Normalized positive solutions for Schr\"odinger equations with potentials in unbounded domains
The paper deals with the existence of positive solutions with prescribed $L^2$ norm for the Schr\"odinger equation $$ -\Delta u+\lambda u+V(x)u=|u|^{p-2}u,\qquad u\in H^1_0(\Omega),\quad\int_\Omega u^2dx=\rho^2,\quad\lambda\in\mathbb{R}, $$ where $\Omega=\mathbb{R}^N$ or $\mathbb{R}^N\setminus\Omega$ is a compact set, $\rho>0$, $V\ge 0$ (also $V\equiv 0$ is allowed), $p\in \left(2,2+\frac 4 N\right)$. The existence of a positive solution $\bar u$ is proved when $V$ verifies a suitable decay assumption $(D_\rho)$, or if $\|V\|_{L^q}$ is small, for some $q\ge \frac N2$ ($q>1$ if $N=2$). No smallness assumption on $V$ is required if the decay assumption $(D_\rho)$ is fulfilled. There are no assumptions on the size of $\mathbb{R}^N\setminus\Omega$. The solution $\bar u$ is a bound state and no ground state solution exists, up to the autonomous case $V\equiv 0$ and $\Omega=\mathbb{R}^N$.