Yakovets, V. P. “Asymptotics of the General Solution to a Linear Singularly Perturbed System of Second-Order Differential Equations With Degeneracy”. Ukrains’kyi Matematychnyi Zhurnal 45, no. 4 (April 25, 1993): 561–575. Accessed December 6, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/5843.