Homogeneous sets, clique-separators, critical graphs, and optimal $χ$-binding functions (2005.02250v3)

Abstract: Given a set $\mathcal{H}$ of graphs, let $f_\mathcal{H}^\star\colon \mathbb{N}{>0}\to \mathbb{N}{>0}$ be the optimal $\chi$-binding function of the class of $\mathcal{H}$-free graphs, that is, $$f_\mathcal{H}^\star(\omega)=\max{\chi(G): G\text{ is } \mathcal{H}\text{-free, } \omega(G)=\omega}.$$ In this paper, we combine the two decomposition methods by homogeneous sets and clique-separators in order to determine optimal $\chi$-binding functions for subclasses of $P_5$-free graphs and of $(C_5,C_7,\ldots)$-free graphs. In particular, we prove the following for each $\omega\geq 1$: (i) $\ f_{{P_5,banner}}^\star(\omega)=f_{3K_1}^\star(\omega)\in \Theta(\omega^2/\log(\omega)),$ (ii) $\ f_{{P_5,co-banner}}^\star(\omega)=f^\star_{{2K_2}}(\omega)\in\mathcal{O}(\omega^2),$ (iii) $\ f_{{C_5,C_7,\ldots,banner}}^\star(\omega)=f^\star_{{C_5,3K_1}}(\omega)\notin \mathcal{O}(\omega),$ and (iv) $\ f_{{P_5,C_4}}^\star(\omega)=\lceil(5\omega-1)/4\rceil.$ We also characterise, for each of our considered graph classes, all graphs $G$ with $\chi(G)>\chi(G-u)$ for each $u\in V(G)$. From these structural results, we can prove Reed's conjecture -- relating chromatic number, clique number, and maximum degree of a graph -- for $(P_5,banner)$-free graphs.