Morse index for the ground state in the energy supercritical Gross--Pitaevskii equation

The ground state of the energy super-critical Gross--Pitaevskii equation with a harmonic potential converges in the energy space to the singular solution in the limit of large amplitudes. The ground state can be represented by a solution curve which has either oscillatory or monotone behavior, depending on the dimension of the system and the power of the focusing nonlinearity. We address here the monotone case for the cubic nonlinearity in the spatial dimensions $d \geq 13$. By using the shooting method for the radial Schrödinger operators, we prove that the Morse index of the ground state is finite and is independent of the (large) amplitude. It is equal to the Morse index of the limiting singular solution, which can be computed from numerical approximations. The numerical results suggest that the Morse index of the ground state is one and that it is stable in the time evolution of the cubic Gross--Pitaevskii equation in dimensions $d \geq 13$.