Obstructions for homomorphisms to odd cycles in series-parallel graphs

Abstract

For a graph $H$, an $H$-colouring of a graph $G$ is a vertex map $\phi:V(G) \to V(H)$ such that adjacent vertices are mapped to adjacent vertices. A graph $G$ is $C_{2k+1}$-critical if $G$ has no $C_{2k+1}$-colouring but every proper subgraph of $G$ has a $C_{2k+1}$-colouring. We prove a structural characterisation of $C_{2k+1}$-critical graphs when $k \geq 2$. In the case that $k = 2$, we use the aforementioned charazterisation to show a $C_3$-free series-parallel graph $G$ has a $C_5$-colouring if either $G$ has neither $C_8$ nor $C_{10}$, or $G$ has no two $5$-cycles sharing a vertex.