Vakarchuk, B. S. “Jackson-Type Inequalities With Generalized Modulus of Continuity and Exact Values of the $n$-Widths for the Classes of $(ψ,β)$-Differentiable Functions in $L_2$. III”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 68, no. 10, Oct. 2016, pp. 1299-1, https://umj.imath.kiev.ua/index.php/umj/article/view/1922.