Vakarchuk, S. B. 2016. “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 68 (6): 723-45.
https://umj.imath.kiev.ua/index.php/umj/article/view/1874.