N.BUTRIS, R.; S.SEETO, A. Existence of a Periodic Solution for Nonlinear Systems of Differential Equations with Retarded Argument. Journal of Education and Science, [S. l.], v. 17, n. 1, p. 10–16, 2005. DOI: 10.33899/edusj.2005.80958. Disponível em: https://edusj.uomosul.edu.iq/index.php/edusj/article/view/41953. Acesso em: 10 sep. 2025.