Feshchenko, I. S. “On Closeness of the Sum of N Subspaces of a Hilbert Space”. Ukrains’kyi Matematychnyi Zhurnal 63, no. 10 (October 25, 2011): 1381–1425. Accessed December 5, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/2814.