[1] S. FEIGELSTOCK: The nilstufe of the direct sum of rank $1$ torsion free groups. Acta Math. Acad. Sci. Hungar. 24 (1973), 269-272. MR 0323921 | Zbl 0271.20026

[2] L. FUCHS: Infinite abelian groups. Vol. II (Academic Press, 1973). MR 0349869 | Zbl 0257.20035

[3] B. J. GARDNER: Some aspects of $T$-nilpotence. Pacific J. Math, (to appear). MR 0360667 | Zbl 0315.16014

[4] R. L. KRUSE D. T. PRICE: Nilpotent rings. (Gordon and Breach, 1969). MR 0266956

[5] R. REE R. J. WISNER: A note on torsion-free nil groups. Proc. Amer. Math. Soc. 7 (1956), 6-8. MR 0077531

[6] T. SZELE: Zur Theorie der Zeroringe. Math. Ann. 121 (1949), 242-246. MR 0033822 | Zbl 0034.31001

[7] T. SZELE: Gruppentheoretische Beziehungen bei gewissen Ringkonstruktionen. Math. Z. 54 (1951), 168-180. MR 0043780 | Zbl 0042.25501

[8] W. J. WICKLESS: Abelian groups which admit only nilpotent multiplications. Pacific J. Math. 40 (1972), 251-259. MR 0304506 | Zbl 0213.30701