@article{DAN_1977_234_2_a7,
     author = {S. S. Kutateladze},
     title = {Extremal operators},
     journal = {Doklady Akademii Nauk},
     pages = {291--293},
     publisher = {mathdoc},
     volume = {234},
     number = {2},
     year = {1977},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1977_234_2_a7/}
}