1. A. G. Babenko, “On the exact constant in the Jackson inequality in L 2,” Mat. Zamet., 39, No. 5, 651–664 (1986).
2. N. I. Chernykh, “On the best approximation of periodic functions by trigonometric polynomials in L 2,” Mat. Zamet., 2, No. 2, 513–522 (1967).
3. N. I. Chernykh, “On the Jackson inequality in L 2,” Trudy Mat. Inst. AN SSSR, 88, 71–74 (1967).
4. M. G. Esmaganbetov, “The widths of classes from L 2[0, 2π] and the minimization of exact constants in Jackson-type inequalities,” Mat. Zamet., 66, No. 6, 816–820 (1999).
5. V. I. Ivanov and O. I. Smirnov, Jackson Constants and Young Constants in the Spaces L p [in Russian], Tula Univ., Tula, 1995.