Horizontal Forms of Chern Type on Complex Finsler Bundles

The aim of this paper is to construct horizontal Chern forms of a holomorphic vector bundle using complex Finsler structures. Also, some properties of these forms are studied.

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Дата:2010
Автор: Ida, C.
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Назва видання:Symmetry, Integrability and Geometry: Methods and Applications
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Цитувати:Horizontal Forms of Chern Type on Complex Finsler Bundles / C. Ida // Symmetry, Integrability and Geometry: Methods and Applications. — 2010. — Т. 6. — Бібліогр.: 18 назв. — англ.

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spelling irk-123456789-1463502019-02-10T01:24:04Z Horizontal Forms of Chern Type on Complex Finsler Bundles Ida, C. The aim of this paper is to construct horizontal Chern forms of a holomorphic vector bundle using complex Finsler structures. Also, some properties of these forms are studied. 2010 Article Horizontal Forms of Chern Type on Complex Finsler Bundles / C. Ida // Symmetry, Integrability and Geometry: Methods and Applications. — 2010. — Т. 6. — Бібліогр.: 18 назв. — англ. 1815-0659 2010 Mathematics Subject Classification: 53B40; 57R20 doi:10.3842/SIGMA.2010.054 http://dspace.nbuv.gov.ua/handle/123456789/146350 en Symmetry, Integrability and Geometry: Methods and Applications Інститут математики НАН України
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection DSpace DC
language English
description The aim of this paper is to construct horizontal Chern forms of a holomorphic vector bundle using complex Finsler structures. Also, some properties of these forms are studied.
format Article
author Ida, C.
spellingShingle Ida, C.
Horizontal Forms of Chern Type on Complex Finsler Bundles
Symmetry, Integrability and Geometry: Methods and Applications
author_facet Ida, C.
author_sort Ida, C.
title Horizontal Forms of Chern Type on Complex Finsler Bundles
title_short Horizontal Forms of Chern Type on Complex Finsler Bundles
title_full Horizontal Forms of Chern Type on Complex Finsler Bundles
title_fullStr Horizontal Forms of Chern Type on Complex Finsler Bundles
title_full_unstemmed Horizontal Forms of Chern Type on Complex Finsler Bundles
title_sort horizontal forms of chern type on complex finsler bundles
publisher Інститут математики НАН України
publishDate 2010
url http://dspace.nbuv.gov.ua/handle/123456789/146350
citation_txt Horizontal Forms of Chern Type on Complex Finsler Bundles / C. Ida // Symmetry, Integrability and Geometry: Methods and Applications. — 2010. — Т. 6. — Бібліогр.: 18 назв. — англ.
series Symmetry, Integrability and Geometry: Methods and Applications
work_keys_str_mv AT idac horizontalformsofcherntypeoncomplexfinslerbundles
first_indexed 2025-07-10T23:30:35Z
last_indexed 2025-07-10T23:30:35Z
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fulltext Symmetry, Integrability and Geometry: Methods and Applications SIGMA 6 (2010), 054, 7 pages Horizontal Forms of Chern Type on Complex Finsler Bundles Cristian IDA Department of Algebra, Geometry and Dif ferential Equations, Transilvania University of Braşov, Str. Iuliu Maniu 50, Braşov 500091, România E-mail: cristian.ida@unitbv.ro Received October 28, 2009, in final form June 30, 2010; Published online July 09, 2010 doi:10.3842/SIGMA.2010.054 Abstract. The aim of this paper is to construct horizontal Chern forms of a holomorphic vector bundle using complex Finsler structures. Also, some properties of these forms are studied. Key words: complex Finsler bundles; horizontal forms of Chern type 2010 Mathematics Subject Classification: 53B40; 57R20 1 Introduction and preliminaries In [7] J.J. Faran posed an open question: Is it possible to define Chern forms of a holomorphic vector bundle using complex Finsler structures? Recently, in [4, 5, 6] and [16] was studied the Chern form of the associated hyperplane line bundle in terms of complex Finsler structures and some of its applications. The main purpose of this note is to obtain horizontal Chern forms of a complex Finsler bundle, following the general construction of Chern forms [8, 10, 15]. Firstly, we define a h′′- cohomology with respect to the horizontal conjugated differential operator and using the partial Bott complex connection [3], we construct horizontal invariant forms of Chern type on the total space. Also, we prove that the corresponding horizontal classes of Chern type do not depend on the complex Finsler structure chosen in a family given by conformal changes of horizontal type. Next, it is proved that these forms live on the associated projectivized bundle and are invariant by a linear family of complex Finsler structures given by infinitesimal deformations. Let π : E → M be a holomorphic vector bundle of rank m over a n-dimensional complex manifold M . Let (U, (zk)), k = 1, . . . , n be a local chart on M and s = {sa}, a = 1, . . . ,m a local frame for the sections of E over U . It is well known that this chart canonically induces another one on E of the form (π−1(U), u = (zk, ηa)), k = 1, . . . , n, a = 1, . . . ,m, where s = ηasa is a section on Ez = π−1(z), for all z ∈ M . If (π−1(U ′), u′ = (z′k, η′a)) is another local chart on E, then the transition laws of these coordinates are z′k = z′k(z), η′a = Ma b (z)ηb, where Ma b (z), a, b = 1, . . . ,m are holomorphic functions on z and detMa b 6= 0. As we already know [3, 4], the total space E has a natural structure of m + n dimen- sional complex manifold because the transition functions Ma b are holomorphic. We consider the complexified tangent bundle TCE of the real tangent bundle TRE and its decomposition TCE = T ′E ⊕ T ′′E, where T ′E and T ′′E = T ′E are the holomorphic and antiholomorphic tan- gent bundles of E, respectively. The vertical holomorphic tangent bundle V ′E = kerπ∗ is the relative tangent bundle of the holomorphic projection π. A local frame field on V ′ uE is {∂/∂ηa}, a = 1, . . . ,m and V ′ uE is isomorphic to the sections module of E in u. mailto:cristian.ida@unitbv.ro http://dx.doi.org/10.3842/SIGMA.2010.054 2 C. Ida A supplementary subbundle of V ′E in T ′E, i.e. T ′E = H ′E ⊕ V ′E is called a complex nonlinear connection on E, briefly c.n.c. A local basis for the horizontal distribution H ′ uE, called adapted for the c.n.c., is {δ/δzk = ∂/∂zk − Na k ∂/∂η a}, k = 1, . . . , n, where Na k (z, η) are the local coefficients of the c.n.c. In the sequel we consider the abreviate notations: ∂k = ∂/∂zk, δk = δ/δzk and ∂̇a = ∂/∂ηa. The adapted basis denoted by {δk = δ/δzk} and {∂̇a = ∂/∂ηa}, for H ′′E = H ′E and V ′′E = V ′E distributions are obtained respectively by conjugation anywhere. Also, we note that the adapted cobases are given by {dzk}, {δηa = dηa + Na k dz k}, {dzk} and {δηa = dηa +Na k dzk} which span the dual bundles H ′∗E, V ′∗E, H ′′∗E and V ′′∗E, respectively. Definition 1 ([9]). A strictly pseudoconvex complex Finsler structure on E is a positive real valued function F : E → R+ ∪ {0} which satisfies the following conditions: 1) F 2 is smooth on E∗ = E \ {0}; 2) F (z, η) ≥ 0 and F (z, η) = 0 if and only if η = 0; 3) F (z, λη) = |λ|F (z, η) for any λ ∈ C∗ = C \ {0}; 4) the complex hessian (hab) = (∂̇a∂̇b(F 2)) is positive definite and determines a hermitian metric tensor on the fibers of the vertical bundle V ′E \ {zero section}. Definition 2. The pair (E,F ) is called a complex Finsler bundle. According to [1, 4, 12], a c.n.c. on (E,F ) depending only on the complex Finsler structure F is the Chern–Finsler c.n.c., locally given by CF Na k = hca∂k∂̇c ( F 2 ) , (1) where (hca) is the inverse of (hac). An important property of the Chern–Finsler c.n.c. (see [4, 12]), is [δj , δk] = ( δk CF Na j −δj CF Na k ) ∂̇a = 0. (2) Throughout this paper we consider adapted frames and coframes with respect to the Chern– Finsler c.n.c. According to [3], a partial connection ∇ of (1, 0) type on V ′E, defined by ∇XY = v′[X,Y ] for any X ∈ Γ(H ′E) and Y ∈ Γ(V ′E), is called the partial Bott complex connection of (E,F ). Here v′ is the natural vertical projector. The (1, 0)-connection form ω = (ωa b ) of ∇ is locally given by ωa b = La bkdz k, La bk = ∂̇b ( CF Na k ) (3) and the (1, 1)-horizontal curvature form Ω = (Ωa b ) of ∇ is locally given by Ωa b = Ωa bjk dzj ∧ dzk, Ωa bjk = −δk(L a bj). (4) Remark 1. We notice that the partial Bott complex connection is in fact the classical Chern– Rund connection, first introduced by H. Rund [14] in the case when E = T ′M is the holomorphic tangent bundle of a complex manifold M . Horizontal Forms of Chern Type on Complex Finsler Bundles 3 h′′-cohomology Similar to [13], let us consider the set Ap,q,r,s(E) of all (p, q, r, s)-forms with complex values on E, locally defined by ϕ = 1 p!q!r!s! ϕIpJqArBs dzIp ∧ dzJq ∧ δηAr ∧ δηBs , where Ip = (i1 . . . ip), Jq = (j1 . . . jq), Ar = (a1 . . . ar), Bs = (b1 . . . bs) and these forms can be nonzero only when they act on p vectors from Γ(H ′E), on q vectors from Γ(H ′′E), on r vectors from Γ(V ′E) and on s vectors from Γ(V ′′E), respectively. By (2), we get the following decomposition of the exterior differential dAp,q,r,s(E) ⊂ Ap+1,q,r,s(E)⊕Ap,q+1,r,s(E)⊕Ap,q,r+1,s(E)⊕Ap,q,r,s+1(E)⊕ ⊕Ap+1,q+1,r−1,s(E)⊕Ap+1,q,r−1,s+1(E)⊕Ap+1,q+1,r,s−1(E)⊕Ap,q+1,r+1,s−1(E) which allows us to define eight morphisms of complex vector spaces if we consider different components in the above decomposition. If we consider the subset Ap,q,0,0(E) ⊂ Ap,q,r,s(E) of all horizontal forms of (p, q, 0, 0) type, which can be nonzero only when they act on p vectors from Γ(H ′E) and q vectors from Γ(H ′′E), then dAp,q,0,0(E) ⊂ Ap+1,q,0,0(E)⊕Ap,q+1,0,0(E)⊕Ap,q,1,0(E)⊕Ap,q,0,1(E). In particular, similar to [17, 18], the horizontal differential operators are defined by d′h : Ap,q,0,0(E) → Ap+1,q,0,0(E), d′′h : Ap,q,0,0(E) → Ap,q+1,0,0(E) where, for any ϕ = 1 p!q!ϕIpJq dzIp ∧ dzJq ∈ Ap,q,0,0(E), we have (d′hϕ)i1...ip+1Jq = p+1∑ k=1 (−1)k−1δik ( ϕi1...îk...ip+1Jq ) and (d′′hϕ)Ipj1...jq+1 = (−1)p q+1∑ k=1 (−1)k−1δjk ( ϕ Ipj1...ĵk...jq+1 ) . (5) We note that by (2), we have d′′h ◦ d′′h = 0. Thus, with respect to the operator d′′h we can define the h′′-cohomology groups of (E,F ) for (p, q, 0, 0)-forms by Hq ( E,Φp,0,0 ) = ker{d′′h : Ap,q,0,0 → Ap,q+1,0,0} Im {d′′h : Ap,q−1,0,0 → Ap,q,0,0} , (6) where Φp,0,0 is the sheaf of germs of (p, 0, 0, 0)-forms d′′h-closed. 2 Horizontal forms of Chern type In this section, using the horizontal curvature form of partial Bott complex connection, we construct horizontal forms of Chern type on a complex Finsler bundle. With the notations from the previous section, we have ωa b ∈ A1,0,0,0(E) and Ωa b ∈ A1,1,0,0(E). Also, by (4) and (5) we have Ω = d′′hω and taking into account the relation d′′h ◦ d′′h = 0, it follows that Ω is a d′′h-closed differential form. 4 C. Ida Let gl(m,C) ≈ Cm×m be the Lie algebra of the linear general group Gl(m,C). A symmetric polynomial f ∈ Sj(Gl(m,C)) is invariant if f ( M−1X1M, . . . ,M−1XjM ) = f(X1, . . . , Xj) for every M ∈ Gl(m,C) and Xj ∈ gl(m,C). Also, it is well known [8, 10, 15], that the algebra of invariant symmetric polynomials on gl(m,C) is generated by the elementary symmetric functions fj ∈ Sj(Gl(m,C)) given by det ( Im − 1 2πi X ) = m∑ j=0 fj(X) = 1− 1 2πi trX + · · ·+ ( − 1 2πi )m detX. (7) Next, we give a known result, but we express it in a form adapted to our study. Proposition 1. At local changes on E, the horizontal curvature of partial Bott complex con- nection changes by the rule Ω′ = M−1ΩM, where M = (Ma b (z)) are the transition functions of the holomorphic bundle E. Proof. By [12], we know that the change rules of the adapted frames and coframes are δj = ∂z′k ∂zj δ′k, ∂̇b = Ma b ∂̇ ′ a, dzj = ∂zj ∂z′k dz′k, δηb = M b aδη ′a (8) and its conjugates. On the other hand (see 7.1.9 in [12]), the local coefficients of the Chern–Finsler c.n.c. change by the rule ∂z′k ∂zj CF N ′a k = Ma b CF N b j − ∂Ma b ∂zj ηb. (9) Finally, it follows by (3), (4), (8) and (9) that Ω′d c = M b c Ωa bM d a . � Now, we consider the (j, j, 0, 0)-invariant forms Ch j (∇) defined by det ( Im − 1 2πi Ω ) = m∑ j=0 Ch j (∇). Then, it follows by (7) Ch j (∇) = (−1)j (2πi)jj! ∑ a,b δ b1...bj a1...ajΩ a1 b1 ∧ · · · ∧ Ωaj bj , where δb1...bj a1...aj are the Kronecker symbols. Because d′′h satisfies the property d′′h(ϕ ∧ ψ) = d′′hϕ ∧ ψ + (−1)deg ϕϕ ∧ d′′hψ for any ϕ ∈ Ap,q,0,0(E) and ψ ∈ Ap′,q′,0,0(E), then by d′′hΩ = 0 we obtain that the differential forms Ch j (∇) are d′′h-closed. Definition 3. Ch j (∇) are called horizontal forms of Chern type of order j of the complex Finsler bundle (E,F ). Horizontal Forms of Chern Type on Complex Finsler Bundles 5 By (6) these forms define the horizontal cohomology classes chj (∇) = [Ch j (∇)] ∈ Hj ( E,Φj,0,0 ) which are called horizontal classes of Chern type of order j of (E,F ). In particular, the first horizontal class of Chern type is represented by the (1, 1, 0, 0)-form Ch 1 (∇) = − 1 2πi Ωa ajk dzj ∧ dzk. Proposition 2. The first horizontal class of Chern type ch1(∇) is invariant by conformal change of horizontal type: F 2 7→ eσ(z)F 2. Proof. Let F 2 7→ F̃ 2 := eσ(z)F 2 be a conformal change of complex Finsler structures on E, where σ(z) is a smooth function on M . Because h̃ab = eσ(z)hab, using (1) we get that the Chern–Finsler c.n.c. changes by the rule CF Ña k = CF Na k + ∂σ ∂zk ηa. Thus, the (1, 0)-connection form of ∇ changes by the rule ω̃ = ω + d′hσ ⊗ I, (10) where I is the identity endomorphism of V ′E. Applying d′′h in the (10) relation, we get the change rule of the (1, 1)-horizontal curvature form of ∇, namely Ω̃ = Ω + d′′hd′hσ ⊗ I which ends the proof. � In fact the above proposition remains valid for all higher horizontal Chern classes, namely: Proposition 3. The horizontal classes of Chern type chj (∇) are invariant by conformal change of horizontal type: F 2 7→ eσ(z)F 2 for any j = 1, . . . ,m. Proof. It is classical that for a square matrix A of order p we have det(A− λI) = p∑ j=0 (−1)j∆p−j(A)λj , where ∆j(A) = 1 j! ∑ α,β δ β1...βj α1...αja α1 β1 · · · aαj βj is the sum of the principal minors of order j of the matrix A (see for instance [15, p. 235]). Now, taking into account d′′h∆j(Ω) = 0, by direct calculations we have Ch j (∇̃) = (−1)j (2πi)j ∆j(Ω + d′′hd′hσ ⊗ I) = (−1)j (2πi)j ( ∆j(Ω) + 1 j! j∑ k=1 ∆j−k(Ω) ∧ (d′′hd′hσ)k ) = Ch j (∇) + d′′h ( (−1)j (2πi)jj! j∑ k=1 ∆j−k(Ω) ∧ d′hσ ∧ (d′′hd′hσ)k−1 ) which says that Ch j (∇̃) and Ch j (∇) are in the same d′′h-cohomology class. � 6 C. Ida It is well known that for every ϕ ∈ Ap,q(M), ϕ∗ = ϕ ◦ π ∈ Ap,q,0,0(E) is the natural lift of ϕ to the total space E. Remark 2. If the complex Finsler structure F comes from a hermitian structure h on E, namely F 2(z, η) = h(η, η) = hab(z)η aηb, then the coefficients of the partial Bott complex connection ∇ are independent of the fiber coordinates ηa, and these coincides with the classical connection coefficients in hermitian bundles. In this case it is easy to see that Ch j (∇) coincide with the classical Chern forms of order j of hermitian bundle (E, h) lifted to the total space E. In the sequel we suppose that the base manifold M is compact. Note that there is a natural C∗ = C \ {0} action on E∗ = E \ {0} and the associated projectivized bundle is defined by PE = E∗/C∗ with the projection p : PE → M . Each fiber Pz(E) = P (Ez) is isomorphic to the (m − 1)-dimensional complex projective space Pm−1(C). The pull-back bundle p−1E is a holomorphic vector bundle of rank m over PE. Thus, the local complex coordinates (z, η) on E may be also considered as a local complex coordinates system for PE as long as η1, . . . , ηm is considered as a homogeneous coordinate system for fibers. All the geometric objects on E which are invariant after replacing η by λη, λ ∈ C∗ are valid on PE. The reason for working on PE rather than on E is that PE is compact if M is compact (see [9, 11]). Now, we simply denote by Ap,q(PE) the set of all horizontal forms of (p, q, 0, 0) type on E whose coefficients are zero homogeneous with respect to fiber coordinates, namely ϕIpJq (z, λη) = ϕIpJq (z, η), for any λ ∈ C∗. By the homogeneity conditions of the complex Finsler structure F (see [3, 4, 12]), the local coefficients of the Chern–Finsler c.n.c. are given by CF Na k = La bkη b (11) and, moreover CF Na k (z, λη) = λ CF Na k (z, η), ∀λ ∈ C∗. (12) Then, it follows by (11) and (12) La bk(z, λη) = La bk(z, η), ∀λ ∈ C∗ which says that the (1, 0)-connection forms ωa b of ∇ live on PE. Similarly, by (4) we get Ωa bjk (z, λη) = Ωa bjk (z, η) for any λ ∈ C∗ which says that the (1, 1)-curvature forms Ωa b of ∇ live on PE. Thus, Ch j (∇) ∈ Aj,j(PE). Finally, we note that properties of horizontal Chern forms in relation with basic properties of classical Chern forms must be studied, as well as the independence of these forms with respect to some families of complex Finsler structures on a holomorphic vector bundle. Another important problem to solve is to describe the obstructions corresponding to these classes. Here we are able to respond only partially to these questions. Applying some results from [2], we get the invariance of horizontal Chern forms by a family of complex Finsler structures given by infinitesimal deformations. We consider a 1-parameter family {F 2 t }, t ∈ R of pseudoconvex complex Finsler structures on a holomorphic vector bundle E. Let us put F 2 0 = F 2 and let ∇0 = ∇ its partial Bott complex connection. The infinitesimal deformation V induced by F 2 t is defined by V := ( ∂F 2 t ∂t ) |t=0 and its components with respect to a fixed frame field s = {sa}, a = 1, . . . ,m of E are given by Vab = ( ∂ht ab ∂t ) |t=0. We put V a b = hcaVbc and we consider it as a section of the bundle End(V ′E). If we consider F 2 t = F 2 + tV for sufficiently small t so that F 2 t remains pseudoconvex then in Theorem 5.2 from [2] it is proved that if ∇V a b = 0 then ∇t = ∇ and Ωt = Ω. Here ∇Z = (d′hZa + Zbωa b )∂̇a, for any Z = Za∂̇a ∈ Γ(V ′E). Thus, we can conclude Horizontal Forms of Chern Type on Complex Finsler Bundles 7 Proposition 4. If ∇V a b = 0 then the horizontal Chern forms Ch j (∇) of (E,F ) are invariant by the linear family of complex Finsler structures given by F 2 t = F 2 + tV . We notice that in Aikou’s paper [2], the partial connection is considered in the pull-back bundle p−1E over PE and the calculations are similar as on V ′E. Acknowledgements The author is grateful to the anonymous referees and would like to thank them for generous sug- gestions and comments. Also, I warmly thank Professor Gheorghe Pitiş for fruitful conversations concerning this topics. References [1] Abate M., Patrizio G., Finsler metrics – a global approach. With applications to geometric function theory, Lectures Notes in Math., Vol. 1591, Springer-Verlag, Berlin, 1994. [2] Aikou T., A note on infinitesimal deformations of complex Finsler structures, An. Ştiinţ. Univ. Al. I. Cuza Iaşi. Mat. (N.S.) 43 (1997), 295–305. [3] Aikou T., Applications of Bott connection to Finsler geometry, in Proceedings of the Colloquium “Steps in Differential Geometry” (Debrecen, 2000), Inst. Math. Inform., Debrecen, 2001, 1–13. [4] Aikou T., Finsler geometry on complex vector bundles. A sampler of Riemann–Finsler geometry, Math. Sci. Res. Inst. Publ., Vol. 50, Cambridge Univ. Press, Cambridge, 2004, 83–105. 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[18] Zhong C., Zhong T., Hodge decomposition theorem on strongly Kähler Finsler manifolds, Sci. China Ser. A 49, (2006), 1696–1714. http://dx.doi.org/10.1023/B:MAHU.0000038969.91179.e4 http://projecteuclid.org/getRecord?id=euclid.nmj/1118795367 http://dx.doi.org/10.1002/mana.19670340104 http://dx.doi.org/10.1007/BF02884722 http://dx.doi.org/10.1007/s11425-006-2055-8 1 Introduction and preliminaries 2 Horizontal forms of Chern type References