@article{TM_1990_192_a5,
     author = {Yu. V. Egorov and V. A. Kondrat'ev},
     title = {On the negative spectrum of an elliptic operator},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     pages = {61--67},
     publisher = {mathdoc},
     volume = {192},
     year = {1990},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/TM_1990_192_a5/}
}