Infinitely many periodic solutions for a class of second-order Hamiltonian systems

In this paper we study the existence of infinitely many periodic solutions for second-order Hamiltonian systems

$$left{ {begin{array}{*{20}c} {ddot u(t) + A(t)u(t) + nabla F(t,u(t)) = 0,} {u(0) - u(T) = dot u(0) - dot u(T) = 0,} end{array} } right.$$

, where F(t, u) is even in u, and ?F(t, u) is of sublinear growth at infinity and satisfies the Ahmad-Lazer-Paul condition.