RomanyukA. S. “On Estimates of Approximation Characteristics of the Besov Classes of Periodic Functions of Many Variables”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 49, no. 9, Sept. 1997, pp. 1250–1261, https://umj.imath.kiev.ua/index.php/umj/article/view/5124.