@article{DE_1971_7_4_a2,
     author = {G. V. Zhdanova},
     title = {Asymptotic series for the eigenvalues and eigenfunctions of second order self-adjoint operators},
     journal = {Differencialʹnye uravneni\^a},
     pages = {603--614},
     publisher = {mathdoc},
     volume = {7},
     number = {4},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1971_7_4_a2/}
}