Graphs with large palette index

TL;DR

This paper introduces a new approach to studying the palette index of graphs, providing conditions for high palette index and constructing families of graphs with maximal and quadratic growth palette indices.

Contribution

It offers a sufficient condition for a graph to have a large palette index and constructs new graph families with maximal and quadratic palette index growth.

Findings

Constructed r-regular graphs with maximum palette index for odd r

First known simple graphs with quadratic palette index growth

Provided a new sufficient condition for high palette index

Abstract

Given an edge-coloring of a graph, the palette of a vertex is defined as the set of colors of the edges which are incident with it. We define the palette index of a graph as the minimum number of distinct palettes, taken over all edge-colorings, occurring among the vertices of the graph. Several results about the palette index of some specific classes of graphs are known. In this paper we propose a different approach that leads to new and more general results on the palette index. Our main theorem gives a sufficient condition for a graph to have palette index larger than its minimum degree. In the second part of the paper, by using such a result, we answer to two open problems on this topic. First, for every $r$ odd, we construct a family of $r$-regular graphs with palette index reaching the maximum admissible value. After that, we construct the first known family of simple graphs whose palette index grows quadratically with respect to their maximum degree.