Robust a posteriori error estimation for nonconforming finite element approximation

The equilibrated residual method for a posteriori error estimation is extended to nonconforming finite element schemes for the approximation of linear second order elliptic equations where the permeability coefficient is allowed to undergo large jumps in value across interfaces between differing media. The estimator is shown to provide a computable upper bound on the error and, up to a constant depending only on the geometry, provides two-sided bounds on the error. The robustness of the estimator is also studied and the dependence of the constant on the jumps in permeability is given explicitly.