Hatcher 0.28

Hatcher, Algebraic Topology, Chapter 0

28. Show that if (X_1,A) satisfies the homotopy extension property, then so does every pair (X_0\sqcup_f X_1,X_0) obtained by attaching X_1 to a space X_0 via a map f:A\to X_0.

Proof.

   \square

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