On some nonlinear Schrödinger equations in ℝ^<i>N</i>

In this paper, we consider the following nonlinear Schrödinger equations with the critical Sobolev exponent and mixed nonlinearities: \[\left\{\begin{aligned} &amp; -\Delta u+\lambda u=t|u|^{q-2}u+|u|^{2^{*}-2}u\quad\text{in }\mathbb{R}^{N},\\ &amp; u\in H^{1}(\mathbb{R}^{N}), \end{aligned}\right.\] where $N\geq 3$ , $t&gt;0$ , $\lambda &gt;0$ and $2&lt; q&lt;2^{*}=\frac {2N}{N-2}$ . Based on our recent study on the normalized solutions of the above equation in [J. Wei and Y. Wu, Normalized solutions for Schrodinger equations with critical Sobolev exponent and mixed nonlinearities, e-print arXiv:2102.04030[Math.AP].], we prove that (1) the above equation has two positive radial solutions for $N=3$ , $2&lt; q&lt;4$ and $t&gt;0$ sufficiently large, which gives a rigorous proof of the numerical conjecture in [J. Dávila, M. del Pino and I. Guerra. Non-uniqueness of positive ground states of non-linear Schrödinger equations. Proc. Lond. Math. Soc. 106 (2013), 318–344.]; (2) there exists $t_q^{*}&gt;0$ for $2&lt; q\leq 4$ such that the above equation has ground-states for $t\geq t_q^{*}$ in the case of $2&lt; q&lt;4$ and for $t&gt;t_4^{*}$ in the case of $q=4$ , while the above equation has no ground-states for $0&lt; t&lt; t_q^{*}$ for all $2&lt; q\leq 4$ , which, together with the well-known results on ground-states of the above equation, almost completely solve the existence of ground-states, except for $N=3$ , $q=4$ and $t=t_4^{*}$ . Moreover, based on the almost completed study on ground-states to the above equation, we introduce a new argument to study the normalized solutions of the above equation to prove that there exists $0&lt;\overline {t}_{a,q}&lt;+\infty$ for $2&lt; q&lt;2+\frac {4}{N}$ such that the above equation has no positive normalized solutions for $t&gt;\overline {t}_{a,q}$ with $\int _{\mathbb {R}^{N}}|u|^{2}{\rm d}x=a^{2}$ , which, together with our recent study in [J. Wei and Y. Wu, Normalized solutions for Schrodinger equations with critical Sobolev exponent and mixed nonlinearities, e-print arXiv:2102.04030[Math.AP].], gives a completed answer to the open question proposed by Soave in [N. Soave. Normalized ground states for the NLS equation with combined nonlinearities: The Sobolev critical case. J. Funct. Anal. 279 (2020) 108610.]. Finally, as applications of our new argument, we also study the following Schrödinger equation with a partial confinement: \[\left\{ \begin{aligned} &amp; -\Delta u+\lambda u+(x_1^{2}+x_2^{2})u=|u|^{p-2}u\quad\text{in }\mathbb{R}^{3},\\ &amp; u\in H^{1}(\mathbb{R}^{3}),\quad \int_{\mathbb{R}^{3}}|u|^{2}{\rm d}x=r^{2}, \end{aligned}\right.\] where $x=(x_1,x_2,x_3)\in \mathbb {R}^{3}$ , $\frac {10}{3}&lt; p&lt;6$ , $r&gt;0$ is a constant and $(u, \lambda )$ is a pair of unknowns with $\lambda$ being

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