Optimal error bounds on an exponential wave integrator Fourier spectral method for the logarithmic Schr\"odinger equation

TL;DR

This paper establishes nearly optimal error bounds for an exponential wave integrator Fourier spectral method applied to the logarithmic Schrödinger equation, improving convergence rates and relaxing regularity requirements.

Contribution

The paper provides the first nearly optimal error bounds for the EWI-FS method on LogSE, applicable under low regularity and with a new stability analysis.

Findings

Error bound of order O(τ|ln τ|^2 + h^2|ln h|) established

Improved convergence rates compared to existing literature

Method applicable to low regularity L∞-potentials

Abstract

We prove a nearly optimal error bound on the exponential wave integrator Fourier spectral (EWI-FS) method for the logarithmic Schr\"odinger equation (LogSE) under the assumption of $H^2$-solution, which is theoretically guaranteed. Subject to a CFL-type time step size restriction $\tau |\ln \tau| \lesssim h^2/|\ln h|$ for obtaining the stability of the numerical scheme affected by the singularity of the logarithmic nonlinearity, an $L^2$-norm error bound of order $O(\tau |\ln \tau|^2 + h^2 |\ln h|)$ is established, where $\tau$ is the time step size and $h$ is the mesh size. Compared to the error estimates of the LogSE in the literature, our error bound either greatly improves the convergence rate under the same regularity assumptions or significantly weakens the regularity requirement to obtain the same convergence rate. Moreover, our result can be directly applied to the LogSE with low regularity $L^\infty$-potential, which is not allowed in the existing error estimates. Two main ingredients are adopted in the proof: (i) an $H^2$-conditional $L^2$-stability estimate, which is established using the energy method to avoid singularity of the logarithmic nonlinearity, and (ii) mathematical induction with inverse inequalities to control the $H^2$-norm of the numerical solution. Numerical results are reported to confirm our error estimates and demonstrate the necessity of the time step size restriction imposed. We also apply the EWI-FS method to investigate soliton collisions in one dimension and vortex dipole dynamics in two dimensions.