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Let $A$ be a retract of $X$. Show that if $i:A\to X$ is the inclusion operator , then $i_*:H_n(A) \to H_n(X)$ is an isomorphism

(It's easy to see that it's a monomorphism. But why is it an epimorphism?)

Thanks!

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    it's not. Every $X$ retracts to a point. You might mean deformation retract.2012-02-16

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