Feshchenko, I.S. (2011) “On closeness of the sum of n subspaces of a Hilbert space”, Ukrains’kyi Matematychnyi Zhurnal, 63(10), pp. 1381–1425. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/2814 (Accessed: 5 December 2025).