@article{GDZPPN002068346, title = {Covering homotopy 3-spheres.}, journal = {Commentarii mathematici Helvetici}, volume = {67}, year = {1992}, pages = {287-292}, zbl = {0759.57002}, url = {http://dml.mathdoc.fr/item/GDZPPN002068346} }
Piergallini, R. Covering homotopy 3-spheres.. Commentarii mathematici Helvetici, Tome 67 (1992) pp. 287-292. http://gdmltest.u-ga.fr/item/GDZPPN002068346/