Konohrai, A. F. “Estimates for the Approximate Characteristics of the Classes $B_{p, \theta}^{\Omega}$ of Periodic Functions of Two Variables With Given Majorant of Mixed Moduli of Continuity”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 63, no. 2, Feb. 2011, pp. 176-8, https://umj.imath.kiev.ua/index.php/umj/article/view/2709.