Nonlinear wave equations model the propagation of waves in a wide range of Nonlinear wave equations model the propagation of waves in a wide range of
physical systems, from acoustics to electromagnetics, from seismic motions to vibrating string and elastic membranes, where oscillatory phenomena occur.
Because of this intrinsic oscillatory physical structure, it is natural, from a mathematical point of view, to investigate the question of the existence
of oscillations, namely periodic and quasi-periodic solutions, for the equations governing such physical systems. This is the central question of
this Thesis.
