KHRISTOV, E.; MISHEV, J. On an application of the Crum-Krein transformations. Ann. Sofia Univ. Fac. Math. and Inf., [S. l.], v. 88, p. 485–501, 1996. Disponível em: https://stipendii.uni-sofia.bg/index.php/fmi/article/view/401. Acesso em: 21 nov. 2024.