@article{10_4153_CJM_1980_104_3,
     author = {Hartley, B. and Pickel, P. F.},
     title = {Free {Subgroups} in the {Unit} {Groups} of {Integral} {Group} {Rings}},
     journal = {Canadian journal of mathematics},
     pages = {1342--1352},
     year = {1980},
     volume = {32},
     number = {6},
     doi = {10.4153/CJM-1980-104-3},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1980-104-3/}
}