1. S. Karlin, Total Positivity, Vol. I, Standford University Press, 1968.
2. G. Pólya and G. Szegö, Aufgaben und Lehrsätze aus der analysis, Vol. II, Dover Reprint, 1945.
3. C. R. Hobby and J. R. Rice, Approximation from a Curve of Functions,Arch. Rational Mech. Anal.,24 (1967), 91–106.
4. P. J. Davis, Interpolation and Approximation, Blaisdell Publishing Company, 1963.
5. D. Braess, Approximation mit Exponentialsummen, Computing, Vol. 2, p. 309–321, 1967.