Karandjulov, L. “Asymptotic Solution of Definite Class of Singularly Perturbed Linear Boundary-Value Problems for Ordinary Differential Equations”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 91, Dec. 1999, pp. 79-95, https://stipendii.uni-sofia.bg/index.php/fmi/article/view/271.