@article{DAN_1991_318_1_a1,
     author = {V. D. Budaev},
     title = {On the {Bessel} inequality for systems of root functions of},
     journal = {Doklady Akademii Nauk},
     pages = {16--20},
     publisher = {mathdoc},
     volume = {318},
     number = {1},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1991_318_1_a1/}
}