A. A. Yusenko, “Quadruples of orthoprojectors connected by a linear relationship”, Ukr. Mat. Zhurn., vol. 62, no. 2, pp. 255–264, Feb. 2010, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/2861