@article{DE_1987_23_10_a12,
     author = {M. D. Kovalenko},
     title = {Biorthogonal expansions in eigenfunctions. {I}},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1764--1772},
     publisher = {mathdoc},
     volume = {23},
     number = {10},
     year = {1987},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1987_23_10_a12/}
}