Vitrychenko, I. E. “On a Special Critical Case of Stability of a Nonautonomous Essentially Nonlinear System”. Ukrains’kyi Matematychnyi Zhurnal, vol. 57, no. 12, Dec. 2005, pp. 1711–1718, https://umj.imath.kiev.ua/index.php/umj/article/view/3721.