[5] GRAEV M. I.: Free topological groups. Izv. Akad. Nauk. SSSR Ser. Mat. 12, 1948, 279-324 (Russian), English translation, Amer. Math. Soc, Transl. 35, 1951. Reprint, Amer. Math. Soc. Transl. 1, 8, 1962, 305-364. MR 0025474

[6] ISBELL J. R.: Uniform Spaces. Amer. Math. Soc, Providence 1964. MR 0170323 | Zbl 0124.15601

[7] KELLEY J.: General Topology. Van Nostrand, Princeton 1955. MR 0070144 | Zbl 0066.16604

[8] MARKOV A. A.: On free topological groups. C. R. (Doklady) Acad. Sci. URSS (N. S.) 31, 1941, 299-301. Bull. Acad. Sci. URSS Ser. Math. (Izv. Akad. Nauk. SSR) 9, 1945, 3-64. (Russian, English summary). English transl. Amer. Math. Soc. Transl. 30, 1950, 11-88. Reprint, Amer. Math. Soc Transl. 1, 8, 1962, 195-272. MR 0004634

[9] MARXEN D.: Uniform semigroups. Math. Ann., 202, 1973, 27-36. MR 0447459 | Zbl 0234.22003

[10] MARXEN D.: Free uniform semigroups and free uniform groups, (to appear). MR 0399310 | Zbl 1225.76281

[11] MORRIS S. A., THOMPSON H. B.: Invariant metrics on free topological groups. Bull. Aust. Math. Soc, 9, 1973, 83-88. MR 0323943 | Zbl 0255.22002