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Abelian Groups - László Fuchs [Springer]

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12 Elongations of p-<strong>Groups</strong> 407<br />

be stationary. This means that E is †-cyclic. The rest follows from the singular<br />

compactness theorem.<br />

ut<br />

After all this preparation we can now prove:<br />

Theorem 12.6 (Mekler–Shelah [1]). In the Constructible Universe, every Crawley<br />

group is †-cyclic.<br />

Proof. Let E be a separable p-group that is not †-cyclic. Owing to Lemma 12.5,we<br />

can choose an !-elongation A of Z.p/ by E, and a group G such that p ! G Š Z.p/<br />

and jGj 2 @ 0<br />

.D@ 1 / hold true, and there is a homomorphism W A ! G with<br />

.p ! A/ ¤ 0. But by Lemma 12.4 there exists an !-elongation A 0 of Z.p/ by E such<br />

that no homomorphism W A 0 ! G exists with .p ! A 0 / ¤ 0. Hence A Š A 0 is<br />

impossible, and consequently, E is not a Crawley group.<br />

ut<br />

Megibben [7] proved: MA+ : CH implies that all @ 1 -separable p-groups of<br />

cardinality @ 1 are Crawley groups; by Theorem 7.1 in Chapter 10 such groups<br />

do exist in ZFC without being †-cyclic. Consequently, Crawley’s problem is<br />

undecidable in ZFC. We forgo the details of the proof.<br />

F Notes. By Mekler–Shelah [1], it is consistent that there exists a Crawley group of<br />

cardinality @ 1 which is not @ 1 -separable. Also, assuming V = L, there exist a separable, non-<br />

†-cyclic p-group E of cardinality @ 1 and elongations A . < 2 @1 / of Z.p/ by E such that if<br />

W A ! A . ¤ / is a homomorphism, then .p ! A / D 0. Mekler [1] shows that it is<br />

consistent with GCH that there exist non-†-cyclic Crawley groups, so the Crawley problem is<br />

undecidable in ZFC+GCH.<br />

Eklof–Huber–Mekler [1] define a p-group A totally Crawley if A=p A is Crawley for all limit<br />

ordinals . It is undecidable in ZFC that for countable lengths, every totally Crawley group is a<br />

direct sum of countable groups.<br />

Warfield [5] points out that !-elongations by non-†-cyclic groups may be abundant. For<br />

instance, there are 2 2@ 0<br />

non-isomorphic !-elongations by a separable p-group A of cardinality<br />

2 @0 with countable basic subgroup; the elongated group can be any reduced p-group that admits<br />

!-elongations by A.<br />

Exercises<br />

(1) (Nunke) A totally projective p-group E of limit length is uniquely -<br />

elongating.<br />

(2) Summands in a uniquely elongating group are likewise uniquely elongating<br />

provided they are of the same length.<br />

(3) Let E be a p-group of length , a limit cardinal, and N a fully invariant subgroup<br />

of the same length. If E is uniquely -elongating, then so is N.<br />

(4) (Richman) A separable p-group A is Crawley if and only if, for any two<br />

dense subgroups of codimension 1 in AŒp, one is carried into the other by an<br />

automorphism of A.<br />

(5) (Nunke) Let 0 ! G ! A ! E ! 0 be an exact sequence. The connecting map<br />

Tor.E; Z.p// ! G ˝ Z.p/ defines a homomorphism W EŒp ! G=pG. If is

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