Multiphase solutions and their reductions for a nonlocal nonlinear Schr\"odinger equation with focusing nonlinearity

Abstract

A nonlocal nonlinear Schr\"odinger equation with focusing nonlinearity is considered which has been derived as a continuum limit of the Calogero-Sutherland model in an integrable classical dynamical system. The equation is shown to stem from the compatibility conditions of a system of linear PDEs, assuring its complete integrability. We construct a nonsingular $N$-phase solution ($N$: positive integer) of the equation by means of a direct method. The features of the one- and two-phase solutions are investigated in comparison with the corresponding solutions of the defocusing version of the equation. We also provide an alternative representation of the $N$-phase solution in terms of solutions of a system of nonlinear algebraic equations. Furthermore, the eigenvalue problem associated with the $N$-phase solution is discussed briefly with some exact results. Subsequently, we demonstrate that the $N$-soliton solution can be obtained simply by taking the long-wave limit of the $N$-phase solution. The similar limiting procedure gives an alternative representation of the $N$-soliton solution as well as the exact results related to the corresponding eigenvalue problem.