Normalized solutions to Schrödinger equations in the strongly sublinear regime

Abstract: We look for solutions to the Schr\"odinger equation [ -\Delta u + \lambda u = g(u) \quad \text{in } \mathbb{R}^N ] coupled with the mass constraint $\int_{\mathbb{R}^N}|u|^2\,dx = \rho^2$, with $N\ge2$. The behaviour of $g$ at the origin is allowed to be strongly sublinear, i.e., $\lim_{s\to0}g(s)/s = -\infty$, which includes the case [ g(s) = \alpha s \ln s^2 + \mu |s|^{p-2} s ] with $\alpha > 0$ and $\mu \in \mathbb{R}$, $2 < p \le 2^*$ properly chosen. We consider a family of approximating problems that can be set in $H^1(\mathbb{R}^N)$ and the corresponding least-energy solutions, then we prove that such a family of solutions converges to a least-energy one to the original problem. Additionally, under certain assumptions about $g$ that allow us to work in a suitable subspace of $H^1(\mathbb{R}^N)$, we prove the existence of infinitely many solutions.