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@article{CM_1954-1956__12__134_0, author = {de Groot, J. and Dekker, T.}, title = {Free subgroups of the orthogonal group}, journal = {Compositio Mathematica}, pages = {134--136}, publisher = {Kraus Reprint}, volume = {12}, year = {1954-1956}, mrnumber = {67120}, zbl = {0057.26102}, language = {en}, url = {http://geodesic.mathdoc.fr/item/CM_1954-1956__12__134_0/} }
de Groot, J.; Dekker, T. Free subgroups of the orthogonal group. Compositio Mathematica, Tome 12 (1954-1956), pp. 134-136. http://geodesic.mathdoc.fr/item/CM_1954-1956__12__134_0/