RomanyukA. S. “On Kolmogorov Widths of Classes $B^r_{p, \theta}$ of Periodic Functions of Many Variables With Low Smoothness in the Space $L_q$”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 46, no. 7, July 1994, pp. 915–926, https://umj.imath.kiev.ua/index.php/umj/article/view/5683.