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In this paper, we study the existence and multiplicity of periodic solutions of the following second-order Hamiltonian systems ¨ x(t) + V ′ (t, x(t)) = 0, where t ∈ R, x ∈ R N and V ∈ C 1 (R × R N , R). By using a symmetric mountain pass theorem, we obtain a new criterion to guarantee that second-order Hamiltonian systems has infinitely many periodic(More)