Hristova, S. G., and D. D. Bainov. “Method for Solving the Periodic Problem for Integro-Differential Equations”. Anal. Numér. Théor. Approx. 19, no. 2 (August 1, 1990): 137–142. Accessed September 3, 2025. https://www.ictp.acad.ro/jnaat/journal/article/view/1990-vol19-no2-art7.