PUTRUS, R.; JAMEEL, G. The periodic solution for some systems of nonlinear integro-differential equations of Volterra type. Journal of Education and Science, [S. l.], v. 21, n. 2, p. 57–77, 2008. DOI: 10.33899/edusj.2008.51289. Disponível em: https://edusj.uomosul.edu.iq/index.php/edusj/article/view/42550. Acesso em: 10 sep. 2025.