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Topological degree is the only homotopy invariant of continuous maps to spheres From Wikipedia, the free encyclopedia
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 the n-sphere and be continuous. Then if and only if f and g are homotopic.
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