Dauylbayev, M. K., and A. B. Uaissov. “Asymptotic Behavior of the Solutions of Boundary-value problems for Singularly Perturbed Integro-Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 71, no. 11, Nov. 2019, pp. 1466-79, https://umj.imath.kiev.ua/index.php/umj/article/view/1529.