Interaction between solitons in a stable medium

is encountered in various physical problems. Gross [1J and Pitaevskil' [2J, and later on Tsuzuki [3J, used this equation to describe the oscillations of a Bose gas at zero temperature. The problem of the evolution of the complex envelope of a monochromatic wave in a nonlinear medium [4,5J also leads to Eq. (1). Finally, interpreting t as a longitudinal coordinate, Eq. (1) can be treated as a two-dimensional variant of the well known "parabolic" equation[6J that describes stationary wave beams in a nonlinear medium.