1. A. I. Stepanets, Methods of Approximation Theory [in Russian], Part 1, Institute of Mathematics, Ukrainian National Academy of Sciences, Kiev (2002).
2. A. Zygmund, Trigonometric Series [Russian translation], Vol. 2, Mir, Moscow (1965).
3. N. P. Korneichuk, Exact Constants in Approximation Theory [in Russian], Nauka, Moscow (1987).
4. S. N. Bernstein, “On the trigonometric interpolation by the least-squares method,” Dokl. Akad. Nauk SSSR, 4, 1–8 (1934).
5. S. M. Nikol’skii, “Asymptotic estimate for the remainder in the approximation by interpolating trigonometric polynomials,” Dokl. Akad. Nauk SSSR, 31, No. 3, 215–218 (1941).