Ettayb , J. “Pseudospectra of the Direct Sum of Linear Operators in Ultrametric Banach Spaces”. Bulletin of the Karaganda University. Mathematics Series 116, no. 4 (December 30, 2024): 107–118. Accessed July 2, 2025. https://mts.buketov.edu.kz/index.php/mathematics-vestnik/article/view/886.