Vitrychenko, I. E. “On the Instability of One Nonautonomous Essentially Nonlinear Equation of Thenth Order”. Ukrains’kyi Matematychnyi Zhurnal, vol. 51, no. 6, June 1999, pp. 835–841, https://umj.imath.kiev.ua/index.php/umj/article/view/4670.