On the control of high Sobolev norms for the Wave equation on ${\mathbb{T}}^d$ over exponentially long times

Abstract

We consider a one-parameter family of nonlinear wave equations on the $d$-dimensional torus, with polynomial nonlinearities of arbitrary degree $q+1$, where $q\ge 1$. We investigate the long-time behavior of high Sobolev $H^s$-norms of solutions in different settings. In the one-dimensional case, and for almost any value of the mass parameter $\mathtt{m}>0$, we prove exponentially long stability times for small initial data. The proof relies on normal form techniques together with suitable \emph{weak} Diophantine conditions. In higher space dimensions, for initial data $u_0\in H^s$, $s\ge s_1+1$, satisfying suitable smallness conditions on the \emph{low} Sobolev norm $H^{s_1}$ and on the $L^2$-norm, we prove a polynomial upper bound on the possible growth of the high Sobolev $H^s$-norm, over finite but exponentially long time scales in the regularity parameter $s_1$. The key ingredient consists in establishing suitable \emph{a priori} tame estimates for the solution. The result applies in \emph{any} space dimension $d\ge 1$ and for \emph{all} values of the mass parameter $\mathtt{m}\ge 0$.