1. On decompositions of the sphere;Adams;Journ. London Math. Soc,1954
2. Decompositions of a sphere;Dekker;Proc. Int. Congr. Math,1954
3. Decompositions of a sphere;Dekker;Fund. Math,1955
4. Orthogonal isomorphic representations of free groups;Groot;Can. J. Math,1956
5. Free subgroups of the orthogonal group;Groot;Comp. Math,1954