@article{MZ_1972__126_171733,
     author = {Norman Purzitsky},
     title = {Two-Generator {Discrete} {Free} {Products.}},
     journal = {Mathematische Zeitschrift},
     pages = {209--223},
     publisher = {mathdoc},
     volume = {126},
     year = {1972},
     zbl = {0221.20064},
     url = {http://geodesic.mathdoc.fr/item/MZ_1972__126_171733/}
}