# Gauss-Bonnet Theorem

## Theorem

Let $M$ be a compact $2$-dimensional Riemannian manifold with boundary $\partial M$.

Let $\Kappa$ be the Gaussian curvature of $M$.

Let $k_g$ be the geodesic curvature of $\partial M$.

Then:

$\ds \int_M \kappa \rd A + \int_{\partial M} k_g \rd s = 2 \pi \map \chi M$

where:

$\d A$ is the element of area of the surface
$\d s$ is the line element along $\partial M$
$\map \chi M$ is the Euler characteristic of $M$.

## Source of Name

This entry was named for Carl Friedrich Gauss and Pierre Ossian Bonnet.

## Historical Note

Carl Friedrich Gauss was aware of the Gauss-Bonnet Theorem but never published it.

It was Pierre Ossian Bonnet who first published, in $1848$, a special case.