@article{DAN_1957_114_6_a6,
     author = {S. G. Krein},
     title = {On correctness classes for certain boundary problems},
     journal = {Doklady Akademii Nauk},
     pages = {1162--1165},
     publisher = {mathdoc},
     volume = {114},
     number = {6},
     year = {1957},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1957_114_6_a6/}
}