Hatcher 2.1.30

Hatcher, Algebraic Topology, Chapter 2, Section 1

30. In each of the commutative diagrams in Figure 1 below, assume that all maps but one are isomorphisms. Show that the remaining map must be an isomorphism as well.

Hatcher 2.1.30
Figure 1
The commutative diagrams mentioned above

Proof.

\square

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