1. D. Jackson, Über die Genauigkeit der Annaherung Stetiger Funktionen Durch Ganze Rationale Funktionen Gegebenen Grades und Trigonometrische Summen Gegebener Ordnung, Thesis, Gottingen (1911).
2. A. Zygmund, “Smooth functions,” Duke Math. J., 12, No. 1, 47–76 (1945).
3. N. I. Akhiezer, Lectures in Approximation Theory [in Russian], Nauka, Moscow (1965).
4. S. B. Stechkin, “On the order of the best approximations of continuous functions,” Izv. Akad. Nauk SSSR, Ser. Mat., 15, No. 3, 219–242 (1951).
5. V. K. Dzyadyk, Introduction to the Theory of Uniform Approximation of Functions by Polynomials [in Russian], Nauka, Moscow (1977).