@article{DAN_1970_191_6_a10,
     author = {P. A. Frolov},
     title = {Topological structure of normally solvable elliptic operators with constant coefficients in the plane},
     journal = {Doklady Akademii Nauk},
     pages = {1238--1240},
     publisher = {mathdoc},
     volume = {191},
     number = {6},
     year = {1970},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1970_191_6_a10/}
}