1. Agmon, S.: Lectures on Elliptic Boundary Value Problems. Prepared for publication by B. Frank Jones, Jr. with the assistance of George W. Batten, Jr. Van Nostrand Mathematical Studies, No. 2. D. Van Nostrand Co., Inc., Princeton (1965)
2. Antonietti, P.F., Beirão da Veiga, L., Verani, M.: A mimetic discretization of elliptic obstacle problems. Math. Comput. 82(283), 1379–1400 (2013)
3. Apel, T.: Anisotropic finite elements: local estimates and applications. In: Advances in Numerical Mathematics. B. G. Teubner, Stuttgart (1999)
4. Brenner, S.C., Scott, L.R.: The Mathematical Theory of Finite Element Methods. Texts in Applied Mathematics, vol. 15, 2nd edn. Springer, New York (2002)
5. Clément, P.: Approximation by finite element functions using local regularization. ESAIM Math. Model. Numer. Anal. 9(R-2), 77–84 (1975)