M. Berti, P. Bolle
Cantor families of periodic solutions of wave equations with 
$C^k% nonlinearities
(353K, pdf)

ABSTRACT.  We prove bifurcation of Cantor families of periodic solutions 
for wave equatins with nonlinearities of class $C^k$. 
It requires a modified Nash-Moser iteration scheme with 
interpolation estimates for the invers of the linearized 
operators and for the composition operators.