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Abelian Groups - László Fuchs [Springer]

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5 B 1 -andB 2 -<strong>Groups</strong> 549<br />

(F) Homogeneous B 2 -groups are completely decomposable. Indeed, if the finite<br />

rank Butler pure subgroups G in the definition of B 2 -groups (recall: B C1 D<br />

B C G in (14.4)) are homogeneous of the same type t, then by Corollary 1.5<br />

they are completely decomposable. Furthermore, then the intersection B \G <br />

is a completely decomposable summand in G , G D .B \G /˚H for some<br />

H which is likewise t-homogeneous completely decomposable. Therefore,<br />

B C1 D B ˚ H ; and B is the direct sum of the H . < /.<br />

(G) Prebalanced extensions of B 2 -groups by B 2 -groups are B 2 -groups. By<br />

Lemma 2.7, a prebalanced A is decent if G=A is B 2 , so a chain of decent<br />

subgroups of A can be continued by such a chain of G=A to reach G.<br />

(H) Let 0 ! A ! B ! C ! 0 be a prebalanced-exact sequence, and B a<br />

B 1 -group. C is a B 1 -group exactly if A is TEP in B. Cp. Lemma 3.3.<br />

If we wish, we may admit in (14.4) factors of countable rank:<br />

Lemma 5.1 (Dugas–Rangaswamy [1]). A torsion-free group B of cardinality is<br />

aB 2 -group if and only if it admits a smooth chain (14.4) such that<br />

(i) B C1 =B is countable for every

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