Ignatyev, A.O. (1992) “Application of the direct Lyapunov’s method to the investigation of integral sets ”, Ukrains’kyi Matematychnyi Zhurnal, 44(10), pp. 1342–1348. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/8230 (Accessed: 5 December 2025).