Khristov, E. and Mishev, J. (1996) “On an application of the Crum-Krein transformations”, Ann. Sofia Univ. Fac. Math. and Inf, 88, pp. 485–501. Available at: https://ftl5.uni-sofia.bg/index.php/fmi/article/view/401 (Accessed: 4 December 2024).