NIEMINEN, T. A condition for the selfadjointness of a linear operator. Annales Fennici Mathematici, [S. l.], n. 316, 1963. DOI: 10.5186/aasfm.1963.316. Disponível em: https://afm.journal.fi/article/view/134002. Acesso em: 26 nov. 2024.