BlatovI. A., PokornayaI. Y., and StryginV. V. “Collocation Method for Solving Singularly Perturbed Boundary-Value Problems by Using Cubic Splines”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 46, no. 4, Apr. 1994, pp. 411–417, https://umj.imath.kiev.ua/index.php/umj/article/view/5738.