@article{DE_1980_16_10_a14,
     author = {V. N. Shapovalov},
     title = {Separation of variables in a second-order linear differential equation},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1864--1874},
     publisher = {mathdoc},
     volume = {16},
     number = {10},
     year = {1980},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1980_16_10_a14/}
}