Isotropic and anisotropic error estimation of the mixed finite element method for second order operators in divergence form

Abstract : This paper presents an a posteriori residual error estimator for the mixed FEM of second order operators using isotropic or anisotropic meshes in $R^d$, $d$=2 or 3. The reliability and efficiency of our estimator is established without any regularity assumptions on the solution of our problem.