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Abelian Groups - László Fuchs [Springer]

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368 11 p-<strong>Groups</strong> with Elements of Infinite Height<br />

.iii/ 0 ! B.u/ ! A.u/ ! C.u/ ! 0 is exact for all increasing sequences u D<br />

. 0 ; 1 ;:::; n ;:::/of ordinals and 1;<br />

.iv/ .p AŒp/ D p CŒp holds for each .<br />

Proof. The equivalence of .i/ and .ii/ has already been established above.<br />

To prove .ii/ ) .iii/; note that the map B.u/ ! A.u/ is monic, since h B .p n b/ D<br />

h A .p n b/ for all b 2 B and n 0. It is also clear that A.u/ C.u/. Inorderto<br />

prove that this is not a proper inclusion, we use induction on the order of c 2 C.u/,<br />

and show that p CŒp k .p AŒp k / for k 1. Fork D 1, pickc 2 p CŒp, and<br />

argue that there is an a 2 p A with a D c. Nowpa 2 p C1 A \ B D p C1 B<br />

implies that pb D pa for some b 2 p B.Since˛ maps a b 2 p AŒp upon c,<br />

the reverse inclusion follows. Assume the assertion is true for k 1 and for all .<br />

If c 2 p CŒp k .k >1/, then there is x 2 A such that .x/ D c, and by induction<br />

hypothesis, some y 2 p C1 AŒp k 1 satisfies .y/ D pc 2 p C1 CŒp k 1 . Choose<br />

an a 0 2 p AŒp k with pa 0 D y, and note that .x a 0 / D c .a 0 / 2 p CŒp.<br />

Consequently, c .a 0 / D .a 1 / for some a 1 2 p AŒp, andc D .a 0 C a 1 / with<br />

a 0 C a 1 2 p AŒp k . Hence .iii/ is exact at C.u/.<br />

.iv/ is a special case of .iii/.<br />

For the implication .iv/ ) .ii/; it remains to check that B\p A D p B for all .<br />

This being trivially true for D 0, suppose it holds for .Toverifyitfor C1,pick<br />

a b 2 B \ p C1 A and an a 0 2 p A with pa 0 D b.As.a 0 / 2 p CŒp, we can find an<br />

a 1 2 p AŒp such that .a 1 / D .a 0 /.Thena D a 0 a 1 2 B \ p A D p B, whence<br />

b D pa 2 p C1 B follows. The induction step for limit ordinals is automatic. ut<br />

The Extension Lemma We have come to a crucial lemma about extending<br />

maps that do not decrease heights. The idea is borrowed from Hill [11].<br />

Lemma 5.6. Given a commutative diagram<br />

0 −−−−→ N −−−−→ G<br />

⏐ ⏐<br />

ψ↓<br />

↓φ<br />

0 −−−−→ B −−−−→ A<br />

α<br />

−−−−→ C −−−−→ 0<br />

with exact rows, suppose that<br />

(i) B is a balanced subgroup of A;<br />

(ii) does not decrease heights .heights in N are computed in G/;<br />

(iii) there is a g 2 G proper with respect to N satisfying pg 2 N.<br />

Then can be extended to a map<br />

W hN; gi !A<br />

which does not decrease heights either, and satisfies ˛ .g/ D .g/.

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