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 (JES), [S. l.], v. 17, n. 1, p. 10–16, 2005. DOI: 10.33899/edusj.2005.80958. Disponível em: https://ojs.uomosul.edu.iq/index.php/edusj/article/view/23827. Acesso em: 6 aug. 2025.