ℋ-matrix approximability of the inverses of FEM matrices

We study the question of approximability for the inverse of the FEM stiffness matrix for (scalar) second order elliptic boundary value problems by blockwise low rank matrices such as those given by the H-matrix format introduced by Hackbusch (Computing 62(2):89–108, 1999). We show that exponential convergence in the local block rank r can be achieved. We also show that exponentially accurate LU -decompositions in the H-matrix format are possible for the stiffness matrices arising in the FEM. Our analysis avoids any coupling of the block rank r to the mesh width h. We also cover fairly general boundary conditions of mixed Dirichlet– Neumann–Robin boundary conditions. Mathematics Subject Classification 65F05 · 65N30 · 65F30 · 65F50