On the clique number of integral circulant graphs

The concept of gcd-graphs is introduced by Klotz and Sander; they arise as a generalization of unitary Cayley graphs. The gcd-graph X(n)(d(1),..., d(k)) has vertices 0, 1,..., n - 1, and two vertices x and y are adjacent iff gcd(x - y, n) is an element of D = {d(1), d(2),..., d(k)}. These graphs are exactly the same as circulant graphs with integral eigenvalues characterized by So. In this work we deal with the clique number of integral circulant graphs and investigate the conjecture proposed in [W. Klotz, T. Sander, Some properties of unitary Cayley graphs, The Electronic journal of Combinatorics 14 (2007) #R45] that the clique number divides the number of vertices in the graph X(n)(D). We completely solve the problem of finding the clique number for integral circulant graphs with exactly one and two divisors. For k >= 3, we construct a family of counterexamples and disprove the conjecture in this case. (C) 2009 Elsevier Ltd. All rights reserved.