@article{MZ_1967__99_170769,
     author = {Oddvar Iden},
     title = {Free {Planes.} {I.}},
     journal = {Mathematische Zeitschrift},
     pages = {117--122},
     publisher = {mathdoc},
     volume = {99},
     year = {1967},
     url = {http://geodesic.mathdoc.fr/item/MZ_1967__99_170769/}
}