Evtukhov, V. M., and M. A. Belozerova. “Asymptotic Representations of the Solutions of Essentially Nonlinear Nonautonomous Second-Order Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal, vol. 60, no. 3, Mar. 2008, pp. 310–331, https://umj.imath.kiev.ua/index.php/umj/article/view/3158.