On an application of the Crum-Krein transformations. Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, [S. l.], v. 88, p. 485–501, 1996. Disponível em: https://annual.uni-sofia.bg/index.php/fmi/article/view/401. Acesso em: 15 dec. 2025.