Koshmanenko, V. D., and S. Ota. “On Characteristic Properties of Singular Operators”. Ukrains’kyi Matematychnyi Zhurnal 48, no. 11 (November 25, 1996): 1484-1493. Accessed May 19, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5202.