@article{10_4064_cm88_1_3,
     author = {Jan Krempa},
     title = {On free subgroups of units
in quaternion algebras},
     journal = {Colloquium Mathematicum},
     pages = {21--27},
     publisher = {mathdoc},
     volume = {88},
     number = {1},
     year = {2001},
     doi = {10.4064/cm88-1-3},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/cm88-1-3/}
}