BUTRIS, R. 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://ojs.uomosul.edu.iq/index.php/edusj/article/view/24420. Acesso em: 7 aug. 2025.