Akobirshoev, M. O., and M. Sh. Shabozov. 2009. “On Exact Values of Quasiwidths for Some Classes of Differentiable Periodic Functions of Two Variables”. Ukrains’kyi Matematychnyi Zhurnal 61 (6), 855-64. https://umj.imath.kiev.ua/index.php/umj/article/view/3063.