@article{DE_1978_14_3_a1,
     author = {S. K. Voronina},
     title = {A necessary condition for the system of eigen- and associated functions of an ordinary differential operator to be a basis in $L_2(0,1)$},
     journal = {Differencialʹnye uravneni\^a},
     pages = {407--417},
     publisher = {mathdoc},
     volume = {14},
     number = {3},
     year = {1978},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1978_14_3_a1/}
}