A. O. Ignat'ev, “On the existence of Lyapunov functions in problems of stability of integral sets”, Ukr. Mat. Zhurn., vol. 45, no. 7, pp. 932–941, Jul. 1993, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5886