Strong coloring 2-regular graphs: Cycle restrictions and partial colorings

Abstract

Let $H$ be a graph with $\Delta(H) \leq 2$, and let $G$ be obtained from $H$ by gluing in vertex-disjoint copies of $K_4$. We prove that if $H$ contains at most one odd cycle of length exceeding $3$, or if $H$ contains at most $3$ triangles, then $\chi(G) \leq 4$. This proves the Strong Coloring Conjecture for such graphs $H$. For graphs $H$ with $\Delta=2$ that are not covered by our theorem, we prove an approximation result towards the conjecture.