Classification and Nondegeneracy of Cubic Nonlinear Schrödinger System in $\mathbb{R}$

Abstract

We study the following one-dimensional cubic nonlinear Schrödinger system: \[ u_i''+2\Big(\sum_{k=1}^Nu_k^2\Big)u_i=-μ_iu_i \ \,\ \mbox{in}\, \ \mathbb{R} , \ \ i=1, 2, \cdots, N, \] where $μ_1\leqμ_2\leq\cdots\leqμ_N<0$ and $N\ge 2$. In this paper, we mainly focus on the case $N=3$ and prove the following results: (i). The solutions of the system can be completely classified; (ii). Depending on the explicit values of $μ_1\leqμ_2\leqμ_3<0$, there exist two different classes of normalized solutions $u=(u_1, u_2, u_3)$ satisfying $\int _{R}u_i^2dx=1$ for all $i=1, 2, 3$, which are completely different from the case $N=2$; (iii). The linearized operator at any nontrivial solution of the system is non-degenerate. The conjectures on the explicit classification and nondegeneracy of solutions for the system are also given for the case $N>3$. These address the questions of [R. Frank, D. Gontier and M. Lewin, CMP, 2021], where the complete classification and uniqueness results for the system were already proved for the case $N=2$.