Geodesic
On the Splitting of Modules and Abelian Groups
Canadian journal of mathematics, Tome 26 (1974) no. 1, pp. 68-77

Voir la notice de l'article provenant de la source Cambridge University Press

In a fundamental paper on torsion-free abelian groups, R. Baer [1] proved that the group P of all sequences of integers with respect to componentwise addition is not free. This means precisely that P is not a direct sum of infinite cyclic groups. However, E. Specker proved in [9] that P has the property that any countable subgroup is free. Since an infinite abelian group G is called -free if each subgroup of rank less than is free, these results are equivalent to: P is -free but not free. 
Hill, Paul. On the Splitting of Modules and Abelian Groups. Canadian journal of mathematics, Tome 26 (1974) no. 1, pp. 68-77. doi: 10.4153/CJM-1974-007-6
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