1. R.M. Dudley, “Continuity of homomorphisms”, Duke Math. J. 28 (1961), 587–594. MR254141.
2. М.И. Граеа [M.1. Graev], "Свободныв топологические сруппы" [Free topological groups], Izv. Akad. Nauk SSSR Ser. Mat. 12 (1948), 279-324
3. Amer. Math. Soc. Transi. 35 (1951), pp. 61
4. reprinted Amer. Math. Soc. Transl. (1) 8 (1962), 305-364. MR10,11.
5. М.И. Граеа [M.I. Graev], “О свободных произведниях тополоичесних групп” [On free products of topological groups], Izv. Akad. Nauk SSSR Ser. Mat. 14 (1950), 343–354. MR12,158.