Kruglov, V. E. “Solution of a Linear Second-Order Differential Equation With Coefficients Analytic in the Vicinity of a Fuchsian Zero Point”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 64, no. 10, Oct. 2012, pp. 1381-93, https://umj.imath.kiev.ua/index.php/umj/article/view/2666.