Mixed Finite Element Methods for Problems with Robin Boundary Conditions

We derive new a-priori and a-posteriori error estimates for mixed nite element discretizations of second-order elliptic problems with general Robin boundary conditions, parameterized by a non-negative and piecewise constant function ε ≥ 0. The estimates are robust over several orders of magnitude of ε, ranging from pure Dirichlet conditions to pure Neumann conditions. A series of numerical experiments is presented that verify our theoretical results.