Ghostpeakons and Characteristic Curves for the Camassa-Holm, Degasperis-Procesi and Novikov Equations

TL;DR

This paper derives explicit formulas for characteristic curves of multipeakon solutions in the Camassa-Holm, Degasperis-Procesi, and Novikov equations, including ghostpeakons with zero amplitude, enhancing understanding and visualization of these wave solutions.

Contribution

It introduces a method to obtain ghostpeakon characteristic curves by taking limits in spectral data, extending previous solutions to include zero-amplitude peakons.

Findings

Explicit formulas for characteristic curves including ghostpeakons.

Method to incorporate zero-amplitude peakons via spectral data limits.

Application to shockpeakon formation during peakon-antipeakon collisions.

Abstract

We derive explicit formulas for the characteristic curves associated with the multipeakon solutions of the Camassa-Holm, Degasperis-Procesi and Novikov equations. Such a curve traces the path of a fluid particle whose instantaneous velocity equals the elevation of the wave at that point (or the square of the elevation, in the Novikov case). The peakons themselves follow characteristic curves, and the remaining characteristic curves can be viewed as paths of 'ghostpeakons' with zero amplitude; hence, they can be obtained as solutions of the ODEs governing the dynamics of multipeakon solutions. The previously known solution formulas for multipeakons only cover the case when all amplitudes are nonzero, since they are based upon inverse spectral methods unable to detect the ghostpeakons. We show how to overcome this problem by taking a suitable limit in terms of spectral data, in order to force a selected peakon amplitude to become zero. Moreover, we use direct integration to compute the characteristic curves for the solution of the Degasperis-Procesi equation where a shockpeakon forms at a peakon-antipeakon collision. In addition to the theoretical interest in knowing the characteristic curves, they are also useful for plotting multipeakon solutions, as we illustrate in several examples.