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    Equivalence of two definitions of a contractible space.

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    We have defined space X to be contractible in two ways:

    Definition 1: X is contractible if it is homotopy equivalent to a point; and
    Definition 2: X is contractible if the identity map of X is null-homotopic.

    Show that these two definitions are equivalent.

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    https://brainmass.com/math/linear-algebra/equivalence-of-two-definitions-of-a-contractible-space-355942

    Solution Preview

    First, write down the definitions:

    Two spaces, X and Y are homotopic, if there are two maps, f: X->Y and g:Y->X such the fg is homotopic to id_X and gf is homotopic to id_Y.
    A map f is null-homotopic if it is homotopic to a constant map.

    Suppose, ...

    Solution Summary

    The equivalence of two definitions of a contractible space is proved.

    $2.19

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