Example of an infinite abelian but non-cyclic group whose automorphism group is cyclic.

Can anyone give me an example of:

• An infinite abelian but non-cyclic group whose automorphism group is cyclic.
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In fact, there are non-cyclic abelian groups of every possible (infinite) cardinality satisfying the conditions. Of course, they cannot be finitely generated. –  Steve D Sep 1 '11 at 15:58

Let $G$ be the (non-finitely generated) additive group of rational numbers with square-free denominators. Then the only automorphism of $G$ is negation.

From here.

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@Graham: Thanks a lot. –  Chandrasekhar Sep 1 '11 at 16:00
For any cotorsion-free ring $A$ there exist abelian groups $G$ of arbitrarily large cardinalities such that $$A\cong\mathop{\rm End}(G)$$
In particular if you need $\mathop{\rm Aut}(G)$ to be cyclic then you take $A$ such that the units of $A$ form a cylic group (e.g. $A=\mathbb{Z}$). Even more generally, you can replace abelian groups with $R$-modules, where $R$ is any commutative, cotorsion-free ring. Then you obtain $A$ - any cotorsion-free $R$-algebra.
cotorsion-free above means no nontrivial homomorphisms of the underlying groups: $\mathbb{Z}^\wedge_p\to A$ for all primes $p$.