@article{DAN_1968_178_4_a7,
     author = {L. E. Isaev},
     title = {A certain class of operators with spectrum concentrated at zero},
     journal = {Doklady Akademii Nauk},
     pages = {783--785},
     publisher = {mathdoc},
     volume = {178},
     number = {4},
     year = {1968},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1968_178_4_a7/}
}