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), 723-745.
https://umj.imath.kiev.ua/index.php/umj/article/view/1874