Incidence coloring of graphs with high maximum average degree

TL;DR

This paper investigates bounds on the number of colors needed for incidence coloring of graphs with high maximum average degree, extending previous results to new degree and average degree ranges.

Contribution

It provides new bounds for incidence coloring of graphs with maximum average degree between 4 and 6, including specific results for graphs with maximum degree at least 7.

Findings

Graphs with max average degree less than 4 and max degree at least 7 are incidence (Δ+3)-colorable.

Graphs with max average degree less than 6 are incidence (Δ+7)-colorable.

General bounds are established for graphs with max average degree less than k, requiring Δ+k-1 colors.

Abstract

An incidence of an undirected graph G is a pair $(v,e)$ where $v$ is a vertex of $G$ and $e$ an edge of $G$ incident with $v$. Two incidences $(v,e)$ and $(w,f)$ are adjacent if one of the following holds: (i) $v = w$, (ii) $e = f$ or (iii) $vw = e$ or $f$. An incidence coloring of $G$ assigns a color to each incidence of $G$ in such a way that adjacent incidences get distinct colors. In 2005, Hosseini Dolama \emph{et al.}~\citep{ds05} proved that every graph with maximum average degree strictly less than $3$ can be incidence colored with $\Delta+3$ colors. Recently, Bonamy \emph{et al.}~\citep{Bonamy} proved that every graph with maximum degree at least $4$ and with maximum average degree strictly less than $\frac{7}{3}$ admits an incidence $(\Delta+1)$-coloring. In this paper we give bounds for the number of colors needed to color graphs having maximum average degrees bounded by different values between $4$ and $6$. In particular we prove that every graph with maximum degree at least $7$ and with maximum average degree less than $4$ admits an incidence $(\Delta+3)$-coloring. This result implies that every triangle-free planar graph with maximum degree at least $7$ is incidence $(\Delta+3)$-colorable. We also prove that every graph with maximum average degree less than 6 admits an incidence $(\Delta + 7)$-coloring. More generally, we prove that $\Delta+k-1$ colors are enough when the maximum average degree is less than $k$ and the maximum degree is sufficiently large.