@article{MZ_1971__119_171499,
     author = {Norman L. Johnson},
     title = {A {Note} on {Free} {Planar} {Extensions.}},
     journal = {Mathematische Zeitschrift},
     pages = {281--282},
     publisher = {mathdoc},
     volume = {119},
     year = {1971},
     zbl = {0199.24303},
     url = {http://geodesic.mathdoc.fr/item/MZ_1971__119_171499/}
}