VitrychenkoI. E. 1997. “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 49 (5), 720–724. https://umj.imath.kiev.ua/index.php/umj/article/view/5054.