[1] A. L. S. CORNER: A note on rank and direct decompositions of torsion-free abelian groups. Proc. Cambridge Philos. Soc. I in 57 (1961), 230-233, and II in 66 (1969), 239-240. Zbl 0100.02903
[2] L. FUCHS: Infinite Abelian Groups. Academic Press, N. Y., Vol. I (1970), Vol. II (1973). Zbl 0209.05503