@article{CMH_1957__32_139154,
     author = {John Milnor},
     title = {On the existence of a connection with curvature zero.},
     journal = {Commentarii mathematici Helvetici},
     pages = {215--223},
     year = {1957},
     volume = {32},
     zbl = {0196.25101},
     url = {http://geodesic.mathdoc.fr/item/CMH_1957__32_139154/}
}