Babenko V, Parfinovych N, Skorokhodov D. The best approximation of closed operators by bounded operators in Hilbert spaces. Carpathian Math. Publ. [Internet]. 2022 Dec. 30 [cited 2024 Nov. 22];14(2):453-6. Available from: https://vps.pnu.edu.ua/index.php/cmp/article/view/6161