BilichenkoR. O. and BabenkoV. F. (2009) “Approximation of unbounded operators by bounded operators in a Hilbert space”, Ukrains’kyi Matematychnyi Zhurnal, 61(2), pp. 147-153. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/3010 (Accessed: 4December2024).