Normalized ground states for NLS equations with mass critical nonlinearities

We study normalized solutions $(\mu,u)\in \mathbb{R} \times H^1(\mathbb{R}^N)$ to %the \r\nnonlinear Schrödinger equations\r\n \[ -\Delta u + \mu u = g(u)\quad \hbox{in}\ \mathbb{R}^N, \qquad\r\n \frac{1}{2}\int_{\mathbb{R}^N} u^2 dx = m,\r\n \]\r\nwhere $N\geq 2$ and the mass $m>0$ is given. Here, $g$ has an $L^2$-critical growth, both at the origin and at infinity, that is, $g(s)\sim |s|^{p-1}s$ as $s\sim 0$ and $s\sim\infty$, where $p=1+\frac{4}{N}$.\r\nWe continue the analysis started in \cite{CGIT24}, where we found two (possibly distinct) minimax values $\underline{b} \leq 0 \leq \overline{b}$ of the Lagrangian functional.\r\nIn this paper, we furnish explicit examples of $g$ satisfying $\underline{b}<0<\overline{b}$, $\underline{b}=0<\overline{b}$, and $\underline{b}<0=\overline{b}$; notice that $\underline{b}=0=\overline{b}$ in the power case $g(t)=|t|^{p-1}t$. Moreover, we deal with the existence and non-existence of a solution with minimal energy.\r\nFinally, we discuss the assumptions required on $g$ to obtain the existence of a positive solution for perturbations of $g$.