Krein, S. G., and I. V. Sapronov. “One Class of Solutions of Volterra Equations With Regular Singularity”. Ukrains’kyi Matematychnyi Zhurnal, vol. 49, no. 3, Mar. 1997, pp. 424–432, https://umj.imath.kiev.ua/index.php/umj/article/view/5015.