N.BUTRIS, R.; ر.; ADEL AZIZ, M.; میرنا. The existence and approximation of the periodic solutions for system of first order nonlinear differential equationsby using Lebesgue integrable. Journal of Education and Science (JES), [S. l.], v. 21, n. 1, p. 61–77, 2008. DOI: 10.33899/edusj.2008.51296. Disponível em: https://rjps.uomosul.edu.iq/index.php/edusj/article/view/17286. Acesso em: 4 aug. 2025.