Lower dimensional invariant tori with prescribed frequency for nonlinear wave equation

Abstract

In this paper, one-dimensional (1D) nonlinear wave equation u t t − u x x + m u + u 3 = 0 , subject to Dirichlet boundary conditions is considered. We show that for each given m > 0 , and each prescribed integer b > 1 , the above equation admits a Whitney smooth family of small-amplitude quasi-periodic solutions with b-dimensional Diophantine frequencies, which correspond to b-dimensional invariant tori of an associated infinite-dimensional dynamical system. In particular, these Diophantine frequencies are the small dilation of a prescribed Diophantine vector. The proof is based on a partial Birkhoff normal form reduction and an improved KAM method.