## Section: New Results

### Quasi invariant modified Sobolev norms for semi-linear reversible PDEs.

Participant : Erwan Faou.

In this joint work [30] with B. Grébert (University of Nantes), 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 ) then the Sobolev norm of the solution is bounded by 2 during very long time (of order ^{-r} with r arbitrary). It turns out that this theorem applies to a large class of reversible semi linear PDEs including the non linear Schrödinger (NLS) 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 techniques that we use to prove the existence of quasi invariant pseudo norms is much more simple and direct.