1.
N. Butris R, ر. Periodic solutions of second-order nonlinear system of integro-differential equations unsolvable for the highest derivative. EDUSJ [Internet]. 2008 Mar. 1 [cited 2025 Aug. 4];21(1):78-89. Available from: https://rjps.uomosul.edu.iq/index.php/edusj/article/view/17274