@article{MZM_1971_9_4_a4,
     author = {I. I. Golichev},
     title = {Discreteness of the spectrum and completeness of systems of characteristic and associated vectors of second-order nonself-adjoint differential operators},
     journal = {Matemati\v{c}eskie zametki},
     pages = {401--408},
     publisher = {mathdoc},
     volume = {9},
     number = {4},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1971_9_4_a4/}
}