Coordinates at small energy and refined profiles for the Nonlinear Schr\"odinger Equation

In this paper we give a new and simplified proof of the theorem on selection of standing waves for small energy solutions of the nonlinear Schr"odinger equations (NLS) that we gave in \cite{CM15APDE}. We consider a NLS with a Schr"odinger operator with several eigenvalues, with corresponding families of small standing waves, and we show that any small energy solution converges to the orbit of a time periodic solution plus a scattering term. The novel idea is to consider the “refined profile”, a quasi–periodic function in time which almost solves the NLS and encodes the discrete modes of a solution. The refined profile, obtained by elementary means, gives us directly an optimal coordinate system, avoiding the normal form arguments in \cite{CM15APDE}, giving us also a better understanding of the Fermi Golden Rule.