1. V. M. Tikhomirov, Usp. Mat. Nauk 15(3), 81–120 (1960).
2. K. I. Babenko, Izv. Akad. Nauk SSSR, Ser. Mat. 22, 631–640 (1958).
3. L. V. Taikov, Mat. Zametki 22, 285–295 (1977).
4. A. Pinkus, n-Widths in Approximation Theory (Springer-Verlag, Berlin, 1985).
5. N. Ainulloev and L. V. Taikov, Mat. Zametki 40(3), 341–351 (1986).