1. A.M. Aghdam, A. Najafizadeh, Square subgroups of rank two abelian groups. Colloq. Math. 117(1), 19–28 (2009)
2. R.A. Beaumont, R.J. Wisner, Rings with additive group which is a torsion-free group of rank two. Acta. Sci. Math. Szeged 20, 105–116 (1959)
3. S. Feigelstock, On the type set of groups and nilpotence. Comment. Math. Univ. St. Pauli 25, 159–165 (1976)
4. S. Feigelstock, The absolute annihilator of a group modulo a subgroup. Publ. Math. Debr. 23, 221–224 (1979)
5. L. Fuchs, Infinite Abelian Groups, vol. II (Academic Press, New York, 1973)