A Birkhoff Normal Form Theorem for Partial Differential Equations on torus

TL;DR

This paper establishes a Birkhoff normal form theorem for Hamiltonian PDEs on a torus, enabling long-term stability analysis in Sobolev spaces for nonlinear wave and Schrödinger equations.

Contribution

It introduces an abstract normal form theorem applicable to PDEs on tori, with novel stability results for specific equations.

Findings

Polynomially long time stability in Sobolev spaces

Sub-exponentially long time stability for nonlinear wave and Schrödinger equations

Normal form is complete up to arbitrary finite order

Abstract

We prove an abstract Birkhoff normal form theorem for Hamiltonian partial differential equations on torus. The normal form is complete up to arbitrary finite order. The proof is based on a valid non-resonant condition and a suitable norm of Hamiltonian function. Then as two examples, we apply this theorem to nonlinear wave equation in one dimension and nonlinear Schr\"{o}dinger equation in high dimension. Consequently, the polynomially long time stability is proved in Sobolev spaces $H^s$ with the index $s$ being much smaller than before. Further, by taking the iterative steps depending on the size of initial datum, we prove sub-exponentially long time stability for these two equations.