On the spread of random graphs

Abstract

The spread of a connected graph G was introduced by Alon, Boppana and Spencer (1998) and measures how tightly connected the graph is. It is defined as the maximum over all Lipschitz functions f on V(G) of the variance of f(X) when X is uniformly distributed on V(G). We investigate the spread for certain models of sparse random graph; in particular for random regular graphs G(n,d), for Erd\H{o}s-R\'enyi random graphs G_{n,p} in the supercritical range p>1/n, and for a 'small world' model. For supercritical G_{n,p}, we show that if p=c/n with c>1 fixed then with high probability the spread of the giant component is bounded, and we prove corresponding statements for other models of random graphs, including a model with random edge-lengths. We also give lower bounds on the spread for the barely supercritical case when p=(1+o(1))/n. Further, we show that for d large, with high probability the spread of G(n,d) becomes arbitrarily close to that of the complete graph K_n.