Linked from
The 15 pages that link to Gauss–Bonnet theorem, each with the reason it gives.
Euler characteristicRelated: It connects this discrete invariant to geometry and curvature.
Spherical geometryRelated: It links a spherical triangle’s angle excess to the area it encloses.
Gaussian curvatureRelated: It turns local curvature values into a global topological constraint.
Atiyah–Singer index theoremRelated: It is recovered as an index formula for the de Rham operator.
Hjelmslev's theoremRelated: For hyperbolic triangles, it makes angle defect proportional to area.