Riemann-Hurwitz formula

In algebraic geometry the Riemann-Hurwitz formula states that if C,D are smooth algebraic curves, and $f:C\to D$ is a finite map of degree $d$ then the number of branch points of $f$, denoted by $B$, is given by

$2({\mbox{genus}}(C)-1)=2d({\mbox{genus}}(D)-1)+B$.

a triangulated gluing diagram for the Riemann sphere, and it's pullback to a torus double cover, which is ramified over the vertices of the triangulation

Over a field in general characteristic, this theorem is a consequence of the Riemann-Roch theorem. Over the complex numbers, the theorem can be proved by choosing a triangulation of the curve $D$ such that all the branch points of the map are nodes of the triangulation. One then considers the pullback of the triangulation to the curve $C$ and computes the Euler characteristics of both curves.