Laura Di Gregorio
Infinite dimensional hamiltonian systems and 
nonlinear wave equation: periodic orbits with long minimal period
(1885K, postcript)

ABSTRACT.  We prove existence of infinitely many small amplitude periodic 
solutions for the nonlinear wave equation. Such solutions bifurcate from resonant finite 
dimensional invariant tori of the fourth order Birkhoff normal form of the 
associated hamiltonian system. As a consequence the origin is an 
accumulation point for these periodic orbits of longer and longer 
minimal period.