1. Baumslag, G.: Groups with the Same Lower Central Sequence as a Relatively Free Group I. The Groups. Trans. Amer. Math. Soc., 129, 308-321 (1967).
2. Baumslag, G.: Groups with the Same Lower Central Sequence as a Relatively Free Group. II Properties. Trans. Amer. Math. Soc. 142, 507-538 (1969).
3. Baumslag, G.: Parafree Groups. Progress in Math. 248, 1-14 (2005).
4. Baumslag, G., Cleary, S.: Parafree one-relator Groups. Journal of Group Theory. 9, 191-201 (2006).
5. Baumslag, G., Cleary, S., Havas, G.: Experimenting with infinite group. Experimental Math. 13, 495-502 (2004).