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Spatial plane waves for the nonlinear Schrödinger equation: local existence and stability results

We consider the Cauchy problem for the nonlinear Schrödinger equation on $\mathbb{R}^2$, $iu_t + u_{xx} + u_{yy} + λ|u|^σu =0$, $λ\in \mathbb{R}$, $σ>0$. We introduce new functional spaces over which the initial value problem is well-posed. Their construction is based on \textit{spatial plane waves} (cf. arXiv:1510.08745). These spaces contain $H^1(\mathbb{R}^2)$ and do not lie within $L^2(\mathbb{R}^2)$. We prove several global well-posedness and stability results over these new spaces, including a new global well-posedness result of $H^1$ solutions with indefinitely large $H^1$ and $L^2$ norms. Some of these results are proved using a new functional transform, the \textit{plane wave transform}. We develop a suitable theory for this transform, prove several properties and solve classical linear PDE's with it, highlighting its wide range of application.