Circular colouring and algebraic no-homomorphism theorems

In this paper, we apply some new algebraic no-homomorphism theorems in conjuction with some new chromatic parameters to estimate the circular chromatic number of graphs. To show the applicability of the general results, as a couple of examples, we generalize a well known inequality for the fractional chromatic number of graphs and we also show that the circular chromatic number of the graph obtained from the Petersen graph by excluding one vertex is equal to 3. Also, we focus on the Johnson-Holroyd-Stahl conjecture about the circular chromatic number of Kneser graphs and we propose in approach to this conjecture. In this regard, we introduce a new related conjecture on Kneser graphs and we also provide some supporting evidence. (C) 2006 Elsevier Ltd. All rights reserved.