PUTRUS, R.; ISAAC, A. The existence and approximation of the periodic solution of a system of nonlinear differential-differential equations of the first order with independent variable delay with boundary conditions. Journal of Education and Science, [S. l.], v. 19, n. 1, p. 190–202, 2007. DOI: 10.33899/edusj.2007.51143. Disponível em: https://edusj.uomosul.edu.iq/index.php/edusj/article/view/42437. Acesso em: 10 sep. 2025.