The nonlinear Schr\"odinger equation in the half-space

TL;DR

This paper investigates the existence and multiplicity of positive solutions to a nonlinear Schrödinger equation in a half-space, revealing a critical threshold for the boundary value that determines solution behavior.

Contribution

It establishes a precise threshold boundary value that dictates whether solutions exist, do not exist, or are multiple, depending on the dimension and boundary conditions.

Findings

Existence of infinitely many solutions for boundary value c below threshold c_p in dimensions ≥ 2.

Non-existence of solutions for boundary value c above threshold c_p.

Explicit determination of the threshold c_p depending only on p.

Abstract

The present paper is concerned with the half-space Dirichlet problem \begin{equation} \tag{$P_c$} \label{problem-abstract} -\Delta v + v = |v|^{p-1}v,\ \mbox{ in } \mathbb{R}^N_{+}, \qquad v = c,\ \mbox{ on } \partial \mathbb{R}^N_{+},\ \qquad \lim_{x_N \to \infty} v(x',x_N) = 0 \mbox{ uniformly in }x' \in \mathbb{R}^{N-1}, \end{equation} where $\mathbb{R}^N_{+} := \{\,x \in \mathbb{R}^N: x_N > 0\, \}$ for some $N \geq 1$ and $p > 1$, $c > 0$ are constants. We analyse the existence, non-existence and multiplicity of bounded positive solutions to \eqref{problem-abstract}. We prove that the existence and multiplicity of bounded positive solutions to \eqref{problem-abstract} depend in a striking way on the value of $c > 0$ and also on the dimension $N$. We find an explicit number $c_p \in (1,\sqrt{e})$, depending only on $p$, which determines the threshold between existence and non-existence. In particular, in dimensions $N \geq 2$, we prove that, for $0 < c < c_p$, problem \eqref{problem-abstract} admits infinitely many bounded positive solutions, whereas, for $c > c_p$, there are no bounded positive solutions to \eqref{problem-abstract}.