On the energy exchange between resonant modes in nonlinear Schrödinger equations

We consider the nonlinear Schrödinger equation $iψ_t= -ψ_{xx}\pm 2\cos 2x \ |ψ|^2ψ,\quad x\in S^1,\ t\in \R$ and we prove that the solution of this equation, with small initial datum $ψ_0=\e (\cos x+\sin x)$, will periodically exchange energy between the Fourier modes $e^{ix}$ and $e^{-ix}$. This beating effect is described up to time of order $\e^{-9/4}$ while the frequency is of order $\e^2$. We also discuss some generalizations.

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