Abstract. Finite difference schemes in the spatial variable for degenerate stochastic parabolic PDEs are investigated. Sharp results on the rate of Lp and almost sure convergence of the finite difference approximations are presented and results on Richardson extrapolation are established for stochastic parabolic schemes under smoothness assumptions.
IntroductionWe consider finite difference schemes to stochastic partial differential equations (SPDEs). The stochastic PDEs we are interested in are linear second order stochastic parabolic equations in the whole R d in the spatial variable. They may degenerate and become first order stochastic or deterministic PDEs. The finite difference schemes which we investigate are spatial discretizations of such SPDEs. One can view them as (possibly degenerate) infinite systems of stochastic differential equations, whose components describe the time evolution of approximate values at the grid points of the solutions to SPDEs. Adapting the approach of [13] we view stochastic finite difference schemes, like in [7] and [3], as stochastic equations for random fields on the whole R d not only on grids.Our aim is to investigate the rate of convergence in the supremum norm of the finite difference approximations. We show that under the stochastic parabolicity condition, if the coefficients and the data are sufficiently smooth, then the solutions to the finite difference schemes admit power series expansions in terms of h, the mesh-size of the grid. The coefficients in these power series are random fields, independent of h, and for any p > 0 the p-th moments of the sup norm of the remainder term is estimated by a power of h. This is Theorem 2.2. Hence for h → 0 we get the convergence (and the sharp rate) of the solutions of the finite difference schemes to a random field which is the solution to the corresponding SPDE. Moreover, by Richardson extrapolation we get that the rate of convergence can be accelerated to any high order if one takes appropriate mixtures of approximations corresponding to different grid sizes. In Theorem 2.4 we obtain convergence estimates for any (high) p-th moments of the sup norm of the approximation error. Hence in 2000 Mathematics Subject Classification. 65M15, 35J70, 35K65.