Libor Barto, Marcin Kozik, Todd Niven. Graphs, polymorphisms and the complexity of homomorphism problems. In Richard E. Ladner, Cynthia Dwork, editors, Proceedings of the 40th Annual ACM Symposium on Theory of Computing, Victoria, British Columbia, Canada, May 17-20, 2008. pages 789-796, ACM, 2008. [doi]
Abstract is missing.