Abstract:A proper vertex coloring of a graph G is achromatic (respectively harmonious) if every two colors appear together on at least one (resp. at most one) edge. The largest (resp. the smallest) number of colors in an achromatic (resp. a harmonious) coloring of G is called the achromatic (resp. harmonious chromatic) number of G and denoted by ψ (G) (resp. h(G)). For a finite set of positive integers D and a positive integer n, a circulant graph, denoted by C D n , is an undirected graph on the set of vertices {0, 1, . . . , n − 1} that has an edge i j if and only if either i − j or j − i is a member of D (where substraction is computed modulo n). For any fixed set D, we show that ψ (C D n ) is asymptotically equal to √ 2 |D| n, with the error term O(log n). We also prove that h(C D n ) is asymptotically equal to
ACHROMATIC AND HARMONIOUS COLORINGS OF CIRCULANT GRAPHS 19√ 2 |D| n, with the error term O(n 1 4 log n). As corollaries, we get results that improve, for a fixed k, the previously best estimations on the lengths of a shortest k-radius sequence over an n-ary alphabet (i.e., a sequence in which any two distinct elements of the alphabet occur within distance k of each other) and a longest packing k-radius sequence over an n-ary alphabet (which is a dual counterpart of a k-radius sequence). C 2017 Wiley Periodicals, Inc.