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Abelian Groups - László Fuchs [Springer]

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9 Cotorsion Hull and Torsion-Free Cover 295<br />

We continue assuming G cotorsion. If 0 ! A ! B ! C ! 0 is a torsionsplitting<br />

exact sequence and T D tC, then evidently, every map W A ! G extends<br />

to a map 0 W B 0 ! G where B 0 B; B 0 =A Š T. AsB=B 0 is torsion-free, by what<br />

has been proved in the preceding paragraph, 0 extends to some W B ! G.<br />

Conversely, suppose G has the injective property relative to all torsion-splitting<br />

exact sequences. Without loss of generality, G may be assumed reduced. By Corollary<br />

6.7, there is an exact sequence 0 ! G ! G ! D ! 0 with G cotorsion and<br />

D torsion-free divisible. Since G has the injective property with respect to this exact<br />

sequence, the sequence splits and G is a summand of the cotorsion group G (hence<br />

G D G ).<br />

ut<br />

Thus torsion groups are projective, and cotorsion groups are injective objects for<br />

the torsion-splitting exact sequences.<br />

Cotorsion Hull The embedding A ! A of a reduced group A in a reduced<br />

cotorsion group deserves special attention. A is actually the cotorsion hull of A in<br />

the following sense: A is the minimal cotorsion group containing A with torsionfree<br />

quotient. In fact, we have<br />

Proposition 9.3.<br />

(i) Let A be a reduced group and A D Ext.Q=Z; A/: Any homomorphism W A!G<br />

into a reduced cotorsion group G extends uniquely to W A ! G.<br />

(ii) The correspondence A 7! A is functorial: every homomorphism ˛ W A ! B has<br />

a unique extension ˛ W A ! B making the following square commutative:<br />

α<br />

A −−−−→ B<br />

⏐ ⏐<br />

↓ ↓<br />

A • α<br />

−−−−→ •<br />

B •<br />

Proof.<br />

(i) The exact sequence 0 ! A !A ! A =A ! 0 leads to the exact sequence<br />

0 D Hom.A =A; G/ ! Hom.A ; G/ !Hom.A; G/ ! Ext.A =A; G/ D 0:<br />

We infer that is an isomorphism, proving (i).<br />

(ii) follows in the same way, using B in place of G.<br />

ut<br />

Proposition 9.4. A torsion-splitting exact sequence e W 0 ! A ! B ! C ! 0 of<br />

reduced groups induces a splitting exact sequence<br />

e W 0 ! A ! B ! C ! 0:

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