1. [BV] D. Braess, R. Verfürth: A posteriori error estimators for the Raviart-Thomas element. Preprint 175/1994 Fakultät für Mathematik der Ruhr-Universität Bochum.
2. Springer Series in Computational Mathematics;Brezzi, Franco,1991
3. Studies in Mathematics and its Applications, Vol. 4;Ciarlet, Philippe G.,1978
4. Approximation by finite element functions using local regularization;Clément, Ph.;Rev. Fran\c{c}aise Automat. Informat. Recherche Op\'{e}rationnelle S\'{e}r.,1975
5. [EEHJ] K. Eriksson, D. Estep, P. Hansbo, C. Johnson: Introduction to adaptive methods for differential equations. Acta Numerica (1995) 105—158.