Coloring some $(P_6,C_4)$-free graphs with $\Delta-1$ colors

TL;DR

This paper proves the Borodin-Kostochka Conjecture for specific classes of graphs that do not contain certain paths, cycles, and subgraphs, extending previous results in graph coloring theory.

Contribution

It establishes the conjecture for ($P_6,C_4,H$)-free graphs where $H$ is either $K_7$ or $C_5^+$, broadening the scope of known cases.

Findings

Borodin-Kostochka Conjecture holds for ($P_6,C_4,K_7$)-free graphs.

Borodin-Kostochka Conjecture holds for ($P_6,C_4,C_5^+$)-free graphs.

Generalizes previous results by Gupta and Pradhan.

Abstract

The Borodin-Kostochka Conjecture states that for a graph $G$, if $\Delta(G)\geq9$, then $\chi(G)\leq\max\{\Delta(G)-1,\omega(G)\}$. We use $P_t$ and $C_t$ to denote a path and a cycle on $t$ vertices, respectively. Let $C=v_1v_2v_3v_4v_5v_1$ be an induced $C_5$. A {\em $C_5^+$} is a graph obtained from $C$ by adding a $C_3=xyzx$ and a $P_2=t_1t_2$ such that (1) $x$ and $y$ are both exactly adjacent to $v_1,v_2,v_3$ in $V(C)$, $z$ is exactly adjacent to $v_2$ in $V(C)$, $t_1$ is exactly adjacent to $v_4,v_5$ in $V(C)$ and $t_2$ is exactly adjacent to $v_1,v_4,v_5$ in $V(C)$, (2) $t_1$ is exactly adjacent to $z$ in $\{x,y,z\}$ and $t_2$ has no neighbors in $\{x,y,z\}$. In this paper, we show that the Borodin-Kostochka Conjecture holds for ($P_6,C_4,H$)-free graphs, where $H\in \{K_7,C_5^+\}$. This generalizes some results of Gupta and Pradhan in \cite{GP21,GP24}.