Average Degree in Graph Powers

Abstract

The kth power of a simple graph G, denoted G^k, is the graph with vertex set V(G) where two vertices are adjacent if they are within distance k in G. We are interested in finding lower bounds on the average degree of G^k. Here we prove that if G is connected with minimum degree d > 2 and |V(G)| > (8/3)d, then G^4 has average degree at least (7/3)d. We also prove that if G is a connected d-regular graph on n vertices with diameter at least 3k+3, then the average degree of G^{3k+2} is at least (2k+1)(d+1) - k(k+1) (d+1)^2/n - 1. Both of these results are shown to be essentially best possible; the second is best possible even when n/d is arbitrarily large.