Let Z(ps) be the residue class ring of integers modulo p(s), where p is a prime number and s is a positive integer. The Grassmann graph over Z(ps), denoted by G(n, m, p(s)), has the vertex set all m-subspaces of Z(ps)(n) (n > m >= 1), and two vertices are adjacent if and only if their intersection is of dimension m - 1. We characterize the automorphisms of G(n, m, p(s)) as follows. Let n >= 2m >= 4 and let phi is an element of Aut(G(n, m, p(s))). Then either phi(X) = XU for all X is an element of V(G(n, m, p(s))), or n = 2m and phi(X)= (XU)(perpendicular to) for all X is an element of V(G(2m, ...
