Graded mesh approximation in weighted Sobolev spaces and elliptic equations in 2D

We study the approximation properties of some general finiteelement spaces constructed using improved graded meshes. In our results, either the approximating function or the function to be approximated (or both) are in a weighted Sobolev space. The finite-element spaces that we define are obtained from conformally invariant families of finite elements (no affine invariance is used), stressing the use of elements that lead to higher regularity finite-element spaces. We prove that for a suitable grading of the meshes, one obtains the usual optimal approximation results. We provide a construction of these spaces that does not lead to long, “skinny” triangles. Our results are then used to obtain hm-quasi-optimal rates of convergence for the FEM approximation of solutions of strongly elliptic interface/boundary value problems.