@article{DE_1973_9_1_a6,
     author = {V. A. Il'in},
     title = {Conditions for the convergence of spectral decompositions that correspond to selfadjoint extensions of elliptic operators. {IV.~Theorems} of negative type for an arbitrary extension of a general second order selfadjoint elliptic operator},
     journal = {Differencialʹnye uravneni\^a},
     pages = {49--73},
     publisher = {mathdoc},
     volume = {9},
     number = {1},
     year = {1973},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1973_9_1_a6/}
}