@article{DAN_1963_151_5_a4,
     author = {M. G. Gasymov and B. M. Levitan},
     title = {On the sum of the differences of the eigenvalues of two singular {Sturm{\textendash}Liouville} operators},
     journal = {Doklady Akademii Nauk},
     pages = {1014--1017},
     year = {1963},
     volume = {151},
     number = {5},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1963_151_5_a4/}
}