# nLab flat connection

Contents

### Context

#### $\infty$-Chern-Weil theory

∞-Chern-Weil theory

∞-Chern-Simons theory

∞-Wess-Zumino-Witten theory

## Theorems

#### Differential cohomology

differential cohomology

# Contents

## Definition

A connection on a fibre bundle is flat if its curvature is zero.

The same definition of flatness holds for connections in various algebraic setups and for connections on quasicoherent sheaves.

The condition of flatness is usually expressed via the Maurer-Cartan equation. Flat connections on bundles are also referred to as local systems (by the Riemann-Hilbert correspondence discussed below).

## Properties

### Riemann-Hilbert correspondence

That flat connections are equivalently representations of the fundamental groupoid of the base space/local systems is known as the Riemann-Hilbert correspondence.

### Flatness as integrability

In geometry one says instead of flat connection, integrable connection. The reason is roughly the following: in the theory of systems of differential equations the flatness of the corresponding connection is the condition of the integrability of the system.

(…elaborate on this with equations)

The condition of flatness is usually expressed via the Maurer-Cartan equation, which is in integrable systems theory often called zero curvature equation. For example, the Lax equations can always be written in the form of the zero curvature equation.

### Over a complex manifold

Over a complex manifold/complex variety the Koszul-Malgrange theorem identifies holomorphic flat connections on complex vector bundles with holomorphic vector bundles. See there for more

### Over a Riemann surface

#### Existence

In (Milnor) it is shown that a vector bundle over a surface of genus $g$ admits flat connections iff its Euler class is less than $g$ by an absolute value (see also Wood, Bundles with totally disconnected structure group). Sullivan gives a refinement.

The Narasimhan-Seshadri theorem identifies moduli spaces of flat connections over a Riemann surface with that of certain stable vector bundles.

### Isometric embeddings

Maurer-Cartan equation is called also structure equation when used to treat the conditions for isometric embeddings of Riemannian submanifolds in an Euclidean space.

## References

On the equivalence between flat connections and group homomorphisms out of the fundamental group (over each connected component):

• Pierre Deligne, §I.1 of: Equations différentielles à points singuliers réguliers, Lecture Notes Math. 163, Springer (1970) $[$publications.ias:355$]$

• Alexandru Dimca, Prop. 2.5.1 of: Sheaves in Topology, Universitext, Springer (2004) $[$doi:10.1007/978-3-642-18868-8$]$

For the generalization of this statement to non-flat connections see the references at parallel transport here.

• John Milnor, On the existence of a connection with curvature zero, Comm. Math. Helv. 32 (1957/58) 215-223 $[$dml:139154$]$