Improved bounds for acyclic coloring parameters

TL;DR

This paper presents improved bounds for acyclic coloring parameters of graphs, providing new upper limits for the acyclic chromatic number and index using randomized algorithms that reduce the number of colors needed.

Contribution

The paper introduces novel bounds for acyclic chromatic number and index, with algorithmic randomized methods that improve upon previous results by reducing color requirements.

Findings

Acyclic chromatic index is at most 2Δ-1.

Acyclic chromatic number is at most approximately (4^{-1/3} + ε)Δ^{4/3} + Δ + 1 for large Δ.

Algorithms avoid cycles with homochromatic pairs, reducing color usage.

Abstract

The {\em acyclic chromatic number} of a graph is the least number of colors needed to properly color its vertices so that none of its cycles has only two colors. The {\em acyclic chromatic index} is the analogous graph parameter for edge colorings. We first show that the acyclic chromatic index is at most $2\Delta-1$, where $\Delta$ is the maximum degree of the graph. We then show that for all $\epsilon >0$ and for $\Delta$ large enough (depending on $\epsilon$), the acyclic chromatic number of the graph is at most $\lceil(4^{-1/3} +\epsilon) {\Delta}^{4/3} \rceil +\Delta+ 1$. Both results improve long chains of previous successive advances. Both are algorithmic, in the sense that the colorings are generated by randomized algorithms. Previous randomized algorithms assume the availability of enough colors to guarantee properness deterministically and use additional colors in dealing with the bichromatic cycles in a randomized fashion. In contrast, our algorithm initially generates colorings that are not necessarily proper; it only aims at avoiding cycles where all pairs of edges, or vertices, that are one edge, or vertex, apart in a traversal of the cycle are homochromatic (of the same color). When this goal is reached, the algorithm checks for properness and if necessary it repeats until properness is attained. Thus savings in the number of colors is attained.