The BEM-based finite element method is reviewed and extended with higher order basis functions on general polygonal meshes. These functions are defined implicitly as local solutions of the underlying homogeneous problem with constant coefficients. They are treated by means of boundary integral formulations and are approximated using the boundary element method in the numerics. To obtain higher order convergence, a new approximation of the material coefficient is proposed since previous strategies are not sufficient. Following recent ideas, error estimates are proved which guarantee quadratic convergence in the H 1 -norm and cubic convergence in the L 2 -norm. The numerical realization is discussed and several experiments confirm the theoretical results.
Introduction.In the field of numerical methods for partial differential equations there is an increasing interest in nonsimplicial meshes. Several applications in solid mechanics, biomechanics, and geological science show a need for general elements within a finite element simulation. Discontinuous Galerkin methods [9] and mimetic finite difference methods [3] are able to handle such meshes. Nevertheless, these two strategies yield nonconforming approximations in their original versions which are not in the Sobolev space in which the exact solution lies.In 1975, Wachspress [26] proposed the construction of conforming rational basis functions on convex polygons with any number of sides. In recent years, several improved basis functions on polygonal elements have been introduced and applied in linear elasticity [24] or computer graphics [15,17], for example. There are even the first attempts which seek to introduce quadratic finite elements on polygons [20]. The recent publication [2] extends the nodal mimetic discretization strategy to arbitrary order on polygonal meshes.In [7] and the detailed work [6], a new kind of conforming finite element method on polygonal meshes has been proposed which uses basis functions that fulfill the differential equation locally. In the local problems constant material parameters and vanishing right-hand sides are prescribed. In case of the diffusion equation harmonic basis functions are recovered. This method has been studied in several articles concerning convergence [12,13] and residual error estimates for adaptive mesh refinement [27]. In the following, the theory is extended to a higher order method on polygonal meshes.The outline of this article is as follows. In section 2, some notation as well as a model problem are introduced. We review the first order method proposed in [7] and give a definition for regular meshes. The BEM-based FEM is extended to a higher order scheme in section 3. Afterward, we introduce interpolation operators in section 4 and prove interpolation properties which yield error estimates for the finite element