Moroz, M. V. “On the Existence of Periodic Solutions of a System of Two Differential Equations With Pulse Influence”. Ukrains’kyi Matematychnyi Zhurnal 54, no. 1 (January 25, 2002): 133–137. Accessed April 11, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4047.