On the chromatic profile for tripartite graphs and beyond

TL;DR

This paper determines the possible minimum degree thresholds for 2-colorability in H-free graphs with chromatic number 3, providing a finite set of values and a structural characterization for each.

Contribution

It completely characterizes the set of thresholds for 2-colorability in graphs with chromatic number 3 and introduces the vertex-extendable threshold concept.

Findings

The set of possible thresholds is finite and discrete.

Complete structural characterization for each threshold value.

Extension of classical results to color-critical graphs with odd girth equal to 3.

Abstract

Let $H$ be a graph and let $\delta_{\chi}(H,r)$ denote the infimum of $c$ such that every $H$-free graph with minimum degree at least $cn$ is $r$-colorable. The \textit{chromatic profile} of $H$ is defined to be the values of $\delta_{\chi}(H,r)$ as $r$ varies. Erd\H{o}s and Simonovits described this graph parameter as ``too complicated", and Allen, B\"ottcher, Griffiths, Kohayakawa, and Morris posed its determination for every graph $H$ as an open problem \cite[Problem~45]{ABGKM2013}, emphasizing its expected difficulty. In this paper, we resolve the case $r=2$ for every graph $H$ with $\chi(H)=3$. We show that the set of possible values of $\delta_{\chi}(H,2)$ with $\chi(H)=3$ is finite and discrete: $$\{\delta_{\chi}(H,2):\chi(H)=3\}=\left\{\frac{1}{2},\frac{2}{5},\frac{2}{7},\frac{1}{4},\frac{2}{9},\frac{1}{5},\frac{2}{11},\frac{1}{6}\right\}.$$ Furthermore, we provide a complete structural characterization of the graphs $H$ associated with each threshold value. Moreover, we extend the classical chromatic profile result for triangle to color-critical graphs $H$ with $g_{\mathrm{odd}}(H)=\chi(H)=3$. Our approach introduces a useful auxiliary parameter. Motivated by the notion of vertex-extendability of Liu, Mubayi, and Reiher \cite{liu2023unified}, we define the {\it vertex-extendable threshold} of $H$, denoted by $\delta_{\mathrm{ext}}(H,r)$, as the infimum of $c\in (0,1)$ so that for every $H$-free graph $G$ on $n$ vertices, the existence of a vertex $v \in V(G)$ with $\chi(G - v) \leq r$ combined with $\delta(G)\ge cn$ implies that $G$ is $r$-colorable. A key structural consequence is that $\delta_{\chi}(H,2) = \max\left\{\delta_\chi(C_{2k+1},2),\delta_{\mathrm{ext}}(H,2)\right\},$ where $H$ is a color-critical graph with $\chi(H)=3$ and $g_{\mathrm{odd}}(H)=2k+1$ for $k\geq 2$.