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Homogeneous sets, clique-separators, critical graphs, and optimal $χ$-binding functions

Given a set $\mathcal{H}$ of graphs, let $f_\mathcal{H}^\star\colon \mathbb{N}_{>0}\to \mathbb{N}_{>0}$ be the optimal $χ$-binding function of the class of $\mathcal{H}$-free graphs, that is, $$f_\mathcal{H}^\star(ω)=\max\{χ(G): G\text{ is } \mathcal{H}\text{-free, } ω(G)=ω\}.$$ In this paper, we combine the two decomposition methods by homogeneous sets and clique-separators in order to determine optimal $χ$-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 $ω\geq 1$: (i) $\ f_{\{P_5,banner\}}^\star(ω)=f_{3K_1}^\star(ω)\in Θ(ω^2/\log(ω)),$ (ii) $\ f_{\{P_5,co-banner\}}^\star(ω)=f^\star_{\{2K_2\}}(ω)\in\mathcal{O}(ω^2),$ (iii) $\ f_{\{C_5,C_7,\ldots,banner\}}^\star(ω)=f^\star_{\{C_5,3K_1\}}(ω)\notin \mathcal{O}(ω),$ and (iv) $\ f_{\{P_5,C_4\}}^\star(ω)=\lceil(5ω-1)/4\rceil.$ We also characterise, for each of our considered graph classes, all graphs $G$ with $χ(G)>χ(G-u)$ 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.