Khristov, Evgeni, and Jordan Mishev. “On an Application of the Crum-Krein Transformations”. Ann. Sofia Univ. Fac. Math. and Inf. 88 (December 12, 1996): 485–501. Accessed December 4, 2024. https://ftl5.uni-sofia.bg/index.php/fmi/article/view/401.