On the average size of independent sets in triangle-free graphs

TL;DR

This paper establishes tight bounds on the average size and total number of independent sets in triangle-free graphs with bounded degree, using the hard-core model, with implications for Ramsey theory.

Contribution

It provides the first asymptotically tight bounds on the expected size and count of independent sets in triangle-free graphs, connecting statistical physics models to combinatorial graph properties.

Findings

Expected size of a random independent set is at least $(1+o_d(1)) \frac{\log d}{d}n$.

Total number of independent sets is at least $\exp[(\frac{1}{2}+o_d(1)) \frac{\log^2 d}{d}n$.

Results are tight, exemplified by random $d$-regular graphs.

Abstract

We prove an asymptotically tight lower bound on the average size of independent sets in a triangle-free graph on $n$ vertices with maximum degree $d$, showing that an independent set drawn uniformly at random from such a graph has expected size at least $(1+o_d(1)) \frac{\log d}{d}n$. This gives an alternative proof of Shearer's upper bound on the Ramsey number $R(3,k)$. We then prove that the total number of independent sets in a triangle-free graph with maximum degree $d$ is at least $\exp \left[\left(\frac{1}{2}+o_d(1) \right) \frac{\log^2 d}{d}n \right]$. The constant $1/2$ in the exponent is best possible. In both cases, tightness is exhibited by a random $d$-regular graph. Both results come from considering the hard-core model from statistical physics: a random independent set $I$ drawn from a graph with probability proportional to $\lambda^{|I|}$, for a fugacity parameter $\lambda>0$. We prove a general lower bound on the occupancy fraction (normalized expected size of the random independent set) of the hard-core model on triangle-free graphs of maximum degree $d$. The bound is asymptotically tight in $d$ for all $\lambda =O_d(1)$. We conclude by stating several conjectures on the relationship between the average and maximum size of an independent set in a triangle-free graph and give some consequences of these conjectures in Ramsey theory.