Kuzhel', A. V. “Evolution of the Concept of the Characteristic Function of a Linear Operator”. Ukrains’kyi Matematychnyi Zhurnal 45, no. 6 (June 25, 1993): 731–743. Accessed December 5, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/5862.