VitrychenkoI. E. “On a Special Critical Case of Stability of a Nonautonomous Essentially Nonlinear System”. Ukrains’kyi Matematychnyi Zhurnal 57, no. 12 (December 25, 2005): 1711–1718. Accessed July 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3721.