PETRUS, R.; JAMEEL, G. The existence and convergence of the periodic solution of the system of complementary equations - nonlinear differential of the second class type Volterra. Journal of Education and Science, [S. l.], v. 18, n. 1, p. 124–137, 2006. DOI: 10.33899/edusj.2006.78661. Disponível em: https://ojs.uomosul.edu.iq/index.php/edusj/article/view/41535. Acesso em: 7 sep. 2025.