Di Beo, Luca, and Alexandr Prishlyak. “Flows With Minimal Number of Singularities in the Boy’s Surface”. Proceedings of the International Geometry Center 15, no. 1 (May 23, 2022): 32-49. Accessed March 29, 2024. //journals.ontu.edu.ua/index.php/geometry/article/view/2225.