Sixth order compact finite difference schemes for Poisson interface with sources

Let Gamma be a smooth curve inside a two-dimensional rectangular region Omega. In this paper, we consider the Poisson interface problem -del(2)u = f in Omega\Gamma with Dirichlet boundary condition such that f is smooth in Omega\Gamma and the jump functions [u] and [del u.(n) over right arrow] across Gamma are smooth along Gamma. This Poisson interface problem includes the weak solution of -del(2)u = f + g delta(Gamma) in Omega as a special case. Because the source term f is possibly discontinuous across the interface curve Gamma and contains a delta function singularity along the curve Gamma, both the solution u of the Poisson interface problem and its flux del u.(n) over right arrow are often discontinuous across the interface. To solve the Poisson interface problem with singular sources, in this paper we propose a sixth order compact finite difference scheme on uniform Cartesian grids. Our proposed compact finite difference scheme with explicitly given stencils extends the immersed interface method (IIM) to the highest possible accuracy order six for compact finite difference schemes on uniform Cartesian grids, but without the need to change coordinates into the local coordinates as in most papers on IIM in the literature. Also in contrast with most published papers on IIM, we explicitly provide the formulas for all involved stencils and therefore, our proposed scheme can be easily implemented and is of interest to practitioners dealing with Poisson interface problems. Note that the curve Gamma splits Omega into two disjoint subregions Omega(+) and Omega(-)-. The coefficient matrix A in the resulting linear system Ax = b, following from the proposed scheme, is independent of any source term f, jump condition g delta(Gamma), interface curve Gamma and Dirichlet boundary conditions, while only b depends on these factors and is explicitly given, according to the configuration of the nine stencil points in Omega(+) or Omega(-). The constant coefficient matrix.. facilitates the parallel implementation of the algorithm in case of a large size matrix and only requires the update of the right hand side vector b for different Poisson interface problems. Due to the flexibility and explicitness of the proposed scheme, it can be generalized to obtain the highest order compact finite difference scheme for non-uniform grids as well. We prove the order 6 convergence for the proposed scheme using the discrete maximum principle. Our numerical experiments confirm the sixth accuracy order of the proposed compact finite difference scheme on uniform meshes for the Poisson interface problems with various singular sources.