1. Ahlberg, J. H., Nilson, E. N.: The approximation of linear functionals. SIAM J. Numer. Anal.3, 173?182 (1966)
2. Ahlberg, J. H., Nilson, E. N., Walsh, J. L.: The theory of splines and their applications. New York: Academic Press 1967
3. Davis, P. J.: Interpolation and approximation. New York: Blaisdell 1963
4. Boor, C. de: Bicubic spline interpolation. J. Math. and Phys.41, 212?218 (1962)
5. Boor, C. de, Lynch, R. E.: On splines and their minimum properties. J. Math. Mech.15, 953?969 (1966)