VitrychenkoI. E. “Critical Cases of Stability of One Nonautonomous Essentially Nonlinear Equation of the <em class="a-Plus-plus">n</em&gt;Th Order”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 49, no. 5, May 1997, pp. 720–724, https://umj.imath.kiev.ua/index.php/umj/article/view/5054.