In mathematics, a holomorphic vector bundle is a complex vector bundle over a complex manifold X such that the total space E is a complex manifold and the projection map π : E → X is holomorphic. Fundamental examples are the holomorphic tangent bundle of a complex manifold, and its dual, the holomorphic cotangent bundle. A holomorphic line bundle is a rank one holomorphic vector bundle.
By Serre's GAGA, the category of holomorphic vector bundles on a smooth complex projective variety X (viewed as a complex manifold) is equivalent to the category of algebraic vector bundles (i.e., locally free sheaves of finite rank) on X.
Definition through trivializationEdit
Specifically, one requires that the trivialization maps
are holomorphic maps. The holomorphic structure on the tangent bundle of a complex manifold is guaranteed by the remark that the derivative (in the appropriate sense) of a vector-valued holomorphic function is itself holomorphic.
The sheaf of holomorphic sectionsEdit
Let E be a holomorphic vector bundle. A local section s : U → E|U is said to be holomorphic if, in a neighborhood of each point of U, it is holomorphic in some (equivalently any) trivialization.
This condition is local, meaning that holomorphic sections form a sheaf on X. This sheaf is sometimes denoted , or abusively by E. Such a sheaf is always locally free of the same rank as the rank of the vector bundle. If E is the trivial line bundle then this sheaf coincides with the structure sheaf of the complex manifold X.
There are line bundles over whose global sections correspond to homogeneous polynomials of degree (for a positive integer). In particular, corresponds to the trivial line bundle. If we take the covering then we can find charts defined by
We can construct transition functions defined by
Now, if we consider the trivial bundle we can form induced transition functions . If we use the coordinate on the fiber, then we can form transition functions
for any integer . Each of these are associated with a line bundle . Since vector bundles necessarily pull back, any holomorphic submanifold has an associated line bundle , sometimes denoted .
Suppose E is a holomorphic vector bundle. Then there is a distinguished operator defined as follows. In a local trivialisation of E, with local frame , any section may be written for some smooth functions . Define an operator locally by
where is the regular Cauchy-Riemann operator of the base manifold. This operator is well-defined on all of E because on an overlap of two trivialisations with holomorphic transition function , if where is a local frame for E on , then , and so
because the transition functions are holomorphic. This leads to the following definition: A Dolbeault operator on a smooth complex vector bundle is an -linear operator
- (Cauchy-Riemann condition) ,
- (Leibniz rule) For any section and function on , one has
Theorem: Given a Dolbeault operator on a smooth complex vector bundle , there is a unique holomorphic structure on such that is the associated Dolbeault operator as constructed above.
With respect to the holomorphic structure induced by a Dolbeault operator , a smooth section is holomorphic if and only if . This is similar morally to the definition of a smooth or complex manifold as a ringed space. Namely, it is enough to specify which functions on a topological manifold are smooth or complex, in order to imbue it with a smooth or complex structure.
The sheaves of forms with values in a holomorphic vector bundleEdit
If denotes the sheaf of C∞ differential forms of type (p, q), then the sheaf of type (p, q) forms with values in E can be defined as the tensor product
These sheaves are fine, meaning that they admit partitions of unity. A fundamental distinction between smooth and holomorphic vector bundles is that in the latter, there is a canonical differential operator, given by the Dolbeault operator defined above:
Cohomology of holomorphic vector bundlesEdit
If E is a holomorphic vector bundle, the cohomology of E is defined to be the sheaf cohomology of . In particular, we have
the space of global holomorphic sections of E. We also have that parametrizes the group of extensions of the trivial line bundle of X by E, that is, exact sequences of holomorphic vector bundles 0 → E → F → X × C → 0. For the group structure, see also Baer sum as well as sheaf extension.
By Dolbeault's theorem, this sheaf cohomology can alternatively be described as the cohomology of the chain complex defined by the sheaves of forms with values in the holomorphic bundle . Namely we have
The Picard groupEdit
In the context of complex differential geometry, the Picard group Pic(X) of the complex manifold X is the group of isomorphism classes of holomorphic line bundles with group law given by tensor product and inversion given by dualization. It can be equivalently defined as the first cohomology group of the sheaf of non-vanishing holomorphic functions.
Hermitian metrics on a holomorphic vector bundleEdit
Let E be a holomorphic vector bundle on a complex manifold M and suppose there is a hermitian metric on E; that is, fibers Ex are equipped with inner products <·,·> that vary smoothly. Then there exists a unique connection ∇ on E that is compatible with both complex structure and metric structure, called the Chern connection; that is, ∇ is a connection such that
- (1) For any smooth sections s of E, where π0,1 takes the (0, 1)-component of an E-valued 1-form.
- (2) For any smooth sections s, t of E and a vector field X on M,
Indeed, if u = (e1, …, en) is a holomorphic frame, then let and define ωu by the equation , which we write more simply as:
If u' = ug is another frame with a holomorphic change of basis g, then
and so ω is indeed a connection form, giving rise to ∇ by ∇s = ds + ω · s. Now, since ,
That is, ∇ is compatible with metric structure. Finally, since ω is a (1, 0)-form, the (0, 1)-component of is .
Let be the curvature form of ∇. Since squares to zero by the definition of a Dolbeault operator, Ω has no (0, 2)-component and since Ω is easily shown to be skew-hermitian, it also has no (2, 0)-component. Consequently, Ω is a (1, 1)-form given by
- Kobayashi, S. (2014). Differential geometry of complex vector bundles (Vol. 793). Princeton University Press.
- Kycia, Radosław Antoni. "The Poincare Lemma, Antiexact Forms, and Fermionic Quantum Harmonic Oscillator". Results in Mathematics. 75 (3): 122. doi:10.1007/s00025-020-01247-8. ISSN 1422-6383.
- For example, the existence of a Hermitian metric on E means the structure group of the frame bundle can be reduced to the unitary group and Ω has values in the Lie algebra of this unitary group, which consists of skew-hermitian metrices.