BOGNÁR, J. Non-negativity properties of operators in spaces with indefinite metric. Annales Fennici Mathematici, [S. l.], n. 336/10, 1964. DOI: 10.5186/aasfm.1964.336-10. Disponível em: https://afm.journal.fi/article/view/134032. Acesso em: 26 nov. 2024.