VakarchukS. B. 2016. “Jackson-Type Inequalities With Generalized Modulus of Continuity and Exact Values of the $n$-Widths of the Classes of $(ψ,β)$-Differentiable Functions in $L_2$. II”. Ukrains’kyi Matematychnyi Zhurnal 68 (8), 1021-36. https://umj.imath.kiev.ua/index.php/umj/article/view/1899.