Runge-Kutta time semidiscretizations of semilinear PDEs with non-smooth data.

We study semilinear evolution equations [Formula: see text] posed on a Hilbert space [Formula: see text], where is normal and generates a strongly continuous semigroup, is a smooth nonlinearity from [Formula: see text] to itself, and [Formula: see text], [Formula: see text], [Formula: see text]. In particular the one-dimensional semilinear wave equation and nonlinear Schrödinger equation with periodic, Neumann and Dirichlet boundary conditions fit into this framework. We discretize the evolution equation with an A-stable Runge-Kutta method in time, retaining continuous space, and prove convergence of order [Formula: see text] for non-smooth initial data [Formula: see text], where [Formula: see text], for a method of classical order , extending a result by Brenner and Thomée for linear systems. Our approach is to project the semiflow and numerical method to spectral Galerkin approximations, and to balance the projection error with the error of the time discretization of the projected system. Numerical experiments suggest that our estimates are sharp.