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Complex vector bundle

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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 scalers. Conversely, any real vector bundle E can be promoted to a complex vector bundle, the "complexification" EC of E: its fibers are

.

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

The basic invariant of a complex vector bundle is a Chern class.

Complex structure

A complex vector bundle can be thought of 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:

such that J acts as the square root i of -1 on fibers: if is the map on fiber-level, then as a linear map. If E is a complex vector bundle, then the complex structure J can be defined by setting to be the scalar multiplication by . 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,

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 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: is conjugate-linear and E and its conjugate are isomorphic as real vector bundle.

The k-th Chern class of is given by

.

In particular, E and are not isomorphic in general.

If E has a hermitian metric, then is isomorphic to the dual bundle through the metric, where we wrote 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:

.

See also

References

  • Milnor, John Willard; Stasheff, James D. (1974), Characteristic classes, Annals of Mathematics Studies, vol. 76, Princeton University Press; University of Tokyo Press, ISBN 978-0-691-08122-9