Distance graphs with missing multiples in the distance sets

Given positive integers m, k, and s with m > ks, let D_m,k,s represent the set {1, 2, …, m} − {k, 2k, …, sk}. The distance graph G(Z, D_m,k,s) has as vertex set all integers Z and edges connecting i and j whenever |i − j| ∈ D_m,k,s. The chromatic number and the fractional chromatic number of G(Z, D_m,k,s) are denoted by χ(Z, D_m,k,s) and χf(Z, D_m,k,s), respectively. For s = 1, χ(Z, D_m,k,1) was studied by Eggleton, Erdős, and Skilton 6], Kemnitz and Kolberg 12], and Liu 13], and was solved lately by Chang, Liu, and Zhu 2] who also determined χf(Z, D_m,k,1) for any m and k. This article extends the study of χ(Z, D_m,k,s) and χf(Z, D_m,k,s) to general values of s. We prove χf(Z, D_m,k,s) = χ(Z, D_m,k,s) = k if m < (s + 1)k; and χf(Z, D_m,k,s) = (m + sk + 1)/(s + 1) otherwise. The latter result provides a good lower bound for χ(Z, D_m,k,s). A general upper bound for χ(Z, D_m,k,s) is obtained. We prove the upper bound can be improved to ⌈(m + sk + 1)/(s + 1)⌉ + 1 for some values of m, k, and s. In particular, when s + 1 is prime, χ(Z, D_m,k,s) is either ⌈(m + sk + 1)/(s + 1)⌉ or ⌈(m + sk + 1)/(s + 1)⌉ + 1. By using a special coloring method called the precoloring method, many distance graphs G(Z, D_m,k,s) are classified into these two possible values of χ(Z, D_m,k,s). Moreover, complete solutions of χ(Z, D_m,k,s) for several families are determined including the case s = 1 (solved in 2]), the case s = 2, the case (k, s + 1) = 1, and the case that k is a power of a prime. © 1999 John Wiley & Sons, Inc. J Graph Theory 30: 245–259, 1999