@article{DAN_1961_137_4_a0,
     author = {M. Sh. Birman},
     title = {Perturbation of the spectrum of a singular elliptic operator under variation of the boundary and boundary conditions},
     journal = {Doklady Akademii Nauk},
     pages = {761--763},
     publisher = {mathdoc},
     volume = {137},
     number = {4},
     year = {1961},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1961_137_4_a0/}
}