BabenkoV. F., and KonarevaS. V. “Jackson – Stechkin-Type Inequalities for the Approximation of Elements of Hilbert Spaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 70, no. 9, Sept. 2018, pp. 1155-6, https://umj.imath.kiev.ua/index.php/umj/article/view/1626.