PUTRUS, R.; رعد; ISAAC, A.; آن. 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 (JES), [S. l.], v. 19, n. 1, p. 190–202, 2007. DOI: 10.33899/edusj.2007.51143. Disponível em: https://rjps.uomosul.edu.iq/edusj/article/view/18289. Acesso em: 4 aug. 2025.