Babenko, V. F., and S. V. Konareva. 2018. “Jackson – Stechkin-Type Inequalities for the Approximation of Elements of Hilbert Spaces”. Ukrains’kyi Matematychnyi Zhurnal 70 (9), 1155-65. https://umj.imath.kiev.ua/index.php/umj/article/view/1626.