Zernov, A. E., & Kuzina, Y. V. (2002). Asymptotic Behavior of Solutions of the Cauchy Problem x′ = f(t, x, x′), x(0) = 0. Ukrains’kyi Matematychnyi Zhurnal, 54(12), 1698-1703. https://umj.imath.kiev.ua/index.php/umj/article/view/4209