Samoilenko, Y. S., and O. V. Strilets. “On Simple $n$-Tuples of Subspaces of a Hilbert Space”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 61, no. 12, Dec. 2009, pp. 1668-03, https://umj.imath.kiev.ua/index.php/umj/article/view/3130.