@article{SM_1970_11_4_a4,
     author = {G. I. \`Eskin},
     title = {Degenerate elliptic pseudodifferential equations of principal type},
     journal = {Sbornik. Mathematics},
     pages = {539--582},
     publisher = {mathdoc},
     volume = {11},
     number = {4},
     year = {1970},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1970_11_4_a4/}
}