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Uniform and optimal error estimates of an exponential wave integrator sine pseudospectral method for the nonlinear Schrodinger equation with wave operator

We propose an exponential wave integrator sine pseudospectral (EWI-SP) method for the nonlinear Schrödinger equation (NLS) with wave operator (NLSW), and carry out rigorous error analysis. The NLSW is NLS perturbed by the wave operator with strength described by a dimensionless parameter $\varepsilon\in(0,1]$. As $\varepsilon\to0^+$, the NLSW converges to the NLS and for the small perturbation, i.e. $0<\varepsilon\ll1$, the solution of the NLSW differs from that of the NLS with a function oscillating in time with $O(\varepsilon^2)$-wavelength at $O(\varepsilon^2)$ and $O(\varepsilon^4)$ amplitudes for ill-prepared and well-prepared initial data, respectively. This rapid oscillation in time brings significant difficulties in designing and analyzing numerical methods with error bounds uniformly in $\varepsilon$. In this work, we show that the proposed EWI-SP possesses the optimal uniform error bounds at $O(τ^2)$ and $O(τ)$ in $τ$ (time step) for well-prepared initial data and ill-prepared initial data, respectively, and spectral accuracy in $h$ (mesh size) for the both cases, in the $L^2$ and semi-$H^1$ norms. This result significantly improves the error bounds of the finite difference methods for the NLSW. Our approach involves a careful study of the error propagation, cut-off of the nonlinearity and the energy method. Numerical examples are provided to confirm our theoretical analysis.