1. A. G. Babenko, N. I. Chernykh, and V. T. Shevaldin, “The Jackson-Stechkin inequality in L2 with a trigonometric modulus of continuity,” Mat. Zametki, 65, No. 6, 928–932 (1999); English transl.: Math. Notes, 65, No. 5-6, 777–781 (1999).
2. F. Dai and Z. Ditzian, “Littlewood-Paley theory and sharp Marchaud inequality,” Acta Sci. Math. (Szëged), 71, 65–90 (2005).
3. F. Dai, Z. Ditzian, and S. Tikhonov, “Sharp Jackson inequalities,” J. Approxim. Theory, 151, 86–112 (2008).
4. R. A. DeVore and G. G. Lorentz, Constructive Approximation, Springer-Verlag, Berlin (1993).
5. Z. Ditzian, “On the Marchaud-type inequality,” Proc. Amer. Math. Soc., 103, 198–202 (1988).