FESHCHENKO, I. S. On closeness of the sum of n subspaces of a Hilbert space. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 63, n. 10, p. 1381–1425, 2011. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/2814. Acesso em: 6 dec. 2025.