M. T. Terekhin, “Existence of Small Periodic Solutions of Nonlinear Systems of Ordinary Differential Equations”, Ukr. Mat. Zhurn., vol. 53, no. 5, pp. 680–687, May 2001, Accessed: Apr. 13, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4289