A Relaxation/Finite Difference discretization of a 2D Semilinear Heat Equation over a rectangular domain

We consider an initial and Dirichlet boundary value problem for a semilinear, two dimensional heat equation over a rectangular domain. The problem is discretized in time by a version of the Relaxation Scheme proposed by C. Besse (C. R. Acad. Sci. Paris S'er. I, vol. 326 (1998)) for the nonlinear Schr"odinger equation and in space by a standard second order finite difference method. The proposed method is unconditionally well-posed and its convergence is established by proving an optimal second order error estimate allowing a mild mesh condition to hold.