Vitrychenko, I. E. “Critical Cases of the π-Stability of a Nonautonomous Quasilinear Equation of the <em class="a-Plus-plus">n</em&gt;Th Order”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 56, no. 2, Feb. 2004, pp. 264-70, https://umj.imath.kiev.ua/index.php/umj/article/view/3750.