@article{DAN_1970_191_2_a1,
     author = {V. A. Il'in and A. F. Filippov},
     title = {The character of the spectrum of a selfadjoint extension of the {Laplace} operator in a bounded region (fundamental},
     journal = {Doklady Akademii Nauk},
     pages = {267--269},
     year = {1970},
     volume = {191},
     number = {2},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1970_191_2_a1/}
}