@article{DAN_1964_158_1_a5,
     author = {M. M. Gekhtman and I. V. Stankevich},
     title = {On the spectrum of non-selfadjoint differential operators},
     journal = {Doklady Akademii Nauk},
     pages = {29--32},
     publisher = {mathdoc},
     volume = {158},
     number = {1},
     year = {1964},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1964_158_1_a5/}
}