PichugovS. A. “Some Properties of the Moduli of Continuity of Periodic Functions in Metric Spaces”. Ukrains’kyi Matematychnyi Zhurnal 68, no. 12 (December 25, 2016): 1657-1664. Accessed January 26, 2021. http://umj.imath.kiev.ua/index.php/umj/article/view/1951.