@article{MZ_1933__37_168466,
     author = {G. Thomsen},
     title = {Zum geometrischen {Spiegelungskalk\"ul.}},
     journal = {Mathematische Zeitschrift},
     pages = {561--565},
     publisher = {mathdoc},
     volume = {37},
     year = {1933},
     zbl = {0007.22105},
     url = {http://geodesic.mathdoc.fr/item/MZ_1933__37_168466/}
}