Terekhin, M.T. (1994) “Nontrivial periodic solutions of a nonautonomous system of second-order differential equations”, Ukrains’kyi Matematychnyi Zhurnal, 46(6), pp. 754–759. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/5703 (Accessed: 10 April 2025).