First and second order approximations for a nonlinear wave equation

Abstract: We consider the nonlinear wave equation (NLW) iv_t-|D|v=|v|^2v on the real line. We show that in a certain regime, the resonant dynamics is given by a completely integrable nonlinear equation called the Szego equation. Moreover, the Szego equation provides a first order approximation for NLW for a large time. The proof is based on the renormalization group method of Chen, Goldenfeld, and Oono. As a corollary, we give an example of solution of NLW whose high Sobolev norms exhibit relative growth. An analogous result of approximation was proved by Gerard and Grellier on the torus using the theory of Birkhoff normal forms. We improve this result by finding the second order approximation on the torus with the help of an averaging method introduced by Temam and Wirosoetisno. We show that the effective dynamics will no longer be given by the Szego equation.