Posteriori error estimators for the Raviart-Thomas element

When error estimators for the Raviart-Thomas element are developed, two difficulties prevent the success of the straightforward application of frequently used arguments. The H(div, Omega)-norm is an anisotropic norm; i.e., it refers to differential operators of different orders. Moreover, the traces of H(div, Omega)-functions are only in H--1/2. Therefore, one does not obtain optimal a posteriori error estimates when using natural norms. This drawback is overcome by using mesh-dependent norms.