Quantitative quasi-invariance of Gaussian measures below the energy level for the 1D generalized nonlinear Schrödinger equation and application to global well-posedness

Abstract We consider the Schrödinger equation on the one dimensional torus with a general odd-power nonlinearity p 5, which is known to be globally well-posed in the Sobolev space H^ (T), for every 1, thanks to the conservation and finiteness of the energy. For regularities σ < 1, where this energy is infinite, we explore a globalization argument adapted to random initial data distributed according to the Gaussian measures µ s, with covariance operator (1-) ˢ, for s in a range (sₚ, 32]. We combine a deterministic local Cauchy theory with the quasi-invariance of Gaussian measures µ 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 Boué-Dupuis variational formula and a Poincaré-Dulac normal form reduction. This approach is similar in spirit to Bourgain’s invariant argument 7 and to arecent work by Forlano-Tolomeo in 18.