@article{GDZPPN00206233X, title = {G Maps and the Projective Class Group}, journal = {Commentarii mathematici Helvetici}, volume = {51}, year = {1976}, pages = {611-626}, zbl = {0365.55005}, url = {http://dml.mathdoc.fr/item/GDZPPN00206233X} }
Petrie, Ted. G Maps and the Projective Class Group. Commentarii mathematici Helvetici, Tome 51 (1976) pp. 611-626. http://gdmltest.u-ga.fr/item/GDZPPN00206233X/