@article{MZ_1972__125_171705,
     author = {Norman L. Johnson},
     title = {Homomorphisms of {Free} {Planes.}},
     journal = {Mathematische Zeitschrift},
     pages = {255--263},
     publisher = {mathdoc},
     volume = {125},
     year = {1972},
     zbl = {0234.50016},
     url = {http://geodesic.mathdoc.fr/item/MZ_1972__125_171705/}
}