Hladysh, B. I., and O. O. Prishlyak. “Deformations in the General Position of the Optimal functions on Oriented Surfaces With Boundary”. Ukrains’kyi Matematychnyi Zhurnal 71, no. 8 (August 25, 2019): 1028-1039. Accessed October 5, 2022. http://umj.imath.kiev.ua/index.php/umj/article/view/1495.