Guido Gentile and Vieri Mastropietro
Construction of periodic solutions of the nonlinear 
wave equation under strong irrationality conditions 
by the Lindstedt series method
(276K, postscrip)

ABSTRACT.  We prove the existence of oscillatory 
solutions of the nonlinear wave equation, under 
irrationality conditions stronger than the usual 
Diophantine one, by perturbative techniques 
inspired by the Lindstedt series method originally introduced 
in classical mechanics to study the existence of invariant 
tori in quasi-integrable Hamiltonian systems