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Abelian Groups - László Fuchs [Springer]

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2 Nice Subgroups 353<br />

We can rephrase the last lemma by saying that in the exact sequence 0 ! N !<br />

A ! C ! 0 the subgroup N is nice if and only if the induced sequence p N !<br />

p A ! p C ! 0 is exact for every (the emphasis being on the exactness at p C).<br />

Example 2.2.<br />

(a) Finite subgroups are trivially nice. More generally, finite extensions of a nice subgroup are<br />

nice. This follows from the simple observation that a finite extension cannot create new cosets<br />

of limit heights.<br />

(b) If A=N is a separable p-group, then N is nice in A.<br />

Example 2.3. For every ordinal , the subgroup p A is nice in the p-group A. Recall that heights<br />

is impossible.<br />

(ii) By the hypotheses of (ii), h A=M .a C M/ implies the existence of a<br />

y 2 M satisfying h A=N .a C y C N/ D , and of a z 2 N satisfying<br />

h A .a C y C z/ D .<br />

ut

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