On the choosability of $H$-minor-free graphs

Abstract

Given a graph $H$, let us denote by $f_\chi(H)$ and $f_\ell(H)$, respectively, the maximum chromatic number and the maximum list chromatic number of $H$-minor-free graphs. Hadwiger's famous coloring conjecture from 1943 states that $f_\chi(K_t)=t-1$ for every $t \ge 2$. In contrast, for list coloring it is known that $2t-o(t) \le f_\ell(K_t) \le O(t (\log \log t)^6)$ and thus, $f_\ell(K_t)$ is bounded away from the conjectured value $t-1$ for $f_\chi(K_t)$ by at least a constant factor. The so-called $H$-Hadwiger's conjecture, proposed by Seymour, asks to prove that $f_\chi(H)=\textsf{v}(H)-1$ for a given graph $H$ (which would be implied by Hadwiger's conjecture). In this paper, we prove several new lower bounds on $f_\ell(H)$, thus exploring the limits of a list coloring extension of $H$-Hadwiger's conjecture. Our main results are: For every $\varepsilon>0$ and all sufficiently large graphs $H$ we have $f_\ell(H)\ge (1-\varepsilon)(\textsf{v}(H)+\kappa(H))$, where $\kappa(H)$ denotes the vertex-connectivity of $H$. For every $\varepsilon>0$ there exists $C=C(\varepsilon)>0$ such that asymptotically almost every $n$-vertex graph $H$ with $\left\lceil C n\log n\right\rceil$ edges satisfies $f_\ell(H)\ge (2-\varepsilon)n$. The first result generalizes recent results on complete and complete bipartite graphs and shows that the list chromatic number of $H$-minor-free graphs is separated from the natural lower bound $(\textsf{v}(H)-1)$ by a constant factor for all large graphs $H$ of linear connectivity. The second result tells us that even when $H$ is a very sparse graph (with an average degree just logarithmic in its order), $f_\ell(H)$ can still be separated from $(\textsf{v}(H)-1)$ by a constant factor arbitrarily close to $2$. Conceptually these results indicate that the graphs $H$ for which $f_\ell(H)$ is close to $(\textsf{v}(H)-1)$ are typically rather sparse.