@article{JDM_1921__30_145606,
     author = {Ludwig Berwald},
     title = {Zur projektiven {Differentialgeometrie} der {Ebene.}},
     journal = {Jahresbericht der Deutschen Mathematiker-Vereinigung},
     pages = {110--121},
     publisher = {mathdoc},
     volume = {30},
     year = {1921},
     zbl = {48.0775.05},
     url = {http://geodesic.mathdoc.fr/item/JDM_1921__30_145606/}
}