1. Ahlberg, J. H., Nilson, E. N., Walsh, J. L.: The Theory of Splines and Their Applications. New York: Academic Press 1967
2. Birkhoff, G., Gordon, W. J.: The draftsman's and related equations. J. of Approximation Theory, (1967)
3. Birkhoff, G., Schultz, M., Varga, R. S.: Smooth Hermite interpolation for rectangles with applications to elliptic differential equations. Numer. Math.11, 232?256 (1968)
4. Caprili, M., Cella, A., Gheri, G.: Spline interpolation techniques for variational methods. Int. J. Solids Struct.6, 565?576 (1973)
5. Carlson, R., Hall, C. A.: Error bounds for bicubic spline interpolation. J. of Approximation Theory7, 41?47 (1973)