1. Topological Group and Related Structures;Arhangel'skiĭ,2008
2. General Topology;Engelking,1989
3. Abstract Harmonic Analysis I;Hewitt,1963
4. The kR-property in free topological groups;Lin;Indag. Math.,2017
5. On free topological groups;Markov;Izv. Akad. Nauk SSSR, Ser. Mat.,1945