Quantitative quasi-invariance of Gaussian measures below the energy level for the 1D generalized nonlinear Schr\"odinger equation and application to global well-posedness
We consider the Schr\"odinger equation on the one dimensional torus with a general odd-power nonlinearity $p \geq 5$, which is known to be globally well-posed in the Sobolev space $H^\sigma(\mathbb{T})$, for every $\sigma \geq 1$, thanks to the conservation and finiteness of the energy. For regularities $\sigma < 1$, where this energy is infinite, we explore a globalization argument adapted to random initial data distributed according to the Gaussian measures $\mu_s$, with covariance operator $(1-\Delta)^s$, for $s$ in a range $(s_p,\frac{3}{2}]$. We combine a deterministic local Cauchy theory with the quasi-invariance of Gaussian measures $\mu_s$, with additional $L^q$-bounds on the Radon-Nikodym derivatives, to prove that the Gaussian initial data generate almost surely global solutions. These $L^q$-bounds are obtained with respect to Gaussian measures accompanied by a cutoff on a renormalization of the energy; the main tools to prove them are the Bou\'e-Dupuis variational formula and a Poincar\'e-Dulac normal form reduction. This approach is similar in spirit to Bourgain's invariant argument and to a recent work by Forlano-Tolomeo.