@article{GDZPPN00241225X, title = {Groupoid Operations and Fibre-Homotopy Equivalence. I.}, journal = {Mathematische Zeitschrift}, volume = {135}, year = {1973}, pages = {207-234}, zbl = {0253.55008}, url = {http://dml.mathdoc.fr/item/GDZPPN00241225X} }
Heath, Philip R. Groupoid Operations and Fibre-Homotopy Equivalence. I.. Mathematische Zeitschrift, Tome 135 (1973) pp. 207-234. http://gdmltest.u-ga.fr/item/GDZPPN00241225X/