@article{CMH_1992__67_3_140269,
     author = {R. Piergallini},
     title = {Covering homotopy 3-spheres.},
     journal = {Commentarii mathematici Helvetici},
     pages = {287--292},
     publisher = {mathdoc},
     volume = {67},
     number = {3},
     year = {1992},
     zbl = {0759.57002},
     url = {http://geodesic.mathdoc.fr/item/CMH_1992__67_3_140269/}
}