Large Complete Minors from a Cheeger Condition
Let $G$ be a graph on $n$ vertices, and let $e_G(S,V(G)\setminus S)$ be the number of edges with exactly one endpoint in $S$. The Cheeger constant and the restricted Cheeger constant of $G$, where $k\ge1$ is real, are, respectively, \[ h(G)=\min_{\substack{\emptyset\ne S\subseteq V(G)\\|S|\le \frac{n}{2}}} \frac{e_G(S,V(G)\setminus S)}{|S|} \text{ and } h_k(G)=\min_{\substack{\emptyset\ne S\subseteq V(G) |S|\le\min\{k,\frac{n}{2}\}}} \frac{e_G(S,V(G)\setminus S)}{|S|}. \] The contraction clique number $\ccl(G)$ is the largest integer $r$ such that $G$ contains the complete graph $K_r$ as a minor. Krivelevich and Nenadov [Complete minors in graphs without sparse cuts, Int. Math. Res. Not. IMRN 12 (2021) 8996--9015] proved that, for every fixed $\eps>0$ and all sufficiently large $n$ and $d$, if $G$ is a graph on $n$ vertices with maximum degree at most $d$, then $h(G)\ge\eps d$ and $h_{\eps n}(G)\ge(\frac{1}{2}+\eps)d$ imply $\ccl(G)=Ω_\eps(\sqrt{\frac{nd}{\log d}})$. They asked to determine if one can guarantee the same lower bound on $\ccl(G)$ without the additional condition on $h_{\eps n}(G)$. They showed that this is the case when $d$ is a constant. We answer this question affirmatively. For every $\eps>0$, there are constants $β=β(\eps)>0$ and $n_0=n_0(\eps)$ such that, whenever $d\ge 3$ is an integer, for every graph $G$ with $n\ge n_0$ vertices and maximum degree at most $d$, if $h(G)\ge\eps d$, then $\ccl(G)\geβ\sqrt{\frac{nd}{\log d}}$. The dependence of this lower bound on $n$ and $d$ is best possible up to a constant factor. As a corollary, a lower bound is derived for the contraction clique number of $d$-regular graphs for which the second largest eigenvalue is bounded away from $d$, compared to earlier $\frac{d}{2}$. The proof combines spectral properties of graphs with an analysis of lazy random walks.