@article{DAN_1977_233_4_a6,
     author = {L. A. Ivanov},
     title = {On lacunae for operators splitting into {Euler{\textendash}Poisson{\textendash}Darboux} factors},
     journal = {Doklady Akademii Nauk},
     pages = {535--538},
     year = {1977},
     volume = {233},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1977_233_4_a6/}
}