@article{IM2_1972_6_5_a7,
     author = {F. A. Berezin},
     title = {Covariant and contravariant symbols of operators},
     journal = {Izvestiya. Mathematics },
     pages = {1117--1151},
     publisher = {mathdoc},
     volume = {6},
     number = {5},
     year = {1972},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1972_6_5_a7/}
}