@article{IM2_1989_32_3_a4,
     author = {S. Z. Levendorskii},
     title = {On~the symbols of degenerate elliptic differential and hypoelliptic pseudodifferential operators},
     journal = {Izvestiya. Mathematics},
     pages = {543--561},
     year = {1989},
     volume = {32},
     number = {3},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1989_32_3_a4/}
}