@article{DAN_1989_305_6_a2,
     author = {V. A. Gurvich and G. B. Shabat},
     title = {Maps of surfaces and their schemes},
     journal = {Doklady Akademii Nauk},
     pages = {1298--1303},
     publisher = {mathdoc},
     volume = {305},
     number = {6},
     year = {1989},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1989_305_6_a2/}
}