Derets, E. V. “On Lower Bounds for the Widths of Classes of Functions Defined by Integral Moduli of Continuity”. Ukrains’kyi Matematychnyi Zhurnal 52, no. 3 (March 25, 2000): 319–328. Accessed April 27, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4421.