A filtered finite difference method for a highly oscillatory nonlinear Klein--Gordon equation

We consider a nonlinear Klein–Gordon equation in the nonrelativistic limit regime with highly oscillatory initial data in the form of a modulated plane wave. In this regime, the solution exhibits rapid oscillations in both time and space, posing challenges for numerical approximation. We propose a filtered finite difference method that achieves second-order accuracy with time steps and mesh sizes that are not restricted in magnitude by the small parameter. Moreover, the method is uniformly convergent in the range from arbitrarily small to moderately bounded scaling parameters. Numerical experiments illustrate the theoretical results.