Putrus, Raad, and Ghada Jameel. “The Periodic Solution for Some Systems of Nonlinear Integro-Differential Equations of Volterra Type”. Journal of Education and Science 21, no. 2 (June 1, 2008): 57–77. Accessed September 10, 2025. https://edusj.uomosul.edu.iq/index.php/edusj/article/view/42550.