On coloring of graphs of girth 2l + 1 without longer odd holes
A hole is an induced cycle of length at least 4. Let $ł\ge 2$ be a positive integer, let ${\cal G}_l$ denote the family of graphs which have girth $2ł+1$ and have no holes of odd length at least $2ł+3$, and let $G\in {\cal G}_ł$. 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 $i\in\{1, \ldots, \lfloor{ł\over 2}\rfloor\}$, then $G[L_i(S)]$ is bipartite for each $i>0$, and consequently $χ(G)\le 4$, where $G[S]$ denotes the subgraph induced by $S$. Let $θ^-$ be the graph obtained from the Petersen graph by deleting three vertices which induce a path, let $θ^+$ be the graph obtained from the Petersen graph by deleting two adjacent vertices, and let $θ$ be the graph obtained from $θ^+$ 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 $θ$ or $θ^-$ but does not induce $θ^+$. As corollaries, $χ(G)\le 3$ for every graph $G$ of ${\cal G}_2$ that induces neither $θ$ nor $θ^-$, and minimal non-3-colorable graphs of ${\cal G}_2$ induce no $θ^+$.