On Modular Edge Colourings of Graphs

TL;DR

This paper improves bounds on the minimum number of colours needed for modular edge colourings of graphs, showing it can be linearly bounded in terms of the modulus with a small additive function, advancing understanding of graph colourings.

Contribution

The paper significantly reduces the multiplicative constant in bounds for modular edge colourings, establishing a near-linear bound with a small additive function, and introduces new bounds for d-degenerate graphs.

Findings

Bound $oldsymbol{oldsymbol{ ext{chi}}'_k(G)}$ by $oldsymbol{k + O(d)}$ for $d$-degenerate graphs.

Established bounds $oldsymbol{oldsymbol{ ext{chi}}'_k(G)} o 7k$ or $9k$ depending on parity of $k$.

Improved previous bounds from $198k - 101$ to $9k + f(k)$ with $f o 0$ as $k o ext{large}$.

Abstract

Given a graph $G$ and an integer $k\geq 2$, let $\chi'_k(G)$ denote the minimum number of colours required to colour the edges of $G$ such that, in each colour class, the subgraph induced by the edges of that colour has all non-zero degrees congruent to $1$ modulo $k$. In 1992, Pyber proved that $\chi'_2(G) \leq 4$ for every graph $G$, and posed the question of whether $\chi'_k(G)$ can be bounded solely in terms of $k$ for every $k\geq 3$. This question was answered in 1997 by Scott, who showed that $\chi'_k(G)\leq5k^2\log k$, and further asked whether $\chi'_k(G) = O(k)$. Recently, Botler, Colucci, and Kohayakawa (2023) answered Scott's question affirmatively proving that $\chi'_k(G) \leq 198k - 101$, and conjectured that the multiplicative constant could be reduced to $1$. A step towards this latter conjecture was made in 2024 by Nweit and Yang, who improved the bound to $\chi'_k(G) \leq 177k - 93$. In this paper, we further improve the multiplicative constant to $9$. More specifically, we prove that there is a function $f\in o(k)$ for which $\chi'_k(G) \leq 7k + f(k)$ if $k$ is odd, and $\chi'_k(G) \leq 9k + f(k)$ if $k$ is even. In doing so, we prove that $\chi'_k(G) \leq k + O(d)$ for every $d$-degenerate graph $G$, which plays a central role in our proof.