@article{CMH_1969__44_139384,
     author = {Hilbert Levitz},
     title = {On the {Finsler} and {Doner-Tarski} {Arithmetical} {Hierarchies.}},
     journal = {Commentarii mathematici Helvetici},
     pages = {89--92},
     year = {1969},
     volume = {44},
     zbl = {0177.01501},
     url = {http://geodesic.mathdoc.fr/item/CMH_1969__44_139384/}
}