ADEL AZIZ, M.; BUTRIS, R. The existence and approximation of the periodic solutions for system of second order nonlinear differential equationsby using Lebesgue integrable. Journal of Education and Science (JES), [S. l.], v. 19, n. 4, p. 80–106, 2007. DOI: 10.33899/edusj.2007.5922. Disponível em: https://rjps.uomosul.edu.iq/index.php/edusj/article/view/18354. Acesso em: 4 aug. 2025.