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Oscillatory solitons of U(1)-invariant mKdV equations II: Asymptotic behavior and constants of motion

The Hirota equation and the Sasa-Satsuma equation are U(1)-invariant integrable generalizations of the modified Korteweg-de Vries equation. These two generalizations admit oscillatory solitons, which describe harmonically modulated complex solitary waves parameterized by their speed, modulation frequency, and phase. Depending on the modulation frequency, the speeds of oscillatory waves (1-solitons) can be positive, negative, or zero, in contrast to the strictly positive speed of ordinary solitons. When the speed is zero, an oscillatory wave is a time-periodic standing wave. Oscillatory 2-solitons with non-zero wave speeds are shown to describe overtake collisions of a fast wave and a slow wave moving in the same direction, or head-on collisions of two waves moving in opposite directions. When one wave speed is zero, oscillatory 2-solitons are shown to describe collisions in which a moving wave overtakes a standing wave. An asymptotic analysis using moving coordinates is carried out to show that, in all collisions, the speeds and modulation frequencies of the individual waves are preserved, while the phases and positions undergo a shift such that the center of momentum of the two waves moves at a constant speed. The primary constants of motion as well as some other features of the nonlinear interaction of the colliding waves are discussed.