Free subgroups of the orthogonal group
Compositio Mathematica, Tome 12 (1954-1956) , pp. 134-136. 
@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},
     zbl = {0057.26102},
     mrnumber = {67120},
     language = {en},
     url = {www.numdam.org/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://www.numdam.org/item/CM_1954-1956__12__134_0/

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Robinson, R.M. [2] Fund. Math. 34 (1947), p. 246-260. | EuDML 213130 | MR 26093 | Zbl 0032.40303

Kurosch, A.G. [3] Gruppentheorie, Berlin (1954), p. 274. | Zbl 0053.01001

Dekker, T. and De Groot, J. [4] Proc. Int. Congr. Math. (1954), II, p. 209.