A MODEL STUDY OF THE QUALITY OF A POSTERIORI ERROR ESTIMATORS FOR FINITE ELEMENT SOLUTIONS OF LINEAR ELLIPTIC PROBLEMS, WITH PARTICULAR REFERENCE TO THE BEHAVIOR NEAR THE BOUNDARY

In References 1{3 we presented a computer-based theory for analysing the asymptotic accuracy (quality of robustness) of error estimators for mesh-patches in the interior of the domain. In this paper we review the approach employed in References 1{3 and extend it to analyse the asymptotic quality of error estimators for mesh-patches at or near a domain boundary. We analyse two error estimators which were found in References 1{3 to be robust in the interior of the mesh (the element residual with p-order equilibrated uxes and (p+1) degree bubble solution or (p + 1) degree polynomial solution (ERpB or ERpPp+1; see References 1{3) and the Zienkiewicz{Zhu Superconvergent Patch Recovery (ZZ-SPR; see References 4{7) and we show that the robustness of these estimators for elements adjacent to the boundary can be signi cantly inferior to their robustness for interior elements. This deterioration is due to the di erence in the de nition of the estimators for the elements in the interior of the mesh and the elements adjacent to the boundary. In order to demonstrate how our approach can be employed to determine the most robust version of an estimator we analysed the versions of the ZZ estimator proposed in References 9{12. We found that the original ZZ-SPR proposed in References 4{7 is the most robust one, among the various versions tested, and some of the proposed `enhancementsʹ can lead to a signi cant deterioration of the asymptotic robustness of the estimator. From the analyses given in References 1{3 and in this paper, we found that the original ZZ estimator (given in References 4{7) is the most robust among all estimators analysed in References 1{3 and in this study.