Y. S. Samoilenko and R. V. Grushevoi, “Systems of one-dimensional subspaces of a Hilbert space”, MFAT, vol. 16, no. 2, pp. 131–139, Jun. 2010, Accessed: Dec. 16, 2025. [Online]. Available: https://zen.imath.kiev.ua/index.php/mfat/article/view/446