1. Agmon, S.: Lectures on elliptic boundary value problems (291 pp.). Princeton: Van Nostrand 1965.
2. Ahlberg, J. H., Nilson, E. N., Walsh, J. L.: The theory of splines and their applications. New York: Academic Press 1967.
3. Aubin, J. P.: Behavior of the error of the approximate solution of boundary value problems for linear elliptic operators by Galerkin's and finite difference methods. Annali de la Scuola Normale de Pisa21, 599?637 (1967).
4. Bers, L., John, F., Schechter, M.: Partial differential equations (343 pp.). New York: Interscience 1964.
5. Blair, J. J.: Error bounds for spline approximation. Unpublished manuscript.