The circular altitude of a graph
  In this paper we investigate a parameter of graphs, called the circular altitude, introduced by Peter Cameron. We show that the circular altitude provides a lower bound on the circular chromatic number, and hence on the chromatic number, of a graph and investigate this parameter for the iterated Mycielskian of certain graphs. 
John Bamberg, Brian Corr, Alice Devillers, Daniel Hawtin, Irene   Pivotto, Eric Swartz
math.CO