StasyukS. A. “Trigonometric Widths of the Classes $B_{p,θ}^{Ω}$ of Periodic Functions of Many Variables”. Ukrains’kyi Matematychnyi Zhurnal 54, no. 5 (May 25, 2002): 700-705. Accessed December 10, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4109.