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All first-order averaging techniques for a posteriori finite element error control on unstructured grids are efficient and reliable

Authors:	C Carstensen

Institution:	Institute for Applied Mathematics and Numerical Analysis, Vienna University of Technology, Wiedner Hauptstraße 8-10, A-1040 Vienna, Austria

Abstract:	All first-order averaging or gradient-recovery operators for lowest-order finite element methods are shown to allow for an efficient a posteriori error estimation in an isotropic, elliptic model problem in a bounded Lipschitz domain $\Omega$ in $\mathbb{R}^d$ . Given a piecewise constant discrete flux $p_h\in P_h$ (that is the gradient of a discrete displacement) as an approximation to the unknown exact flux (that is the gradient of the exact displacement), recent results verify efficiency and reliability of $\begin{displaymath}\eta_M:=\min\{\Vert p_h-q_h\Vert _{L^2(\Omega)}:\,q_h\in\mathcal{Q}_h\} \end{displaymath}$ in the sense that $\eta_M$ is a lower and upper bound of the flux error $\Vert p-p_h\Vert _{L^2(\Omega)}$ up to multiplicative constants and higher-order terms. The averaging space $\mathcal{Q}_h$ consists of piecewise polynomial and globally continuous finite element functions in components with carefully designed boundary conditions. The minimal value $\eta_M$ is frequently replaced by some averaging operator $A: P_h\rightarrow\mathcal{Q}_h$ applied within a simple post-processing to . The result $q_h:=Ap_h\in\mathcal{Q}_h$ provides a reliable error bound with $\eta_M\leq\eta_A:=\Vert p_h-Ap_h\Vert _{L^2(\Omega)}$ . This paper establishes $\eta_A\leq C_{\mbox{\tiny eff}}\,\eta_M$ and so equivalence of $\eta_M$ and $\eta_A$ . This implies efficiency of $\eta_A$ for a large class of patchwise averaging techniques which includes the ZZ-gradient-recovery technique. The bound $C_{\mbox{\tiny eff}}\le 3.88$ established for tetrahedral finite elements appears striking in that the shape of the elements does not enter: The equivalence $\eta_A\approx\eta_M$ is robust with respect to anisotropic meshes. The main arguments in the proof are Ascoli's lemma, a strengthened Cauchy inequality, and elementary calculations with mass matrices.

Keywords:	A posteriori error estimate efficiency finite element method gradient recovery averaging operator mixed finite element method nonconforming finite element method

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