## Mathematics, Department of

#### Date of this Version

2011

#### Citation

the electronic journal of combinatorics 18

#### Abstract

The study of the number of independent sets in a graph has a rich history. Recently, Kahn proved that disjoint unions of *Kr,r’s* have the maximum number of independent sets amongst *r*-regular bipartite graphs. Zhao extended this to all *r*-regular graphs. If we instead restrict the class of graphs to those on a fixed number of vertices and edges, then the Kruskal-Katona theorem implies that the graph with the maximum number of independent sets is the lex graph, where edges form an initial segment of the lexicographic ordering. In this paper, we study three related questions. Firstly, we prove that the lex graph has the maximum number of weighted independent sets for any appropriate weighting. Secondly, we solve the problem of maximizing the number of independents sets in graphs with specified independence number or clique number. Finally, for *m* ≤ *n*, we find the graphs with the minimum number of independent sets for graphs with *n* vertices and *m* edges.