Graphs with least eigenvalue -2 attaining a convex quadratic upper bound for the stability number

Abstract

In this paper we study the conditions under which the stability number of line graphs, generalized line graphs and exceptional graphs attains a convex quadratic programming upper bound. In regular graphs this bound is reduced to the well known Hoffman bound. Some vertex subsets inducing sub graphs with regularity properties are analyzed. Based on an observation concerning the Hoffman bound a new construction of regular exceptional graphs is provided. AMS Mathematics Subject Classification (2000): 05C50.