[1] A. EHRENFEUCHT: 
On a problem of J. H. C. Whitehead concerning abelian groups. Bull. Acad. Polon. Sci. Cl. III., 3 (1955), 127-128. 
MR 0069163 | 
Zbl 0065.01003[3] J. ROTMAN: 
On a problem of Baer and a problem of Whitehead in abelian groups. Acta Math. Acad. Sci. Hung. 12 (1961), 245-254. 
Zbl 0101.01903[4] L. PROCHÁZKA: 
A note on completely decomposable torsion free abelian groups. Comment. Math. Univ. Carolinae 10 (1969), 141-161. 
MR 0246964