@article{IM2_1980_15_1_a0,
     author = {O. D. Apyshev and M. Otelbaev},
     title = {On the spectrum of a~class of differential operators and some imbedding theorems},
     journal = {Izvestiya. Mathematics },
     pages = {1--24},
     publisher = {mathdoc},
     volume = {15},
     number = {1},
     year = {1980},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1980_15_1_a0/}
}