Omel’chenkoP. V. “On Reduction of Block Matrices in a Hilbert Space”. Ukrains’kyi Matematychnyi Zhurnal 61, no. 10 (October 25, 2009): 1338-1347. Accessed November 22, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3105.