Multiple periodic solutions for superquadratic second-order discrete Hamiltonian systems

Some multiplicity results are obtained for periodic solutions of the nonautonomous superquadratic second-order discrete Hamiltonian systemsΔ2u(t-1)+∇F(t,u(t))=0∀t∈Zby using critical point theory, especially, a three critical points theorem proposed by Brezis and Nirenberg.