KOROL', I. I. On Periodic Solutions of One Class of Systems of Differential Equations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 57, n. 4, p. 483–495, 2005. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3615. Acesso em: 5 dec. 2025.