X I C/ can be viewed as a generalization of the Dolbeault cohomology groups for nonKähler, and non-integrable, almost-complex manifolds X . On the other hand, [DLZ10, Theorem 2.3] states that, on a compact four-dimensional almost-complex manifold X , the subgroups HJC .X / and HJ .X / induce always a decomposition of 2 HdR .X I R/: this could be intended as a generalization of the Hodge decomposition theorem for compact four-dimensional almost-complex manifolds. .;/ According to the following result, the groups HJ .X I C/ can be considered as the counterpart of the Dolbeault cohomology groups in the non-Kähler and nonintegrable cases. Theorem 4.2 ([DLZ10, Lemma 2.15, Theorem 2.16]). Let X be a compact complex manifold. If the Hodge and Frölicher spectral sequence degenerates at the first the natural filtration associated with the structure of double complex of step and ; k ^ X; @; @ induces a Hodge decomposition of weight k on HdR .X I C/ for some 1 k 2 N, then X is complex-C -pure-and-full at the kth stage, and .p;q/
HJ
p;q
.X I C/ ' H@ .X /
for every .p; q/ 2 N2 such that p C q D k.
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4 Cohomology of Almost-Complex Manifolds
Recall that, for compact complex surfaces, the Hodge and Frölicher spectral sequence degenerates at the first step, [Kod64],and the natural filtration associated with the structure of double complex of ^; X; @; @ induces a Hodge 2 decomposition of weight 2 on HdR .X I C/, see, e.g., [BHPVdV04, Theorem IV.2.8, Proposition IV.2.9]. (We recall that, in [Rol08], for every n 2 N n f0; 1g, a nilmanifold of real dimension 2n with left-invariant complex structure such that the Hodge and Frölicher spectral sequence does not degenerate at the nth step was constructed, showing that the Hodge and Frölicher spectral sequence can be arbitrarily non-degenerate.) Note also that, for compact complex manifolds satisfying the @@-Lemma, the assumptions of Theorem 4.2 are satisfied, for any k 2 N, by [DGMS75, 5.21], and recall that compact complex manifolds admitting a Kähler metric satisfy the @@-Lemma, [DGMS75, Lemma 5.11]. Therefore, as a corollary of [DLZ10, Lemma 2.15, Theorem 2.16], and of Remark 4.2 and Theorem 4.1, one gets the following result.
Corollary 4.1 ([LZ09, Proposition 2.1], [DLZ10, Theorem 2.16, Proposition 2.17]). One has that: (i) every compact complex surface is complex-C 1 -pure-and-full at the 2nd stage, and hence, in particular, C 1 -pure-and-full and pure-and-full; (ii) every compact complex manifold satisfying the @@-Lemma is complex-C 1 pure-and-full at every stage, and hence complex-pure-and-full at every stage; (iii) every compact complex manifold admitting a Kähler structure is complex-C 1 pure-and-full at every stage, and hence complex-pure-and-full at every stage. Actually, T. Drˇaghici, T.-J. Li, and W. Zhang proved in [DLZ10] the following result, which one can consider as a sort of Hodge decomposition theorem in the non-Kähler case. Theorem 4.3 ([DLZ10, Theorem 2.3]). Every almost-complex structure on a compact four-dimensional manifold is C 1 -pure-and-full and pure-and-full. Proof. The proof of the previous theorem rests on the very special properties of four-dimensional manifolds. For the sake of completeness, we recall here the argument by T. Drˇaghici, T.-J. Li, and W. Zhang in [DLZ10]. Firstly, note that, by Theorem 4.1, it suffices to prove that an almost-complex structure J on a compact four-dimensional manifold is C 1 -full. Suppose that J is not C 1 -full. Fix a Hermitian metric g on X , and denote its associated .1; 1/-form by !. Recall that 2 the Hodge--operator g b^2 X W ^2 X ! ^2 X satisfies g b^2 X D id^2 X , hence it induces a splitting ^2 X D ^C g X ˚ ^g X ;
˚ 2 where ^˙ g X WD ' 2 ^ X W g ' D ˙' , for ˙ 2 fC; g. Setting P^ X WD 2C1 ker D ker L b^ X the space of primitive forms, where is the adjoint
4.2 C 1 -Pure-and-Fullness f
