@article{DE_1970_6_1_a1,
     author = {V. A. Il'in},
     title = {A lower bound of the sum of the squares of the fundamental functions, and of the number of the fundamental values, for an arbitrary $FSF$ of {Laplace's} operator},
     journal = {Differencialʹnye uravneni\^a},
     pages = {7--16},
     publisher = {mathdoc},
     volume = {6},
     number = {1},
     year = {1970},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1970_6_1_a1/}
}