BUTRIS, R.; ر. Existence of a periodic solutions for certain system of nonlinear integro-differential equations. Journal of Education and Science (JES), [S. l.], v. 21, n. 2, p. 103–117, 2008. DOI: 10.33899/edusj.2008.51249. Disponível em: https://rjps.uomosul.edu.iq/index.php/edusj/article/view/18421. Acesso em: 4 aug. 2025.