TerekhinM. T. “Existence of Small Periodic Solutions of Nonlinear Systems of Ordinary Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal 53, no. 5 (May 25, 2001): 680-687. Accessed November 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4289.