RomanyukA. S. 1994. “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 46 (7), 915–926. https://umj.imath.kiev.ua/index.php/umj/article/view/5683.