Prishlyak, O. O. “Conjugacy of Morse Functions on Surfaces With Values on a Straight Line and Circle”. Ukrains’kyi Matematychnyi Zhurnal 52, no. 10 (October 25, 2000): 1421–1425. Accessed April 7, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4547.