Chromatic number of randomly augmented graphs

TL;DR

This paper investigates the chromatic number of perturbed graphs formed by combining a deterministic host graph with a random graph, providing bounds and a coloring algorithm for certain conditions.

Contribution

It extends known results on random graphs to perturbed graphs, offering an asymptotic upper bound on the chromatic number and a constructive coloring method under specific host graph conditions.

Findings

Asymptotic upper bound for chromatic number of perturbed graphs.

Generalizes Bollobás's theorem to perturbed graph models.

Provides a constructive coloring algorithm for certain host graphs.

Abstract

An extension of the Erd\H{o}s-Renyi random graph model $G_{n,p}$ is the model of perturbed graphs introduced by Bohman, Frieze and Martin (Bohman, Frieze, Martin 2003). This is a special case of the model of randomly augmented graphs studied in this paper. An augmented graph denoted by $pert_{H,p}$ is the union of a deterministic host graph and a random graph $G_{n,p}$. Among the first problems in perturbed graphs has been the question how many random edges are needed to ensure Hamiltonicity of the graph. This question was answered in the paper by Bohman, Frieze and Martin. The host graph is often chosen to be a dense graph. In recent years several papers on combinatorial problems in perturbed graphs were published, e.g. on the emergence of powers of Hamiltonian cycles (Dudek, Reiher, Ruci\'nski, Schacht 2020), some positional games played on perturbed graphs (Clemens, Hamann, Mogge, Parczyk, 2020) and the behavior of multiple invariants e.g. fixed clique size (Bohman, Frieze, Krivelevich, Martin, 2004). In this paper we study the chromatic number of randomly augmented graphs. We concentrate on a host graph $H$ with chromatic number $o(n)$, augmented by a $G_{n,p}$ with $n^{-\frac{1}{3} + \delta}\leq p(n) \leq 1-\delta$ for some $\delta \in (0,1)$. Our main result is an upper bound for the chromatic number: we show that asymptotically almost surely $\chi(pert_{H,p}) \leq (1+o(1)) \cdot \frac{n \log(b)}{2 (\log(n) - \log(\chi(H))}$ where $b = (1-p)^{-1}$. This result collapses to the famous theorem of Bollob\'as (1988), when $H$ is the empty host graph, thus our result can be regarded as a generalization of the latter. Our proof is not constructive. Further, we give a constructive coloring algorithm, when the chromatic number of the host graph is at most $\frac{n}{\log(n)^{\alpha}},$ $\alpha>\frac{1}{2}.$