Normalized ground states to a cooperative system of Schrödinger equations with generic $L^2$-subcritical or $L^2$-critical nonlinearity

Abstract: We look for ground state solutions to the Schr\"odinger-type system [ \begin{cases} -\Delta u_j + \lambda_j u_j = \partial_jF(u)\ \int_{\rn} u_j^2 \, dx = a_j^2\ (\lambda_j,u_j) \in \mathbb{R} \times H^1(\mathbb{R}^N) \end{cases} j \in {1,\dots,M} ] with $N,M\ge1$, where $a=(a_1,\dots,a_M) \in ]0,\infty[^M$ is prescribed and $(\lambda,u) = (\lambda_1,\dots,\lambda_M,u_1,\dots u_M)$ is the unknown. We provide generic assumptions about the nonlinearity $F$ which correspond to the $L^2$-subcritical and $L^2$-critical cases, i.e., when the energy is bounded from below for all or some values of $a$. Making use of a recent idea, we minimize the energy over the constraint $\Set{\left|u_j\right|{L^2}\le a_j \text{ for all } j}$ and then provide further assumptions that ensure $|u_j|{L^2}=a_j$.