@article{AM2_1980__21_182422,
     author = {Walter Benz},
     title = {On a functional equation arising from hyperbolic geometry.},
     journal = {Aequationes mathematicae},
     pages = {113--119},
     year = {1980},
     volume = {21},
     zbl = {0443.39005},
     url = {http://geodesic.mathdoc.fr/item/AM2_1980__21_182422/}
}