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Abelian Groups - László Fuchs [Springer]

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12 Elongations of p-<strong>Groups</strong> 405<br />

BŒp ˚ G and CŒp ˚ G are not isometric: the former is a free valuated vector space<br />

(G is free with value !), but the latter is not. ut<br />

Crawley <strong>Groups</strong> A separable torsion group E is said to be a Crawley group if<br />

all !-elongations of G Š Z.p/ by E are isomorphic. Megibben [7] proved that in<br />

ZFC it is undecidable whether or not Crawley groups of cardinality @ 1 are †-cyclic.<br />

We follow the Mekler–Shelah proof in solving the problem in the Constructible<br />

Universe without cardinality restriction.<br />

In the following arguments, H denotes an arbitrary, but fixed, p-group of<br />

cardinality @ 1 such that p ! H Š Z.p/. (For convenience, temporarily we may<br />

choose H D H !C1 , the Prüfer group of length ! C 1, but later a different choice<br />

will be needed.)<br />

Lemma 12.3 (Mekler–Shelah [1]). Let E 0 be a separable p-group containing a<br />

subgroup E of countable index, and 0 ! G ! A ! E ! 0 an !-elongation with<br />

G Dhgi of order p. If E 0 =E is not †-cyclic, then in the commutative diagram<br />

0 −−−−→ G −−−−→ A −−−−→ E −−−−→ 0<br />

⏐ ⏐<br />

∥ ↓ ↓incl<br />

0 −−−−→ G −−−−→ A i −−−−→ E ′ −−−−→ 0<br />

groups A i .i D 1; 2/ can be chosen such that any homomorphism W A ! H with<br />

.g/ ¤ 0 extends to at most one of A i .<br />

Proof. The non-unique existence of the bottom exact sequence is a consequence of<br />

the fact that the induced map Ext.E 0 ; G/ ! Ext.E; G/ is epic, but not necessarily<br />

monic. Suppose an elongation A 1 has been selected. As E 0 =E is countable, but not<br />

†-cyclic (so not separable), it has non-zero elements of infinite heights, i.e. there are<br />

elements x n 2 E 0 nE and e n 2 E .n

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