1. Abelian groups without elements of finite order;Baer, Reinhold;Duke Math. J.,1937
2. Some aspects of groups with unique roots;Baumslag, Gilbert;Acta Math.,1960
3. P. Hall, Nilpotent groups, Canadian Mathematical Congress, University of Alberta, 1957.
4. P. G. Kontorovich, On the theory of noncommutative torsion free groups, Doklady Akad. Nauk SSSR. vol. 59 (1948) p. 213.
5. Sur les groupes nilpotents et les anneaux de Lie;Lazard, Michel;Ann. Sci. Ecole Norm. Sup. (3),1954