Korol’, I. I. “Investigation of the Periodic Solutions of Nonlinear Autonomous Systems in the Critical Case”. Ukrains’kyi Matematychnyi Zhurnal 60, no. 3 (March 25, 2008): 332–339. Accessed April 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3159.