“ON AN APPLICATION OF THE OPERATORS OF CRUMM-ICAEIN TO THE DECOMPOSITION IN PRODUCTS OF THE SOLUTIONS OF TWO EQUATIONS OF SCHRODINGER WITH EQUAL SPECTRA”. Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, vol. 82, no. 2, Dec. 1991, pp. 219-47, https://annual.uni-sofia.bg/index.php/fmi/article/view/545.