AtlasiukO. M., and MikhailetsV. A. “Fredholm One-Dimensional Boundary-Value Problems with parameter in Sobolev Spaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 70, no. 11, Nov. 2018, pp. 1457-65, http://umj.imath.kiev.ua/index.php/umj/article/view/1649.