1. A. I. Kozko and A. V. Rozhdestvenskii, “On Jackson’s inequality in L 2 with a generalized modulus of continuity,” Mat. Sb. 195(8), 3–46 (2004) [Sb. Math. 195 (8), 1073–1115 (2004)].
2. A. I. Kozko and A. V. Rozhdestvenskii, “On Jackson’s inequality with a generalized modulus of continuity,” Mat. Zametki 73(5), 783–788 (2003) [Math. Notes 73 (5), 736–741 (2003)].
3. N. I. Chernykh, “On Jackson’s inequality in L 2,” in Trudy Mat. Inst. Steklov, Vol. 88: Approximation of Functions in the Mean, Collection of papers (Nauka, Moscow, 1967), pp. 71–74 [Proc. Steklov Inst. Math. 88, 75–78 (1967)].
4. V. V. Arestov and N. I. Chernykh, “On the L 2-approximation of periodic functions by trigonometric polynomials,” in Approximation and Functions Spaces (North-Holland, Amsterdam, 1981), pp. 25–43.
5. N. I. Chernykh, “Best approximation of periodic functions by trigonometric polynomials in L 2,” Mat. Zametki 2(5), 513–522 (1967) [Math. Notes 2 (5), 803–808 (1968) (1967)].