@article{DAN_1973_211_3_a2,
     author = {I. S. Kats},
     title = {Density of the spectrum of a string},
     journal = {Doklady Akademii Nauk},
     pages = {520--523},
     publisher = {mathdoc},
     volume = {211},
     number = {3},
     year = {1973},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1973_211_3_a2/}
}