Collapsibility of Random Clique Complexes

Abstract

We prove a sufficient condition for a finite clique complex to collapse to a $k$-dimensional complex, and use this to exhibit thresholds for $(k+1)$-collapsibility in a sparse random clique complex. In particular, if every strongly connected, pure $(k+1)$-dimensional subcomplex of a clique complex $X$ has a vertex of degree at most $2k+1$, then $X$ is $(k+1)$-collapsible. In the random model $X(n,p)$ of clique complexes of an Erd\H{o}s--R\'{e}nyi random graph $G(n,p)$, we then show that for any fixed $k\geq 0$, if $p=n^{-\alpha}$ for fixed $1/(k+1) < \alpha < 1/k$, then a clique complex $X\overset{dist}{=} X(n,p)$ is $(k+1)$-collapsible with high probability.