Evaluation of Inner Products of Implicitly-defined Finite Element Functions on Multiply Connected Planar Mesh Cells

Abstract: Recent advancements in finite element methods allows for the implementation of mesh cells with curved edges. In the present work, we develop the tools necessary to employ multiply connected mesh cells, i.e. cells with holes, in planar domains. Our focus is efficient evaluation the $H^1$ semi-inner product and $L^2$ inner product of implicitly-defined finite element functions of the type arising in boundary element based finite element methods (BEM-FEM) and virtual element methods (VEM). These functions may be defined by specifying a polynomial Laplacian and a continuous Dirichlet trace. We demonstrate that these volumetric integrals can be reduced to integrals along the boundaries of mesh cells, thereby avoiding the need to perform any computations in cell interiors. The dominating cost of this reduction is solving a relatively small Nystrom system to obtain a Dirichlet-to-Neumann map, as well as the solution of two more Nystrom systems to obtain an ``anti-Laplacian'' of a harmonic function, which is used for computing the $L^2$ inner product. We demonstrate that high-order accuracy can be achieved with several numerical examples.