@article{DE_1971_7_4_a9,
     author = {V. A. Il'in and Sh. A. Alimov},
     title = {Conditions for the convergence of spectral decompositions that correspond to self-adjoint extensions of elliptic operators. {I.} {Self-adjoint} extension of the {Laplace} operator with a point spectrum},
     journal = {Differencialʹnye uravneni\^a},
     pages = {670--710},
     publisher = {mathdoc},
     volume = {7},
     number = {4},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1971_7_4_a9/}
}