@article{MZM_1997_61_4_a15,
     author = {O. G. Parfenov},
     title = {Estimates of the spectrum of some differential operators},
     journal = {Matemati\v{c}eskie zametki},
     pages = {634--636},
     publisher = {mathdoc},
     volume = {61},
     number = {4},
     year = {1997},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1997_61_4_a15/}
}