1. Lanczos, C.: Discourse on Fourier Series, Hafner Pub. Co., New York, 1966
2. Stein, E. M., Weiss, G.: Introduction to Fourier Analysis on Euclidean Spaces, Princeton University Press, Princeton, 1971
3. Timan, A. F.: Theory of Approximation of Functions of a Real Variable, Macmillan, New York, 1963
4. Lorentz, G. G.: Approximation of Functions, Holt, 2nd Ed., Chelsea Pub. Co., New York, 1986
5. Chui, C. K.: Approximation induced by a Fourier series. Rocky Mountain J. Math., 4, 643–648 (1974)