1. Jackson, D.: Ueber die Genauigkeit der Annäherung stetiger Funktionen durch game rationale Funktionen gegebenen Grades und trigonometrische Summen gegebenen Ordnung, Göttingen, 1911. Thesis.
2. Nikol’skiǐ, S.M.: Approximation of functions of several variables and imbedding theorems, Springer, 1975 (translated from the Russian).
3. Bernshteǐn, S.N.: ‘On the best approximation of continuous functions by polynomials of a given degree’, in Collected works, Vol. 1, Moscow, 1952, pp 11–104 (in Russian).
4. Korneǐchuk, N.R: Extremal problems in approximation theory, Moscow, 1976 (in Russian).
5. Lorentz, G.G.: Approximation of functions, Holt, Rinehart & Winston, 1966.