@article{ZNSL_1988_167_a9,
     author = {A. N. Protopopov},
     title = {Topological classification of branched coverings of the two-dimensional sphere},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {135--156},
     publisher = {mathdoc},
     volume = {167},
     year = {1988},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1988_167_a9/}
}