ShabozovM. S. “Exact Jackson - Stechkin-Type Inequalities for 2π -Periodic Functions in <i>L </i><sub>2 </sub&gt; And Widths of Some Classes of Functions”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 63, no. 10, Oct. 2011, pp. 1434-40, https://umj.imath.kiev.ua/index.php/umj/article/view/2816.