BILICHENKO, R. O.; BABENKO, V. F. Approximation of unbounded operators by bounded operators in a Hilbert space. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 61, n. 2, p. 147–153, 2009. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3010. Acesso em: 14 apr. 2025.