Ignatyev, A. O. “Application of the Direct Lyapunov’s Method to the Investigation of Integral Sets ”.
Ukrains’kyi Matematychnyi Zhurnal 44, no. 10 (October 1, 1992): 1342–1348. Accessed April 14, 2025.
https://umj.imath.kiev.ua/index.php/umj/article/view/8230.