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Abelian Groups - László Fuchs [Springer]

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4 More on Torsion-Complete <strong>Groups</strong> 319<br />

Theorem 4.4 (Crawley–Jónsson [1]). Torsion-complete p-groups have the<br />

exchange property.<br />

ut<br />

Large Subgroup Topology The balance of this section is devoted to the<br />

topological aspects of torsion-completeness, in particular, to the large subgroup<br />

topology.<br />

Let A be a separable p-group, and fU j g j2J the family of its large subgroups which<br />

we now declare as a subbase of open neighborhoods of 0. This is a linear topology<br />

which we will denote by w. Thus the open subgroups in w are those that contain a<br />

large subgroup, i.e. which satisfy the Pierce condition.<br />

(a) Separable p-groups are Hausdorff in the large subgroup topology w. This is<br />

obvious, since already the intersection of the large subgroups p n A .n 2 N/ is<br />

the first Ulm subgroup A 1 .<br />

(b) The topology w is finer than the p-adic topology. This follows from the fact that<br />

p n A is a large subgroup for each n h i C n i for each i. Then the indicator<br />

of c i is .h i ; h i C 1;:::;h i C n i 1; 1;:::/, thus c i … A(u)foralli, contradicting<br />

the Cauchy property of the subsequence.<br />

Charles [3] defines the inductive topology on a p-group A by declaring those<br />

subgroups G of A to form a subbase of neighborhoods about 0 which satisfy: for<br />

each k 2 N,<br />

GŒp k D p n A \ AŒp k for some n 2 N:

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