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@article{IVM_1958_2_a11, author = {I. S. Kats and M. G. Krein}, title = {Criteria for the discreteness of the spectrum of a~singular string}, journal = {Izvesti\^a vys\v{s}ih u\v{c}ebnyh zavedenij. Matematika}, pages = {136--153}, publisher = {mathdoc}, number = {2}, year = {1958}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/IVM_1958_2_a11/} }
I. S. Kats; M. G. Krein. Criteria for the discreteness of the spectrum of a~singular string. Izvestiâ vysših učebnyh zavedenij. Matematika, no. 2 (1958), pp. 136-153. http://geodesic.mathdoc.fr/item/IVM_1958_2_a11/