Hopf theorem

The Hopf theorem (named after Heinz Hopf) is a statement in differential topology, saying that the topological degree is the only homotopy invariant of continuous maps to spheres.

Let M be an n-dimensional compact connected oriented manifold and ${\displaystyle S^{n}}$ the n-sphere and ${\displaystyle f,g\colon M\to S^{n}}$ be continuous. Then ${\displaystyle \deg(f)=\deg(g)}$ if and only if f and g are homotopic.