Hefter, S. L., and O. L. Piven’. “Infinite Order Differential Operators in the Module of Formal Generalized Functions and in a Ring of Formal Power Series”. Ukrains’kyi Matematychnyi Zhurnal 74, no. 6 (July 7, 2022): 784 - 799. Accessed April 20, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/6955.