Sharp uppper and lower bounds on the number of spanning trees in Cartesian product graphs

Let G1 and G2 be simple graphs and let n1 = |V (G1)|, m1 = |E(G1)|, n2 = |V (G2)| and m2 = |E(G2)|. In this paper we derive sharp upper and lower bounds for the number of spanning trees τ in the Cartesian product G1 G2 of G1 and G2. We show that: τ(G1 G2) ≥ 2(n1−1)(n2−1) n1n2 (τ(G1)n1) n2+1 2 (τ(G2)n2) n1+1 2 and τ(G1 G2) ≤ τ(G1)τ(G2) [ 2m1 n1 − 1 + 2m2 n2 − 1 ](n1−1)(n2−1) . We also characterize the graphs for which equality holds. As a by-product we derive a formula for the number of spanning trees in Kn1 Kn2 which turns out to be nn1−2 1 n n2−2 2 (n1 + n2) (n1−1)(n2−1).