Acyclic Edge Coloring of Triangle Free Planar Graphs

TL;DR

This paper proves that triangle-free planar graphs have an acyclic edge coloring using at most +3 colors, advancing understanding of acyclic chromatic indices for specific graph classes.

Contribution

It establishes an upper bound of +3 for the acyclic chromatic index of graphs satisfying Property A, including triangle-free planar graphs.

Findings

Triangle-free planar graphs satisfy Property A.

Acyclic chromatic index of such graphs is at most +3.

2-fold graphs also satisfy Property A.

Abstract

An $acyclic$ edge coloring of a graph is a proper edge coloring such that there are no bichromatic cycles. The \emph{acyclic chromatic index} of a graph is the minimum number k such that there is an acyclic edge coloring using k colors and is denoted by $a'(G)$. It was conjectured by Alon, Sudakov and Zaks (and much earlier by Fiamcik) that $a'(G)\le \Delta+2$, where $\Delta =\Delta(G)$ denotes the maximum degree of the graph. If every induced subgraph $H$ of $G$ satisfies the condition $\vert E(H) \vert \le 2\vert V(H) \vert -1$, we say that the graph $G$ satisfies $Property\ A$. In this paper, we prove that if $G$ satisfies $Property\ A$, then $a'(G)\le \Delta + 3$. Triangle free planar graphs satisfy $Property\ A$. We infer that $a'(G)\le \Delta + 3$, if $G$ is a triangle free planar graph. Another class of graph which satisfies $Property\ A$ is 2-fold graphs (union of two forests).