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, [S. l.], v. 17, n. 4, p. 85–96, 2005. DOI: 10.33899/edusj.2005.82967. Disponível em: https://edusj.uomosul.edu.iq/index.php/edusj/article/view/41239. Acesso em: 10 sep. 2025.