KovtonyukD. A., and RyazanovV. I. “On the Theory of Prime Ends for Space Mappings”. Ukrains’kyi Matematychnyi Zhurnal 67, no. 4 (April 25, 2015): 467-479. Accessed January 26, 2021. http://umj.imath.kiev.ua/index.php/umj/article/view/1997.