# Solving the 4NLS with white noise initial data

## TL;DR

This paper develops a novel approach to solve the cubic fourth order nonlinear Schrödinger equation on the circle with white noise initial data, introducing a nonlinear decomposition and random gauge transform to handle singularities.

## Contribution

It introduces the 'random-resonant / nonlinear decomposition' and a random gauge transform to construct global solutions with white noise as invariant measure, overcoming classical limitations.

## Key findings

- Constructed global-in-time singular dynamics for the 4NLS with white noise initial data.
- Developed the 'random-resonant / nonlinear decomposition' to isolate singular solution components.
- Established convergence of smooth solutions via a partially iterated Duhamel formulation.

## Abstract

We construct global-in-time singular dynamics for the (renormalized) cubic fourth order nonlinear Schr\"odinger equation on the circle, having the white noise measure as an invariant measure. For this purpose, we introduce the "random-resonant / nonlinear decomposition", which allows us to single out the singular component of the solution. Unlike the classical McKean, Bourgain, Da Prato-Debussche type argument, this singular component is nonlinear, consisting of arbitrarily high powers of the random initial data. We also employ a random gauge transform, leading to random Fourier restriction norm spaces. For this problem, a contraction argument does not work and we instead establish convergence of smooth approximating solutions by studying the partially iterated Duhamel formulation under the random gauge transform. We reduce the crucial nonlinear estimates to boundedness properties of certain random multilinear functionals of the white noise.