Kaplun, Y. I., and V. G. Samoilenko. “On Extendability of Solutions of Differential Equations to a Singular Set”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 55, no. 3, Mar. 2003, pp. 373-8, https://umj.imath.kiev.ua/index.php/umj/article/view/3911.