Forbidden subgraph colorings and the oriented chromatic number

Abstract

We present an improved upper bound of O(d1+1m−1) for the (2,F)-subgraph chromatic number χ2,F(G) of any graph G of maximum degree d. Here, m denotes the minimum number of edges in any member of F. This bound is tight up to a (logd)1/(m−1) multiplicative factor and improves the previous bound presented in Aravind and Subramanian (2011) [5]. Along the way, we state and prove an easy-to-use version of the Lovász Local Lemma.We also obtain a relationship connecting the oriented chromatic number χo(G) of graphs and the (j,F)-subgraph chromatic numbers χj,F(G) introduced and studied in Aravind and Subramanian (2011) [5]. In particular, we relate the oriented chromatic number and the (2,r)-treewidth chromatic number and show that χo(G)≤k((r+1)2r)k−1 for any graph G having (2,r)-treewidth chromatic number at most k. The latter parameter is the least number of colors in any proper vertex coloring which is such that the subgraph induced by the union of any two color classes has treewidth at most r.We also generalize a result of Alon et al. (1996) [3] on acyclic chromatic number of graphs on surfaces (with characteristic −γ≤0) to (2,F)-subgraph chromatic numbers and prove that χ2,F(G)=O(γm/(2m−1)) for some constant m depending only on F. We also show that this bound is nearly tight. We then use this result to show that graphs of genus g have oriented chromatic number at most 2O(g1/2+ϵ) for every fixed ϵ>0. We also refine the proof of a bound on χo(G) obtained by Kostochka et al. (1997) in [10] to obtain an improved bound on χo(G).