N.Butris, Raad, رعد, and Ayad S.Seeto. “Existence of a Periodic Solution for Nonlinear Systems of Differential Equations With Retarded Argument”. Journal of Education and Science (JES) 17, no. 1 (January 1, 2005): 10–16. Accessed August 4, 2025. https://rjps.uomosul.edu.iq/index.php/edusj/article/view/17810.