@article{MZ_1980__170_172903,
     author = {Wilhelm Magnus},
     title = {Rings of {Fricke} {Characters} and {Automorphism} {Groups} of {Free} {Groups.}},
     journal = {Mathematische Zeitschrift},
     pages = {91--104},
     publisher = {mathdoc},
     volume = {170},
     year = {1980},
     zbl = {0433.20033},
     url = {http://geodesic.mathdoc.fr/item/MZ_1980__170_172903/}
}