Hatcher 0.26

Hatcher, Algebraic Topology, Chapter 0

26. Use Corollary 0.20 to show that if (X,A) has the homotopy extension property, then X\times I deformation retracts to X\times\{0\}\cup A\times I. Deduce from this that Proposition 0.18 holds more generally for any pair (X_1,A) satisfying the homotopy extension property.

Proof.

   \square

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