@article{DAN_1964_154_6_a3,
     author = {M. G. Krein and G. K. Langer},
     title = {On the theory of quadratic pencils of self-adjoint operators},
     journal = {Doklady Akademii Nauk},
     pages = {1258--1261},
     publisher = {mathdoc},
     volume = {154},
     number = {6},
     year = {1964},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1964_154_6_a3/}
}