@article{SM_1970_11_3_a0,
     author = {Yu. V. Egorov},
     title = {Nondegenerate subelliptic pseudodifferential operators},
     journal = {Sbornik. Mathematics},
     pages = {291--309},
     publisher = {mathdoc},
     volume = {11},
     number = {3},
     year = {1970},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1970_11_3_a0/}
}