1. Über die beste Annähergung von Funktionen einer gegebenen Funktionklasse;Kolmogorov;Ann. Math.,1936
2. Metric entropy, widths and superpositions of functions;Lorentz;Amer. Math. Monthly,1962
3. The widths of sets in functional spaces and the theory of best approximations;Tihomirov;Uspehi Mat. Nauk.,1960
4. The widths of sets in functional spaces and the theory of best approximations;Tihomirov;Uspehi Mat. Nauk. M. R.,1960
5. Best n-dimensional approximation to sets of functions;Brown,1964