N.BUTRIS, R.; ADEL AZIZ, M. 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://ojs.uomosul.edu.iq/index.php/edusj/article/view/23278. Acesso em: 6 aug. 2025.