@article{DE_1969_5_11_a1,
     author = {V. A. Il'in},
     title = {Fourier series in fundamental systems of functions of the {Beltrami} operator},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1940--1978},
     publisher = {mathdoc},
     volume = {5},
     number = {11},
     year = {1969},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1969_5_11_a1/}
}