@article{CMH_1938__11_138717,
     author = {Paul Finsler},
     title = {\"Uber {Freisysteme} (lineare {Freigebilde).}},
     journal = {Commentarii mathematici Helvetici},
     pages = {62--76},
     year = {1938},
     volume = {11},
     zbl = {0019.32503},
     url = {http://geodesic.mathdoc.fr/item/CMH_1938__11_138717/}
}