@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://www.numdam.org/item/CM_1954-1956__12__134_0/}
}