1. Babu?ka, I., Osborn, J.: Analysis of Finite Element Methods for Second Order Boundary Value Problems Using Mesh Dependent Norms. Numer. Math.34, 41?62 (1980)
2. Babu?ka, I., Osborn, J.: Generalized Finite Element Methods: Their Performance and Their Relation to Mixed Methods. SIAM J. Numer. Anal.20, 510?536 (1982)
3. Banerjee, U.: Lower Norm Estimates for Approximate Solutions of Differential Equations with Non-Smooth Coefficients. (Submitted to Numer. Math.)
4. Ciarlet, P.: The Finite Element Method for Elliptic Problems. Amsterdam: North Holland 1978
5. Falk, R., Osborn, J.: Error Estimates for Mixed Methods. RAIRO, Numer. Anal.14, 249?277 (1980)