Mathematics – Combinatorics
Scientific paper
2009-09-18
Combinatorics, Probability and Computing, 19 (2010), No. 2, 315-320
Mathematics
Combinatorics
5 pages. Accepted by Combin. Probab. Comput
Scientific paper
10.1017/S0963548309990538
We show that the number of independent sets in an N-vertex, d-regular graph is at most (2^{d+1} - 1)^{N/2d}, where the bound is sharp for a disjoint union of complete d-regular bipartite graphs. This settles a conjecture of Alon in 1991 and Kahn in 2001. Kahn proved the bound when the graph is assumed to be bipartite. We give a short proof that reduces the general case to the bipartite case. Our method also works for a weighted generalization, i.e., an upper bound for the independence polynomial of a regular graph.
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