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Abelian Groups - László Fuchs [Springer]

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284 9 <strong>Groups</strong> of Extensions and Cotorsion <strong>Groups</strong><br />

where, for each prime p, G p is a reduced cotorsion group which is a module over the<br />

ring J p of p-adic integers. The G p are uniquely determined fully invariant subgroups.<br />

Proof. In view of the isomorphism Q=Z Š ˚p Z.p 1 /, the preceding theorem<br />

implies G Š Ext.˚p Z.p 1 /; G/ D Q p G p where G p D Ext.Z.p 1 /; G/ is cotorsion.<br />

G p is the intersection of all q n G for primes q ¤ p and all n

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