Hladysh, B. I., and O. O. Prishlyak. “Functions With Nondegenerate Critical Points on the Boundary of the Surface”. Ukrains’kyi Matematychnyi Zhurnal 68, no. 1 (January 25, 2016): 28-37. Accessed March 29, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/1819.