Quasi invariant modified Sobolev norms for semi linear reversible PDEs

TL;DR

This paper develops almost invariant Sobolev-like norms for infinite-dimensional reversible PDEs, enabling long-time bounds on solutions' Sobolev norms under small initial data, applicable to various semi-linear PDEs including NLS.

Contribution

It introduces a new, simpler method to construct quasi-invariant pseudo norms for reversible PDEs, extending long-time stability results beyond Hamiltonian systems.

Findings

Constructed almost invariant pseudo norms close to Sobolev norms.

Proved long-time bounds on Sobolev norms for small initial data.

Applied method to nonlinear Schrödinger equations on tori and coupled NLS systems.

Abstract

We consider a general class of infinite dimensional reversible differential systems. Assuming a non resonance condition on the linear frequencies, we construct for such systems almost invariant pseudo norms that are closed to Sobolev-like norms. This allows us to prove that if the Sobolev norm of index $s$ of the initial data $z_0$ is sufficiently small (of order $\epsilon$) then the Sobolev norm of the solution is bounded by $2\epsilon$ during very long time (of order $\epsilon^{-r}$ with $r$ arbitrary). It turns out that this theorem applies to a large class of reversible semi linear PDEs including the non linear Schr\"odinger equation on the d-dimensional torus. We also apply our method to a system of coupled NLS equations which is reversible but not Hamiltonian. We also notice that for the same class of reversible systems we can prove a Birkhoff normal form theorem that in turn implies the same bounds on the Sobolev norms. Nevertheless the technics that we use to prove the existence of quasi invariant pseudo norms is much more simple and direct.