Moskaleva, Yu. P., V. L. Ostrovskyi, and K. A. Yusenko. 2007. “On Quadruples of Linearly Connected Projections and Transitive Systems of Subspaces”. Methods of Functional Analysis and Topology 13 (1): 43-49. https://zen.imath.kiev.ua/index.php/mfat/article/view/335.