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Abelian Groups - László Fuchs [Springer]

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312 10 Torsion <strong>Groups</strong><br />

(C) B is a basic subgroup of B. In fact, as we saw in Theorem 2.4 in Chapter 6,<br />

B is a basic subgroup in QB, so it is also a basic subgroup in its torsion<br />

subgroup B.<br />

Theorem 3.2. Let B be a †-cyclic p-group of infinite cardinality .ThenjBj @ 0<br />

.<br />

Equality holds if also fin rk B D .<br />

Proof. The stated inequality is an immediate consequence of the representation<br />

of elements of B in the form (10.4): jBj D Q n2N n . P n2N n/ @ 0<br />

: If also<br />

fin rk B D , then we distinguish two cases according as the set of cardinalities<br />

n DjB n j .n 2 N/ contains only finitely many or infinitely many n equal to .<br />

In the second alternative the claim is obvious. In the first case, for the proof we<br />

may ignore any B n of cardinality , and assume that n

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