Ignat'ev, A. O. “Bounds for the Periods of Periodic Solutions of Ordinary Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal 67, no. 11 (November 25, 2015): 1569–1573. Accessed December 5, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/2090.