N.Butris, R., ر., M. Adel Aziz, and میرنا. “The Existence and Approximation of the Periodic Solutions for System of First Order Nonlinear Differential Equationsby Using Lebesgue Integrable”. Journal of Education and Science (JES), vol. 21, no. 1, Mar. 2008, pp. 61-77, doi:10.33899/edusj.2008.51296.