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