MOROZ, M. V. On the Existence of Periodic Solutions of a System of Two Differential Equations with Pulse Influence. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 54, n. 1, p. 133–137, 2002. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4047. Acesso em: 6 dec. 2025.