 Extension-null-homotopy criterion for a sphere (source code)

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= Extension-null-homotopy criterion for a sphere

A map $f:S^{n-1}\to X$ extends to $D^n$ exactly when it is null-homotopic. An extension contracts radially through the disc; conversely, a null-homotopy descends through the cone quotient $CS^{n-1}\cong D^n$.

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