Homoclinic solutions of an infinite-dimensional Hamiltonian system

We consider the system [GRAPHICS] which is an unbounded Hamiltonian system in L-2 (R-N, R-2M). We assume that the constant function (u(o), v(0)) equivalent to (0, 0) is an element of R-2M is a stationary solution, and that H and V are periodic in the t and x variables. We present a variational formulation in order to obtain homoclinic solutions z = (U, V) satisfying z (t, x) --> 0 as \t\ + \x\ --> infinity. It is allowed that V changes sign and that -Delta + V has essential spectrum below (and above) 0. We also treat the case of a bounded domain Omega instead of R-N with Dirichlet boundary conditions.