NIEMINEN, T. On the norm of a linear operator in a Hilbert space. Annales Fennici Mathematici, [S. l.], n. 329, 1963. DOI: 10.5186/aasfm.1963.329. Disponível em: https://afm.journal.fi/article/view/134015. Acesso em: 26 nov. 2024.