Ignat'ev, A.O. (1993) “On the existence of Lyapunov functions in problems of stability of integral sets”,
Ukrains’kyi Matematychnyi Zhurnal, 45(7), pp. 932–941. Available at:
https://umj.imath.kiev.ua/index.php/umj/article/view/5886 (Accessed: 7 April 2025).