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Reference.org
Hopf theorem
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<p>The Hopf theorem (named after <a href="/facts/Heinz_Hopf/PMks10NV">Heinz Hopf</a>) is a statement in <a href="/facts/Differential_topology/Y21eGQe0">differential topology</a>, saying that the <a href="/facts/Degree_of_a_continuous_mapping/yWxq9e4B">topological degree</a> is the only <a href="/facts/Homotopy_invariant/1nWrPxdj">homotopy invariant</a> of <a href="/facts/Continuous_maps/sKbl02pB">continuous maps</a> to <a href="/facts/N-sphere/bFu2S7E2">spheres</a>. </p>