Bobochko, V. N. “Turning Point in a System of Differential Equations With Analytic Operator”. Ukrains’kyi Matematychnyi Zhurnal 48, no. 2 (February 25, 1996): 147–160. Accessed April 16, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/5343.