Fractional chromatic number vs. Hall ratio

Abstract

Given a graph $G$, its Hall ratio $ρ(G)=\max_{H\subseteq G}\frac{|V(H)|}{α(H)}$ forms a natural lower bound on its fractional chromatic number $χ_f(G)$. A recent line of research studied the fundamental question of whether $χ_f(G)$ can be bounded in terms of a (linear) function of $ρ(G)$. In a breakthrough-result, Dvořák, Ossona de Mendez and Wu gave a strong negative answer by proving the existence of graphs with bounded Hall ratio and arbitrarily large fractional chromatic number. In this paper, we solve two natural follow-up problems that were raised by Dvořák et al. The first problem concerns determining the growth of $g(n)$, defined as the maximum ratio $\frac{χ_f(G)}{ρ(G)}$ among all $n$-vertex graphs. Dvořák et al. obtained the bounds $Ω(\log\log n) \le g(n)\le O(\log n)$, leaving an exponential gap between the lower and upper bound. We almost fully resolve this problem by proving that the truth is close to the upper bound, i.e., $g(n)=(\log n)^{1-o(1)}$. The second problem posed by Dvořák et al. asks for the existence of graphs with bounded Hall ratio, arbitrarily large fractional chromatic number and such that every subgraph contains an independent set that touches a constant fraction of its edges. We affirmatively solve this second problem by showing that such graphs indeed exist.