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 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

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 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