A KAM theorem for the Hamiltonian with finite zero normal frequencies and its applications (1908.11072v1)

Abstract: In this paper, we investigate the existence of KAM tori for an infinite dimensional Hamiltonian system with finite number of zero normal frequencies. By constructing a constant quantity we show that, for "most" frequencies in the sense of Lebesgue measure, either if the quantity is zero, there is a KAM tori or if the quantity is not zero, there is no KAM tori in some domain. As application, we show that the nonlinear Schr\"{o}dinger equation with a zero frequency possesses many quasi-periodic solutions.