PUTRUS, R.; JAMEEL, G. The periodic solution for some systems of nonlinear integro-differential equations of Volterra type. Journal of Education and Science (JES), [S. l.], v. 21, n. 2, p. 57–77, 2008. DOI: 10.33899/edusj.2008.51289. Disponível em: https://ojs.uomosul.edu.iq/index.php/edusj/article/view/24428. Acesso em: 7 aug. 2025.