On the lifespan of solutions and control of high Sobolev norms for the completely resonant NLS on tori

We consider a completely resonant nonlinear Schrödinger equation on the d-dimensional torus, for any $d\ge 1$, with polynomial nonlinearity of any degree $2p+1$, $p\ge 1$, which is gauge and translation invariant. We study the behaviour of high Sobolev $H^s$-norms of solutions, $s\ge s_1+1>d/2+2$, whose initial datum $u_0\in H^s$ satisfies an appropriate smallness condition on its low $H^{s_1}$ and $L^2$-norms respectively. We prove a polynomial upper bound on the possible growth of the Sobolev norm $H^s$ over finite but long time scale that is exponential in the regularity parameter $s_1$. As a byproduct we get stability of the low $H^{s_1}$-norm over such time interval. A key ingredient in the proof is the introduction of a suitable “modified energy” that provides an a priori upper bound on the growth. This is obtained by combining para-differential techniques and suitable tame estimates.