Romanyshyn, I. M., and L. A. Synyts’kyi. “Construction of Approximations for a Stationary Solution of a System of Singular Ordinary Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 49, no. 6, June 1997, pp. 774–778, https://umj.imath.kiev.ua/index.php/umj/article/view/5061.