Hatamleh, R., and V. A. Zolotarev. 2016. “Triangular Models of Commutative Systems of Linear Operators Close to Unitary Operators”. Ukrains’kyi Matematychnyi Zhurnal 68 (5), 694-711. https://umj.imath.kiev.ua/index.php/umj/article/view/1872.