@article{JRAM_1973__264_151384,
     author = {Michael Fried},
     title = {On a theorem of {Ritt} and related {Diophantine} problems.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {40--55},
     publisher = {mathdoc},
     volume = {264},
     year = {1973},
     zbl = {0278.12101},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1973__264_151384/}
}