Golub , A. P. “Convergence of Denominators of the Pade Joint Approximations of a Set of Confluent Hypergeometric Functions”. Ukrains’kyi Matematychnyi Zhurnal, vol. 40, no. 6, Nov. 1988, pp. 792-5, https://umj.imath.kiev.ua/index.php/umj/article/view/9294.