Y. Y. Soroka, “Homeotopy groups for nonsingular foliations of the plane”, Ukr. Mat. Zhurn., vol. 69, no. 7, pp. 1000–1008, Jul. 2017, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/1753