Zavizion, G. V., and N. I. ShkiI. “Systems of Singularly Perturbed Degenerate Integro-Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal 51, no. 12 (December 25, 1999): 1694–1703. Accessed June 16, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4772.