Vakarchuk, S. B. “Jackson-Type Inequalities With Generalized Modulus of Continuity and Exact Values of the $n$-Widths of the Classes of $(ψ,β)$-Differential Functions in $L_2$. I”.
Ukrains’kyi Matematychnyi Zhurnal, vol. 68, no. 6, June 2016, pp. 723-45,
https://umj.imath.kiev.ua/index.php/umj/article/view/1874.