Published online by Cambridge University Press: 05 August 2019
The Hamming graph H(d, n) is the Cartesian product of d complete graphs on n vertices. Let ${m=d(n-1)}$ be the degree and
$V = n^d$ be the number of vertices of H(d, n). Let
$p_c^{(d)}$ be the critical point for bond percolation on H(d, n). We show that, for
$d \in \mathbb{N}$ fixed and
$n \to \infty$,
$$p_c^{(d)} = {1 \over m} + {{2{d^2} - 1} \over {2{{(d - 1)}^2}}}{1 \over {{m^2}}} + O({m^{ - 3}}) + O({m^{ - 1}}{V^{ - 1/3}}),$$
$O(m^{-1}V^{-1/3})$ is the width of the critical window. For
$d=4,5,6$ we have
$m^{-3} =
O(m^{-1}V^{-1/3})$, and so the above formula represents the full asymptotic expansion of
$p_c^{(d)}$. In [16] we show that this formula is a crucial ingredient in the study of critical bond percolation on H(d, n) for
$d=2,3,4$. The proof uses a lace expansion for the upper bound and a novel comparison with a branching random walk for the lower bound. The proof of the lower bound also yields a refined asymptotics for the susceptibility of a subcritical Erdös–Rényi random graph.