KoshmanenkoV. D., and OtaS. “On Characteristic Properties of Singular Operators”. Ukrains’kyi Matematychnyi Zhurnal 48, no. 11 (November 25, 1996): 1484-1493. Accessed September 1, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5202.