Flat vector bundle

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In mathematics, a vector bundle is said to be flat if it is endowed with a linear connection with vanishing curvature, i.e. a flat connection.

## de Rham cohomology of a flat vector bundle

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Let  π : E -> X {\displaystyle \pi :E\to X} denote a flat vector bundle, and  ∇ : Γ ( X , E ) -> Γ ( X , Ω X 1 ⊗ E ) {\displaystyle \nabla :\Gamma (X,E)\to \Gamma \left(X,\Omega _{X}^{1}\otimes E\right)} be the covariant derivative associated to the flat connection on E.

Let  Ω X ∗ ( E ) = Ω X ∗ ⊗ E {\displaystyle \Omega _{X}^{*}(E)=\Omega _{X}^{*}\otimes E} denote the vector space (in fact a sheaf of modules over  O X {\displaystyle {\mathcal {O}}_{X}} ) of differential forms on X with values in E. The covariant derivative defines a degree-1 endomorphism d, the differential of  Ω X ∗ ( E ) {\displaystyle \Omega _{X}^{*}(E)} , and the flatness condition is equivalent to the property  d 2 = 0 {\displaystyle d^{2}=0} .

In other words, the graded vector space Ω X ∗ ( E ) {\displaystyle \Omega _{X}^{*}(E)} is a cochain complex. Its cohomology is called the de Rham cohomology of E, or de Rham cohomology with coefficients twisted by the local coefficient system E.

## Flat trivializations

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A trivialization of a flat vector bundle is said to be flat if the connection form vanishes in this trivialization. An equivalent definition of a flat bundle is the choice of a trivializing atlas with locally constant transition maps.

## Examples

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* Trivial line bundles can have several flat bundle structures. An example is the trivial bundle over  C ∖ { 0 } , {\displaystyle \mathbb {C} \backslash \\{0\\},} with the connection forms 0 and  − 1 2 d z z {\displaystyle -{\frac {1}{2}}{\frac {dz}{z}}} . The parallel vector fields are constant in the first case, and proportional to local determinations of the square root in the second.
* The real canonical line bundle Λ t o p M {\displaystyle \Lambda ^{\mathrm {top} }M} of a differential manifold M is a flat line bundle, called the orientation bundle. Its sections are volume forms.
* A Riemannian manifold is flat if and only if its Levi-Civita connection gives its tangent vector bundle a flat structure.

## See also

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* Vector-valued differential forms
* Local system, the more general notion of a locally constant sheaf.
* Orientation character, a characteristic form related to the orientation line bundle, useful to formulate Twisted Poincaré duality
* Picard group whose connected component, the Jacobian variety, is the moduli space of algebraic flat line bundles.
* Monodromy, or representations of the fundamental group by parallel transport on flat bundles.
* Holonomy, the obstruction to flatness.