@article{DE_1991_27_12_a1,
     author = {V. D. Budaev},
     title = {Criteria for the {Bessel} property and the {Riesz} basis property of systems of root functions of differential operators. {I}},
     journal = {Differencialʹnye uravneni\^a},
     pages = {2033--2044},
     publisher = {mathdoc},
     volume = {27},
     number = {12},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1991_27_12_a1/}
}