From $χ$- to $χ_p$-bounded classes

Abstract

$χ$-bounded classes are studied here in the context of star colorings and more generally $χ_p$-colorings. This leads to natural extensions of the notion of bounded expansion class and to structural characterization of these. In this paper we solve two conjectures related to star coloring boundedness. One of the conjectures is disproved and in fact we determine which weakening holds true. We give structural characterizations of (strong and weak) $χ_p$-bounded classes. On the way, we generalize a result of Wood relating the chromatic number of a graph to the star chromatic number of its $1$-subdivision. As an application of our characterizations, among other things, we show that for every odd integer $g>3$ even hole-free graphs $G$ contain at most $φ(g,ω(G))\,|G|$ holes of length $g$.