1. S. Agmon, Lectures on Elliptic Boundary Value Problems. D. Van Nostrand Comp., Inc. Princeton, New Jersey, New York 1965.
2. J. P. Aubin, Approximation des espaces de distributions et des opérateurs diffeérentiels. Bull. Soc. math. France. Mémoire12 1967 139p.
3. I. Babuska, Numerical Solution of Boundary Value Problems by the Perturbated Variational Principle. Techn. Note BN-624, University of Maryland, 1969.
4. I. Babuska, Error-Bounds for Finite Element Method. Techn. Note BN-630. Univ. of Maryland, 1969.
5. I. Babuska, Approximation by Hill Functions. To appear.