ETTAYB , J. Pseudospectra of the direct sum of linear operators in ultrametric Banach spaces. Bulletin of the Karaganda University. Mathematics Series, [S. l.], v. 116, n. 4, p. 107–118, 2024. DOI: 10.31489/2024m4/107-118. Disponível em: https://mts.buketov.edu.kz/index.php/mathematics-vestnik/article/view/886. Acesso em: 2 jul. 2025.