Kc. Chang et al., NONTRIVIAL PERIODIC-SOLUTIONS FOR STRONG RESONANCE HAMILTONIAN-SYSTEMS, Annales de l Institut Henri Poincare. Analyse non lineaire, 14(1), 1997, pp. 103-117
For a Hamiltonian system, in which the Hamiltonian is assumed to have
an asymptotically linear gradient, the existence of nontrivial periodi
c solutions is proved under the assumption that the linearized operato
rs have distinct Maslov indices at 0 and at infinity. Both the lineari
zed operators may be degenerate. In particular, the results cover the
''strong resonance'' case.