On the adaptable chromatic number of graphs

The adaptable chromatic number of a graph G is the smallest integer k such that for any edge k-colouring of G there exists a vertex kcolouring of G in which the same colour never appears on an edge and both its endpoints. (Neither the edge nor the vertex colourings are necessarily proper in the usual sense.) We give an efficient characterization of graphs with adaptable chromatic number at most two, and prove that it is NP-hard to decide if a given graph has adaptable chromatic number at most k, for any k ≥ 3. The adaptable chromatic number cannot exceed the chromatic number; for complete graphs, the adaptable chromatic number seems to be near the square root of the chromatic number. On the other hand, there are graphs of arbitrarily high girth and chromatic number, in which the adaptable chromatic number coincides with the classical chromatic ∗Supported by NSERC †Supported in part by the National Science Council under grant NSC95-2115-M-110013-MY3