Based on the algorithmic proof of Lovász local lemma due to Moser and Tardos, the works of Grytczuk et al. on words, and Dujmović et al. on colorings, Esperet and Parreau developed a framework to prove upper bounds for several chromatic numbers (in particular acyclic chromatic index, star chromatic number and Thue chromatic number) using the so-called entropy compression method.Inspired by this work, we propose a more general framework and a better analysis. This leads to improved upper bounds on chromatic numbers and indices. In particular, every graph with maximum degree ∆ has an acyclic chromatic number at most 3 2 ∆ 4 3 + O(∆). Also every planar graph with maximum degree ∆ has a facial Thue choice number at most ∆ + O(∆ 1 2 ) and facial Thue choice index at most 10. * This research is partially supported by the ANR EGOS, under contract ANR-12-JS02-002-01.
The algorithmLet V ∈ {1, 2, . . . , κ} t be a vector of length t, for some arbitrarily large t ≫ n = |V (G)|. Algorithm ACYCLICCOLORINGGAMMA_G (see below) takes the vector V as input and returns a partial acyclic coloring ϕ : V (G) → {•, 1, 2, . . . , κ} of G (• means that the vertex is uncolored) and a text file R that is called a record in the remaining of the paper. The acyclic coloring ϕ is necessarily partial since we try to color G with a number of colors less than its acyclic chromatic number. For a given vertex v of G, we denote by N (v) the set of neighbors of v.Algorithm 1: ACYCLICCOLORINGGAMMA_G Input : V (vector of length t).Output: (ϕ, R).Write "Uncolor, neighbor u \n" in R 11 else if v belongs to a bicolored cycle of length 2k (k ≥ 2), say (v = u 1 , . . . , u 2k ) then // Bicolored cycle issue 12 for j ← 1 to 2k − 2 do 13 ϕ(u j ) ← • 14 Write "Uncolor, 2k-cycle (v = u 1 , . . . , u 2k ) \n" in R 15 return (ϕ, R)