@article{ELMA_1985__40_141358,
     author = {J. Simonis and J. de van de Craats},
     title = {Coordinate free definitions of cross ratio.},
     journal = {Elemente der Mathematik},
     pages = {63--68},
     year = {1985},
     volume = {40},
     zbl = {0569.51003},
     url = {http://geodesic.mathdoc.fr/item/ELMA_1985__40_141358/}
}