@article{JRAM_1971__251_151162,
     author = {Joseph E. Valentine and E.Z. Andalafte},
     title = {A metric characterization of },
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {142--152},
     publisher = {mathdoc},
     volume = {251},
     year = {1971},
     zbl = {0223.50014},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1971__251_151162/}
}