@article{DE_2001_37_3_a1,
     author = {N. V. Assonova},
     title = {Hilbert and {Bessel} {Properties} of {Systems} of {Root} {Functions} of $2m${th-Order} {Operators}},
     journal = {Differencialʹnye uravneni\^a},
     pages = {299--309},
     publisher = {mathdoc},
     volume = {37},
     number = {3},
     year = {2001},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_2001_37_3_a1/}
}