@article{SM_1967_2_3_a3,
     author = {Yu. V. Egorov},
     title = {Pseudodifferential operators of principal type},
     journal = {Sbornik. Mathematics},
     pages = {319--333},
     publisher = {mathdoc},
     volume = {2},
     number = {3},
     year = {1967},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1967_2_3_a3/}
}