Speaker:   Frantisek Franek
  Department of Computing and Software
  McMaster University


Title: Erdös' conjecture on multiplicities of complete subgraphs for nearly quasirandom graphs

Let kt(G) denote the number of cliques of order t in the graph G and let  ct(G) be the ratio of (kt(G) + kt(G)) and the value C(t,|G|), where G denotes the complement graph of G and C(a,b) denotes the quantity a choose b. In simple terms, it is the ratio of t-cliques and cocliques and the number of t-subsets of G. Finally, let ct(n) = min{ ct(G) : |G| = n } and let ct be the limit of ct(n) for n approaching infinity.

An old conjecture of Erdös related to Ramsey's theorem states that ct =21-C(t,2). It was shown false by A. Thomason. It has been known that ct(G) must be approximatelly equal to ct whenever G is a pseudorandom graph. The aim of this talk is to show that c4(G)c4 if G is a graph arising from a pseudorandom one by a small perturbation. Joint research with. V. Rödl.