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Suppose is a compact two-dimensional orientable Riemannian manifold with boundary . Denote by the Gaussian curvature at points of , and by the geodesic curvature at points of . Then
where is the Euler characteristic of .
The theorem applies in particular if the manifold does not have a boundary, in which case the integral can be omitted.
If one bends and deforms the manifold , its Euler characteristic will not change, while the curvatures at given points will. The theorem requires, somewhat surprisingly, that the total integral of all curvatures will remain the same.
A generalization to dimensions was found in the 1940s, by Allendoerfer, Weil, and Chern. See generalized Gauss-Bonnet theorem and Chern-Weil homomorphism.
Riemannian geometry Theorems