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Abelian Groups - László Fuchs [Springer]

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4 Theorems on Torsion Products 245<br />

(3) (a) If A ˝ C contains a copy of Z.p 1 /, then either A or C has a subgroup<br />

Š Z.p 1 /.<br />

(b) Examine when A ˝ C Š Z.p 1 /.<br />

(4) (a) Prove that A ˝ C Š Z implies A Š C Š Z.<br />

(b) If A ˝ C is a non-trivial free group, then both A and C are free.<br />

(5) If A is torsion-free, then QA Š Hom.Q=Z; A ˝ Q=Z/ (Z-adic completion).<br />

(6) If the sequence 0 ! A ! B ! C ! 0 is pure-exact, then the same holds<br />

for the sequence 0 ! tA ! tB ! tC ! 0 of torsion subgroups. [Hint:<br />

Tor.Z.p 1 /; /.]<br />

4 Theorems on Torsion Products<br />

While tensoring with a p-group implements drastic structural simplification, this is<br />

not the case for the torsion product, though we may notice some smoothing effect.<br />

In this section, we explore some interesting features of Tor.<br />

In order to get more information about Tor, we want to take advantage of its<br />

left exactness. The first and foremost fact to be observed is that if A 0 A and<br />

C 0 C, then Tor.A 0 ; C 0 / Tor.A; C/. This is a powerful property that will be used<br />

throughout without mentioning it explicitly.<br />

Elementary Facts Our first concern is how Tor behaves towards multiplication<br />

by integers.<br />

Lemma 4.1 (Nunke [4]). For every integer n, we have<br />

n Tor.A; C/ D Tor.nA; nC/:<br />

Proof. Starting with the exact sequence 0 ! nA ! A ! A=nA ! 0 and the same<br />

for C, we obtain the exact sequence<br />

0 ! Tor.nA; nC/ ! Tor.A; C/ ! Tor.A=nA; C/ ˚ Tor.A; C=nC/;<br />

using the commutative diagram dual to Corollary 1.11. Hence we conclude that<br />

n Tor.A; C/ Tor.nA; nC/, since the direct sum in the displayed formula is<br />

annihilated by n. To prove the converse inclusion, pick a generator x D .na; m; nc/ 2<br />

Tor.nA; nC/ where mna D 0 D mnc .a 2 A; c 2 C/. Herex D .a; nm; nc/ D<br />

n.a; nm; c/ 2 nTor.A; C/.<br />

ut<br />

Interestingly, in the last lemma the integer n can be replaced by p for any<br />

ordinal .<br />

Lemma 4.2 (Nunke [4]). For all ordinals and p-groups A; C, we have<br />

p Tor.A; C/ D Tor.p A; p C/:

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