Subbotin, Y. N., and S. A. Telyakovskii. “On Relative Widths of Classes of Differentiable Functions. II”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 62, no. 3, Mar. 2010, pp. 423–431, https://umj.imath.kiev.ua/index.php/umj/article/view/2877.