Romanyuk, A. S. “On the Problem of Linear Widths of the Classes $B_{p,θ}^r$ of Periodic Functions of Many Variables”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 66, no. 7, July 2014, pp. 970–982, https://umj.imath.kiev.ua/index.php/umj/article/view/2192.