Contraction of a topological space
= Contraction of a topological space
{title2=$H:X\times[0,1]\to X$}
A contraction is a <homotopy> from the identity map of $X$ to a constant map. Its existence says that $X$ is a <contractible space>. For a <tree>, choosing a root and moving each point along its unique finite path to that root gives a contraction, with continuity understood in the <weak topology of a CW complex>.