1. M. Š. Birman and M. Z. Solomjak: Piecewise polynomial approximations of functions of classes Wp. (Russian) Mat. Sb. 73 (115) (1967), 331–355.
2. A. N. Kolmogorov and W. M. Tichomirow: ɛ-entropy and ɛ-capacity of sets in function spaces. (Russian) Uspehi Mat. Nauk 14 (86) (1959), 3–86. German translation in Arbeiten zur Informations-theorie III, Berlin 1960.
3. H. König: Grenzordnungen von Operatorenidealen I, II. Math. Ann. 212 (1974), 51–77.
4. G. G. Lorentz: Metric entropy and approximation. Bull. Amer. Math. Soc. 72 (1966), 903–937.
5. G. G. Lorentz: Approximation of functions. New York, 1966.