On coloring of graphs of girth 2l + 1 without longer odd holes

TL;DR

This paper investigates the coloring properties of graphs with specific girth and hole restrictions, proving bounds on chromatic number and characterizing certain 3-connected graphs without unstable 3-cutsets.

Contribution

It establishes new bipartiteness and coloring bounds for graphs in the family ${\

Findings

If certain subgraphs are bipartite, the entire graph is 4-colorable.

3-connected graphs without unstable 3-cutsets induce specific Petersen-related graphs.

Graphs avoiding certain induced subgraphs are 3-colorable.

Abstract

A hole is an induced cycle of length at least 4. Let $\l\ge 2$ be a positive integer, let ${\cal G}_l$ denote the family of graphs which have girth $2\l+1$ and have no holes of odd length at least $2\l+3$, and let $G\in {\cal G}_{\l}$. For a vertex $u\in V(G)$ and a nonempty set $S\subseteq V(G)$, let $d(u, S)=\min\{d(u, v):v\in S\}$, and let $L_i(S)=\{u\in V(G) \mbox{ and } d(u, S)=i\}$ for any integer $i\ge 0$. We show that if $G[S]$ is connected and $G[L_i(S)]$ is bipartite for each $i\in\{1, \ldots, \lfloor{\l\over 2}\rfloor\}$, then $G[L_i(S)]$ is bipartite for each $i>0$, and consequently $\chi(G)\le 4$, where $G[S]$ denotes the subgraph induced by $S$. Let $\theta^-$ be the graph obtained from the Petersen graph by deleting three vertices which induce a path, let $\theta^+$ be the graph obtained from the Petersen graph by deleting two adjacent vertices, and let $\theta$ be the graph obtained from $\theta^+$ by removing an edge incident with two vertices of degree 3. For a graph $G\in{\cal G}_2$, we show that if $G$ is 3-connected and has no unstable 3-cutset then $G$ must induce either $\theta$ or $\theta^-$ but does not induce $\theta^+$. As corollaries, $\chi(G)\le 3$ for every graph $G$ of ${\cal G}_2$ that induces neither $\theta$ nor $\theta^-$, and minimal non-3-colorable graphs of ${\cal G}_2$ induce no $\theta^+$.