@article{DAN_1957_112_6_a2,
     author = {M. A. Krasnosel'skii and S. G. Krein and P. E. Sobolevskii},
     title = {On differential equations with unbounded operators in {Hilbert} space},
     journal = {Doklady Akademii Nauk},
     pages = {990--993},
     publisher = {mathdoc},
     volume = {112},
     number = {6},
     year = {1957},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1957_112_6_a2/}
}