@article{SM_1991_69_1_a9,
     author = {Yu. V. Egorov and V. A. Kondrat'ev},
     title = {On the negative spectrum of an elliptic operator},
     journal = {Sbornik. Mathematics},
     pages = {155--177},
     publisher = {mathdoc},
     volume = {69},
     number = {1},
     year = {1991},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1991_69_1_a9/}
}