Derev’yanko, N. V. “Approximation of the Classes $H_p^{Ω}$ of Periodic Functions of Many Variables in the Space $L_p$”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 66, no. 5, May 2014, pp. 634–644, https://umj.imath.kiev.ua/index.php/umj/article/view/2165.