Global well-posedness of quadratic and subquadratic half wave Schr{ö}dinger equations

Abstract: We consider the following $p$ order nonlinear half wave Schr{\"o}dinger equations$$\left(i \partial_{t}+\partial_{x }^2-\left|D_{y}\right|\right) u=\pm|u|^{p-1} u$$on the plane $\mathbb{R}^2$ with $1<p\leq 2$. This equation is considered as a toy model motivated by the study of solutions to weakly dispersive equations. In particular, the global well-posedness of this equation is a difficult problem due to the anisotropic property of the equation, with one direction corresponding to the half-wave operator, which is not dispersive. In this paper, we prove the global well-posedness of this equation in $L_x^2 H_y^s(\mathbb{R}^2) \cap H_x^1 L_y^2(\mathbb{R}^2)$($\frac{1}{2}\leq s \leq 1$), which is the first global well-posedness result of nonlinear half wave Schr{\"o}dinger equations. With the global well-posedness in the energy space for the focusing equation and the study on the solitary wave in [1], we complete the proof of the stability of the set of ground states. Moreover, we consider the half wave Schr{\"o}dinger equations on $\mathbb{R}_{x}\times\mathbb{T}_{y}$, which can also be called the wave guide Schr{\"o}dinger equations on $\mathbb{R}_{x}\times\mathbb{T}_{y}$. Using a similar approach in the analysis of the Cauchy problem of half wave Schr{\"o}dinger equations on $\mathbb{R}^2$, we can also deduce the global well-posedness of $p$ ($1<p\leq2$) order wave guide Schr{\"o}dinger equations in $L_x^2 H_y^s(\mathbb{R}\times\mathbb{T}) \cap H_x^1 L_y^2(\mathbb{R}\times\mathbb{T})$ with $\frac{1}{2}\leq s \leq 1$. With the global well-posedness in the energy space for the focusing wave guide Schr{\"o}dinger equations and the study on the ground states in [2], we complete the proof of the orbital stability of the ground states with small frequencies.