@article{CMH_1944__17_138851,
     author = {Hans H\"afeli},
     title = {Quaternionengeometrie und das {Abbildungsproblem} der regul\"aren {Quaternionenfunktionen.}},
     journal = {Commentarii mathematici Helvetici},
     pages = {135--164},
     publisher = {mathdoc},
     volume = {17},
     year = {1944},
     zbl = {0060.24514},
     url = {http://geodesic.mathdoc.fr/item/CMH_1944__17_138851/}
}