Baranovich, A. M., and Yu. L. Daletskii. “Some Geometric-Differential Models in the Class of Formal Operator Power Series”.
Ukrains’kyi Matematychnyi Zhurnal 48, no. 6 (June 25, 1996): 739–746. Accessed April 16, 2025.
https://umj.imath.kiev.ua/index.php/umj/article/view/5274.