@article{DE_1986_22_12_a5,
     author = {V. A. Il'in},
     title = {Necessary and sufficient conditions for being a {Riesz} basis of root vectors of second-order discontinuous operators},
     journal = {Differencialʹnye uravneni\^a},
     pages = {2059--2071},
     publisher = {mathdoc},
     volume = {22},
     number = {12},
     year = {1986},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1986_22_12_a5/}
}