# Complex vector bundle

In mathematics, a complex vector bundle is a vector bundle whose fibers are complex vector spaces.

Any complex vector bundle can be viewed as a real vector bundle through the restriction of scalars. Conversely, any real vector bundle E can be promoted to a complex vector bundle, the complexification

${\displaystyle E\otimes \mathbb {C} ;}$

whose fibers are ExR C.

Any complex vector bundle over a paracompact space admits a hermitian metric.

The basic invariant of a complex vector bundle is a Chern class. A complex vector bundle is canonically oriented; in particular, one can take its Euler class.

A complex vector bundle is a holomorphic vector bundle if X is a complex manifold and if the local trivializations are biholomorphic.

## Complex structure

A complex vector bundle can be thought of as a real vector bundle with an additional structure, the complex structure. By definition, a complex structure is a bundle map between a real vector bundle E and itself:

${\displaystyle J:E\to E}$

such that J acts as the square root i of -1 on fibers: if ${\displaystyle J_{x}:E_{x}\to E_{x}}$  is the map on fiber-level, then ${\displaystyle J_{x}^{2}=-1}$  as a linear map. If E is a complex vector bundle, then the complex structure J can be defined by setting ${\displaystyle J_{x}}$  to be the scalar multiplication by ${\displaystyle i}$ . Conversely, if E is a real vector bundle with a complex structure J, then E can be turned into a complex vector bundle by setting: for any real numbers a, b and a real vector v in a fiber Ex,

${\displaystyle (a+ib)v=av+J(bv).}$

Example: A complex structure on the tangent bundle of a real manifold M is usually called an almost complex structure. A theorem of Newlander and Nirenberg says that an almost complex structure J is "integrable" in the sense it is induced by a structure of a complex manifold if and only if a certain tensor involving J vanishes.

## Conjugate bundle

If E is a complex vector bundle, then the conjugate bundle ${\displaystyle {\overline {E}}}$  of E is obtained by having complex numbers acting through the complex conjugates of the numbers. Thus, the identity map of the underlying real vector bundles: ${\displaystyle E_{\mathbb {R} }\to {\overline {E}}_{\mathbb {R} }=E_{\mathbb {R} }}$  is conjugate-linear, and E and its conjugate E are isomorphic as real vector bundles.

The k-th Chern class of ${\displaystyle {\overline {E}}}$  is given by

${\displaystyle c_{k}({\overline {E}})=(-1)^{k}c_{k}(E)}$ .

In particular, E and E are not isomorphic in general.

If E has a hermitian metric, then the conjugate bundle E is isomorphic to the dual bundle ${\displaystyle E^{*}=\operatorname {Hom} (E,{\mathcal {O}})}$  through the metric, where we wrote ${\displaystyle {\mathcal {O}}}$  for the trivial complex line bundle.

If E is a real vector bundle, then the underlying real vector bundle of the complexification of E is a direct sum of two copies of E:

${\displaystyle (E\otimes \mathbb {C} )_{\mathbb {R} }=E\oplus E}$

(since VRC = ViV for any real vector space V.) If a complex vector bundle E is the complexification of a real vector bundle E', then E' is called a real form of E (there may be more than one real form) and E is said to be defined over the real numbers. If E has a real form, then E is isomorphic to its conjugate (since they are both sum of two copies of a real form), and consequently the odd Chern classes of E have order 2.