1. Barnhill, R. E., Birkhoff, G., Gordon, W. J.: Smooth interpolation in triangles. General Motors Research Report GMR-1064, Feb. 1971. To appear in Journal on Approximation Theory
2. Birkhoff, G.: Piecewise bicubic interpolation and approximation in polygons. In: Approximations with Special Emphasis on Spline Functions. Edited by I. J. Schoenberg. Academic Press 1969
3. Carlson, R., Hall, C.: Ritz approximations in two-dimensional boundary value problems. Numer. Math.18, 171?181 (1971)
4. Cullen, C. G.: Matrices and linear transformations. Reading: Addison-Wesley 1966
5. Davis, P. J.: Interpolation and approximation. London: Blaisdell 1963