Probing quasi-integrability of the Gross–Pitaevskii equation in a harmonic-oscillator potential

Previous simulations of the one-dimensional Gross-Pitaevskii equation (GPE) with repulsive nonlinearity and a harmonic-oscillator trapping potential hint towards the emergence of quasi-integrable dynamics -- in the sense of quasi-periodic evolution of a moving dark soliton without any signs of ergodicity -- although this model does not belong to the list of integrable equations. To investigate this problem, we replace the full GPE by a suitably truncated expansion over harmonic-oscillator eigenmodes (the Galerkin approximation), which accurately reproduces the full dynamics, and then analyze the system's spectrum. The analysis enables us to interpret the observed quasi-integrability as the fact that the Galerkin approximation's finite-mode dynamics always produces a quasi-discrete power spectrum, with no visible continuous component, the presence of the latter being a necessary manifestation of ergodicity. Undertaking, for the comparison's sake, the same analysis in an infinitely deep potential box, we conclude that it leads, instead, to a clearly continuous power spectrum, corresponding to the non-integrability of the box model.