Mokhonko, A. Z., and A. A. Mokhonko. “Deficiency Values for the Solutions of Differential Equations With Branching Point”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 66, no. 7, July 2014, pp. 939–957, https://umj.imath.kiev.ua/index.php/umj/article/view/2190.