N. BUTRIS, R.; ر. Periodic solutions of second-order nonlinear system of integro-differential equations unsolvable for the highest derivative. Journal of Education and Science (JES), [S. l.], v. 21, n. 1, p. 78–89, 2008. DOI: 10.33899/edusj.2008.51297. Disponível em: https://rjps.uomosul.edu.iq/index.php/edusj/article/view/17274. Acesso em: 4 aug. 2025.