@article{DE_2000_36_2_a0,
     author = {N. V. Assonova and V. D. Budaev},
     title = {The {Hilbert} and {Bessel} properties of systems of eigenfunctions of second-order operators},
     journal = {Differencialʹnye uravneni\^a},
     pages = {147--151},
     publisher = {mathdoc},
     volume = {36},
     number = {2},
     year = {2000},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_2000_36_2_a0/}
}