S. Adly et D. Motreanu, Periodic solutions for second-order differential equations involving nonconvex superpotentials, J GLOB OPT, 17(1-4), 2000, pp. 9-17
The paper establishes the existence of a nonconstant periodic solution of a
general second order nonautonomous Hamiltonian system with discontinuous n
onlinearities. The multiplicity of solutions is also studied.