1. D. M. Arnold and E. L. Lady, Endomorphism rings and direct sums of torsion free abelian groups, Trans. A.M.S. 211(1975), 225–237.
2. R. A. Beaumont and R. S. Pierce, Subrings of algebraic number fields, Acta. Sci. Math. Szeged 22(1961), 202–216.
3. _____, Torsion free rings, Ill. Jour. Math. 5(1961), 61–98.
4. L. Fuchs, Infinite Abelian Groups, Vol. II, Academic Press, New York, 1973.
5. L. Fuchs and F. Loonstra, On direct decompositions of torsion free abelian groups of finite rank, Rend. Sem. Mat. Univ. Padova 44(1970), 75–83.