Kalimbetov, B.T., and Zh.O. Habibullaev. 2015. “Regularization Method for Singularly Perturbed Integro-Differential Systems With Multiple Spectrum”. Bulletin of the Karaganda University. Mathematics Series 80 (4):91-97. https://mts.buketov.edu.kz/index.php/mathematics-vestnik/article/view/66.