# Existence of modified wave operators and infinite cascade result for a half wave Schr{\"o}dinger equation on the plane

## TL;DR

This paper proves the existence of modified wave operators for a half wave Schrödinger equation on the plane and demonstrates solutions with small initial data whose Sobolev norms grow unboundedly over time.

## Contribution

It establishes the existence of modified wave operators for the equation and links this to infinite cascade phenomena, revealing new long-term behavior of solutions.

## Key findings

- Existence of modified wave operators between solutions of the half wave Schrödinger and cubic Szegő equations.
- Construction of solutions with Sobolev norms tending to infinity as time progresses.
- Identification of the equation as one of the rare dispersive equations with unbounded Sobolev norm growth for small data.

## Abstract

We consider the following half wave Schr{\"o}dinger equation,$$\left(i \partial_{t}+\partial_{x }^2-\left|D_{y}\right|\right) U=|U|^{2} U$$on the plane $\mathbb{R}_x \times \mathbb{R}_y$. We prove the existence of modified wave operators between small decaying solutions to this equation and small decaying solutions to the non chiral cubic Szeg\H o equation, which is similar to the the existence result of modified wave operators on $\mathbb{R}_x \times \mathbb{T}_y$ obtained by H. Xu [16]. We then combine our modified wave operators result with a recent cascade result [7] for the cubic Szeg\H o equation by P. G{\'e}rard and A. Pushnitski to deduce that there exists solutions $U$ to the half wave Schr{\"o}dinger equation such that $\|U(t)\|_{L_{x}^{2} H_{y}^{1}}$ tends to infinity as $\log t$ when $t \rightarrow +\infty$. It indicates that the half wave Schr{\"o}dinger equation on the plane is one of the very few dispersive equations admitting global solutions with small and smooth data such that the $H^s$ norms are going to infinity as $t$ tends to infinity.