Derets, E. V. “On Lower Bounds for the Widths of Classes of Functions Defined by Integral Moduli of Continuity”.
Ukrains’kyi Matematychnyi Zhurnal
, vol. 52, no. 3, Mar. 2000, pp. 319-28,
https://umj.imath.kiev.ua/index.php/umj/article/view/4421
.