 
Summary: Graphs and Combinatorics 7, 16 (1991)
Graphsand
Combinatorics
© SpringerVerlag 1991
Ramsey GraphsContainManyDistinct
InducedSubgraphs
N. Alon1 and A. Hajnal2
1 Bellcore,Morristown, NJ 07960,USA and Department ofMathematics, SacklerFacultyof
Exact Sciences,Tel AvivUniversity,Tel Aviv,Israel
2 Mathematical Institute ofthe Hungarian AcademyofSciences,Budapest,Hungary
Abstract.It is shown that anygraph on n verticescontainingno cliqueand no independent set on
t + 1verticeshas at least
2./( 2r2o~2t))
distinct induced subgraphs.
1. Introduction
All graphs considered here are finite, simple and undirected. G. always denotes a
graph on n vertices. For a graph G, let i(G) denote the total number of isomorphism
types of induced subgra_phs of G. We call i(G) the isomorphism number of G. Note
that i(G) = i(G), where G is the complement of G and that if G. has n vertices then
i(G.) > n, as G. contains an induced subgraph on m vertices for each I < m < n. An
