Vitrychenko, I. E. 1999. “On the Instability of One Nonautonomous Essentially Nonlinear Equation of the<em class="a-Plus-plus">n</em&gt;Th Order”. Ukrains’kyi Matematychnyi Zhurnal 51 (6), 835–841. https://umj.imath.kiev.ua/index.php/umj/article/view/4670.