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. EDUSJ [Internet]. 2007 Jan. 1 [cited 2025 Aug. 4];19(1):190-202. Available from: https://rjps.uomosul.edu.iq/index.php/edusj/article/view/18289