@article{SM_1990_66_2_a13,
     author = {S. Z. Levendorskii},
     title = {On types of degenerate elliptic operators},
     journal = {Sbornik. Mathematics},
     pages = {523--540},
     publisher = {mathdoc},
     volume = {66},
     number = {2},
     year = {1990},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1990_66_2_a13/}
}