A graph for which the inertia bound is not tight

Abstract

The inertia bound gives an upper bound on the independence number of a graph by considering the inertia of matrices corresponding to the graph. The bound is known to be tight for graphs on 10 or fewer vertices as well as for all perfect graphs. The question has been asked as to whether the bound is always tight. We show that the bound is not tight for the Paley graph on 17 vertices as well as for the graph obtained from Paley 17 by deleting a vertex.