Abstract

The quadratic nonlinear wave equation on a one-dimensional torus with small initial values located in a single Fourier mode is considered. In this situation, the formation of metastable energy strata has recently been described and their long-time stability has been shown. The topic of the present paper is the correct reproduction of these metastable energy strata by a numerical method. For symplectic trigonometric integrators applied to the equation, it is shown that these energy strata are reproduced even on long time intervals in a qualitatively correct way.

Highlights

  • We consider the nonlinear wave equation∂ttu − ∂xxu + ρu = u2, u = u(x, t) ∈ R, (1)on a one-dimensional torus, x ∈ T = R/(2πZ), with a positive Klein–Gordon parameter ρ

  • We discretize the nonlinear wave equation and answer the question whether this long-time property of the exact solution is inherited by the numerical method

  • We consider a spectral collocation in space combined with a symplectic trigonometric integrator in time

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Summary

Introduction

We discretize the nonlinear wave equation and answer the question whether this long-time property of the exact solution is inherited by the numerical method. We consider a spectral collocation in space combined with a symplectic trigonometric integrator in time We show that this numerical method reproduces the energy strata of the exact solution even. Related questions have been studied for symplectic and non-symplectic methods applied to the nonlinear Schrodinger equation In this case, an initial value consisting of a single Fourier mode yields a solution that continues to consist of a single Fourier mode for all times (plane wave solution). The stability in numerical discretizations of these plane wave solutions under small perturbations of the initial value is an old question [22] and has been studied on short time intervals [4, 7, 19, 20, 22] and on long time intervals [10]

Metastable energy strata revisited
Trigonometric integrators
Metastable energy strata in trigonometric integrators
Numerical experiment
Approximation ansatz
The modulated Fourier expansion on a short time interval
Almost-invariant energies
From short to long time intervals
Expansion of the modulation functions
Construction of modulation functions
Bounds of the modulation functions
Proof of Theorem 4
Bounds of the defect
Bounds of the remainder
Transitions in the almost-invariant energies
Controlling mode energies by almost-invariant energies

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