Simple vertex coloring algorithms

TL;DR

This paper presents a simple, nearly optimal classical algorithm for $(1 + \\epsilon)\\Delta$-coloring of graphs and introduces a quantum algorithm that surpasses classical query bounds, along with a quantum lower bound for graph coloring.

Contribution

It introduces a simple algorithm for near-optimal graph coloring and a quantum algorithm that outperforms classical query complexity bounds.

Findings

Classical algorithm matches best existing query complexity.

Quantum algorithm reduces query complexity below classical lower bounds.

Quantum lower bound of (n) for O() coloring.

Abstract

Given a graph $G$ with $n$ vertices and maximum degree $\Delta$, it is known that $G$ admits a vertex coloring with $\Delta + 1$ colors such that no edge of $G$ is monochromatic. This can be seen constructively by a simple greedy algorithm, which runs in time $O(n\Delta)$. Very recently, a sequence of results (e.g., [Assadi et. al. SODA'19, Bera et. al. ICALP'20, Alon Assadi Approx/Random'20]) show randomized algorithms for $(\epsilon + 1)\Delta$-coloring in the query model making $\tilde{O}(n\sqrt{n})$ queries, improving over the greedy strategy on dense graphs. In addition, a lower bound of $\Omega(n\sqrt n)$ for any $O(\Delta)$-coloring is established on general graphs. In this work, we give a simple algorithm for $(1 + \epsilon)\Delta$-coloring. This algorithm makes $O(\epsilon^{-1/2}n\sqrt{n})$ queries, which matches the best existing algorithms as well as the classical lower bound for sufficiently large $\epsilon$. Additionally, it can be readily adapted to a quantum query algorithm making $\tilde{O}(\epsilon^{-1}n^{4/3})$ queries, bypassing the classical lower bound. Complementary to these algorithmic results, we show a quantum lower bound of $\Omega(n)$ for $O(\Delta)$-coloring.