TerekhinM. T. (2001) “Existence of Small Periodic Solutions of Nonlinear Systems of Ordinary Differential Equations”, Ukrains’kyi Matematychnyi Zhurnal, 53(5), pp. 680-687. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/4289 (Accessed: 1September2024).