Flat vector bundle
# Flat vector bundle

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

Let $\pi :E\to X$ denote a flat vector bundle, and $\nabla :\Gamma (X,E)\to \Gamma \left(X,\Omega _{X}^{1}\otimes E\right)$ be the covariant derivative associated to the flat connection on E.

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

In other words, the graded vector space $\Omega _{X}^{*}(E)$ 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

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

* Trivial line bundles can have several flat bundle structures. An example is the trivial bundle over $\mathbb {C} \backslash \{0\},$ with the connection forms 0 and $-{\frac {1}{2}}{\frac {dz}{z}}$. 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 $\Lambda ^{\mathrm {top} }M$ 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.
