@article{EXMATH_2007_2007_1_a22,
     author = {Santos, J. C. },
     title = {When is a group homomorphism a covering homomorphism? },
     journal = {Extracta mathematicae},
     pages = {325-330},
     publisher = {mathdoc},
     volume = {22},
     number = {3},
     year = {2007},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EXMATH_2007_2007_1_a22/}
}