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    $\Omega X$ is an H-space

    E-HAT-4.3-17

    Area
    Topology
    Topics
    Higher Homotopy Groups
    Status
    draft

    Show that \(\Omega X\) is an H-space with multiplication the composition of loops.

    Solution

    The based loop space \(\Omega X\) has multiplication by concatenation: \[ (\alpha*\beta)(t)= \begin{cases} \alpha(2t),&0\le t\le\tfrac12,\\ \beta(2t-1),&\tfrac12\le t\le1. \end{cases} \] The constant loop is a two-sided unit up to the standard reparametrization homotopies. Concatenating three loops in the two possible parenthesizations differs only by a piecewise-linear reparametrization of \(I\), hence is homotopic through based loops. Thus multiplication is associative up to homotopy. Reversal of loops gives a homotopy inverse. Therefore \[ \boxed{\Omega X\text{ is an H-space}.} \]