Kalimbetov, B.T., and Zh.O. Habibullaev. “Regularization Method for Singularly Perturbed Integro-Differential Systems With Multiple Spectrum”. Bulletin of the Karaganda University. Mathematics Series, vol. 80, no. 4, Dec. 2015, pp. 91-97, https://mts.buketov.edu.kz/index.php/mathematics-vestnik/article/view/66.