Ignat’evA. O. “On the Existence of Lyapunov Functions in Problems of Stability of Integral Sets”. Ukrains’kyi Matematychnyi Zhurnal 45, no. 7 (July 25, 1993): 932–941. Accessed September 1, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5886.