Abstract

We define a kinetic and a potential energy such that the principle of stationary action from Lagrangian mechanics yields a Camassa–Holm system (2CH) as the governing equations. After discretizing these energies, we use the same variational principle to derive a semi-discrete system of equations as an approximation of the 2CH system. The discretization is only available in Lagrangian coordinates and requires the inversion of a discrete Sturm–Liouville operator with time-varying coefficients. We show the existence of fundamental solutions for this operator at initial time with appropriate decay. By propagating the fundamental solutions in time, we define an equivalent semi-discrete system for which we prove that there exists unique global solutions. Finally, we show how the solutions of the semi-discrete system can be used to construct a sequence of functions converging to the conservative solution of the 2CH system.

Highlights

  • The Camassa–Holm (CH) equation ut − utxx + 3uux − 2uxuxx − uuxxx = 0, (1.1)is first known to have appeared in [25], written in an alternative form, as a special case in a hierarchy of completely integrable partial differential equations

  • The discretization is only available in Lagrangian coordinates and requires the inversion of a discrete Sturm–Liouville operator with time-varying coefficients

  • We show the existence of fundamental solutions for this operator at initial time with appropriate decay

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Summary

Introduction

Is first known to have appeared in [25], written in an alternative form, as a special case in a hierarchy of completely integrable partial differential equations. We will not follow this approach here, but rather use the fact that (1.2) can be derived as the governing equation for a different variational problem, where the action functional includes a potential energy term and the variation is performed on the group of diffeomorphisms only. This point of view enables us to derive a discretization which mimics the variational derivation of the continuous case. In (e) we plot the distribution of the potential energy given by (ρ(t, x) − ρ∞), and observe how it grows when the peaks get closer to each other

Derivation of the semi-discrete CH system using a variational approach
Construction of the fundamental solutions of the discrete momentum operator
An equivalent semi-discrete system for global solutions in time
Reformulation of the semi-discrete system
Existence and uniqueness of the solution to the semi-discrete 2CH system
Convergence of the scheme
Construction of the initial data
L2 dξ 2 max

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