Stasyuk, S. A. 2002. “Trigonometric Widths of the Classes $B_{p,θ}^{Ω}$ of Periodic Functions of Many Variables”. Ukrains’kyi Matematychnyi Zhurnal 54 (5), 700-705. https://umj.imath.kiev.ua/index.php/umj/article/view/4109.