VakarchukS. B. “Jackson-Type Inequalities and Exact Values of Widths of Classes of Functions in the Spaces $S^p , 1 ≤ P < ∞$”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 56, no. 5, May 2004, pp. 595–605, https://umj.imath.kiev.ua/index.php/umj/article/view/3780.