Existence and stability of quasi-periodic solutions for derivative wave   equations
  In this note we present a new KAM result which proves the existence of Cantor families of small amplitude, analytic, quasi-periodic solutions of derivative wave equations, with zero Lyapunov exponents and whose linearized equation is reducible to constant coefficients. In turn, this result is derived by an abstract KAM theorem for infinite dimensional reversible dynamical systems. 
Massimiliano Berti, Luca Biasco, Michela Procesi
math.AP math.DS