@article{FAA_1975_9_1_a4,
     author = {G. A. Isaev},
     title = {Numerical range of operator pencils and keldysh multiple completeness},
     journal = {Funkcionalʹnyj analiz i ego prilo\v{z}eni\^a},
     pages = {31--34},
     publisher = {mathdoc},
     volume = {9},
     number = {1},
     year = {1975},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/FAA_1975_9_1_a4/}
}