@article{MS2_1987__61_2_186105,
     author = {Frank Forelli},
     title = {Two-sheeted coverings of the disc.},
     journal = {Mathematica Scandinavica},
     pages = {17--38},
     year = {1987},
     volume = {61},
     number = {2},
     zbl = {0663.30042},
     url = {http://geodesic.mathdoc.fr/item/MS2_1987__61_2_186105/}
}