@article{MAN_1956__131_160501,
     author = {G. Ewald},
     title = {\"Uber den {Begriff} der {Orthogonalit\"at} in der {Kreisgeometrie.}},
     journal = {Mathematische Annalen},
     pages = {463--469},
     publisher = {mathdoc},
     volume = {131},
     year = {1956},
     zbl = {0072.15801},
     url = {http://geodesic.mathdoc.fr/item/MAN_1956__131_160501/}
}