@article{SM_1987_56_1_a13,
     author = {Yu. A. Semenov},
     title = {On the spectral theory of second-order elliptic differential operators},
     journal = {Sbornik. Mathematics},
     pages = {221--247},
     publisher = {mathdoc},
     volume = {56},
     number = {1},
     year = {1987},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1987_56_1_a13/}
}