1. American Mathematical Society Colloquium Publications, Vol. 25;Birkhoff, Garrett,1948
2. Actualit\'{e}s Scientifiques et Industrielles [Current Scientific and Industrial Topics], No. 1132;Bourbaki, N.,1951
3. On a generalization of the Euclidean algorithm;Cohn, P. M.;Proc. Cambridge Philos. Soc.,1961
4. On subsemigroups of free semigroups;Cohn, P. M.;Proc. Amer. Math. Soc.,1962
5. Factorization in non-commutative power series rings;Cohn, P. M.;Proc. Cambridge Philos. Soc.,1962