PETRUS, R.; CETO, A. The periodic solution of a particular system of integrative-differential and nonlinear equations with independent variable delay. Journal of Education and Science (JES), [S. l.], v. 17, n. 4, p. 85–96, 2005. DOI: 10.33899/edusj.2005.82967. Disponível em: https://ojs.uomosul.edu.iq/index.php/edusj/article/view/23099. Acesso em: 6 aug. 2025.