@article{IM2_1990_34_3_a1,
     author = {R. I. Grigorchuk},
     title = {On the topological and metric types of surfaces regularly covering a~closed surface},
     journal = {Izvestiya. Mathematics },
     pages = {517--553},
     publisher = {mathdoc},
     volume = {34},
     number = {3},
     year = {1990},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1990_34_3_a1/}
}