Shabozov, M. Sh. 2011. “Exact Jackson - Stechkin-Type Inequalities for 2π -Periodic Functions in L 2 and Widths of Some Classes of Functions”. Ukrains’kyi Matematychnyi Zhurnal 63 (10): 1434-40. https://umj.imath.kiev.ua/index.php/umj/article/view/2816.