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Abelian Groups - László Fuchs [Springer]

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540 14 Butler <strong>Groups</strong><br />

with C torsion-free is necessarily balanced-exact whenever T is torsion.<br />

Proof. We apply the definition of prebalancedness to A D T and to a pure subgroup<br />

H of G such that T < H and rk H=T D 1. Then the sum H D T C B (for a Butler<br />

group B) must be direct, since T \ B D 0. HereB has to be of rank 1, so T is<br />

balanced in H (and hence in G).<br />

ut<br />

This lemma is applied to record a remarkable property of Butler groups. The<br />

proof for countable rank is as simple as for the finite rank case.<br />

Proposition 2.6. Every pure subgroup B of a countable completely decomposable<br />

group satisfies Bext 1 .B; T/ D 0 for all torsion groups T.<br />

Proof. Let B be as stated, and 0 D B 0 < B 1 < < B n < ::: a chain of pure<br />

subgroups with union B where rk B nC1 =B n D 1.n

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