A. O. Prishlyak and D. P. Lychak, “Morse functions and flows on nonorientable surfaces”, MFAT, vol. 15, no. 3, pp. 251–258, Sep. 2009, Accessed: Dec. 17, 2025. [Online]. Available: https://zen.imath.kiev.ua/index.php/mfat/article/view/422