SerdyukA. S. “Estimates of the Kolmogorov Widths for Classes of Infinitely Differentiable Periodic Functions”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 49, no. 12, Dec. 1997, pp. 1700–1706, https://umj.imath.kiev.ua/index.php/umj/article/view/5176.