Infinitely many homoclinic solutions for second order Hamiltonian systems

In this paper we study the existence of infinitely many homoclinic solutions for second order Hamiltonian systems ü−L(t)u+Wu(t,u)=0, ∀t∈R, where L(t)L(t) is unnecessarily positive definite for all t∈Rt∈R, and W(t,u)W(t,u) is of subquadratic growth as |u|→∞|u|→∞.