Romanyuk, A. S. “Trigonometric Widths of the Classes $B_{p,θ}^ R$ of Functions of Many Variables in the Space $L_q$”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 50, no. 8, Aug. 1998, pp. 1089-97, https://umj.imath.kiev.ua/index.php/umj/article/view/4858.