N.Butris R, ر., Adel Aziz M, میرنا. The existence and approximation of the periodic solutions for system of first order nonlinear differential equationsby using Lebesgue integrable. EDUSJ [Internet]. 2008 Mar. 1 [cited 2025 Aug. 4];21(1):61-77. Available from: https://rjps.uomosul.edu.iq/index.php/edusj/article/view/17286