Resonant averaging for small solutions of stochastic NLS equations

Abstract

We consider the free linear Schr\"odinger equation on a torus $\mathbb T^d$, perturbed by a hamiltonian nonlinearity, driven by a random force and damped by a linear damping: $$ u_t -i\Delta u +i\nu \rho |u|^{2q_*}u = - \nu f(-\Delta) u + \sqrt\nu\,\frac{d}{d t}\sum_{k\in \mathbb Z^d} b_l\beta^k(t)e^{ik\cdot x} \ . $$ Here $u=u(t,x),\ x\in\mathbb T^d$, $0<\nu\ll 1$, $q_*\in\mathbb N$, $f$ is a positive continuous function, $\rho$ is a positive parameter and $\beta^k(t)$ are standard independent complex Wiener processes. We are interested in limiting, as $\nu\to0$, behaviour of distributions of solutions for this equation and of its stationary measure. Writing the equation in the slow time $\tau=\nu t$, we prove that the limiting behaviour of the both is described by the effective equation $$ u_\tau+ f(-\Delta) u = -iF(u)+\frac{d}{d\tau}\sum b_k\beta^k(\tau)e^{ik\cdot x} \, $$ where the nonlinearity $F(u)$ is made out of the resonant terms of the monomial $ |u|^{2q_*}u$. We explain the relevance of this result for the problem of weak turbulence