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Abelian Groups - László Fuchs [Springer]

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282 9 <strong>Groups</strong> of Extensions and Cotorsion <strong>Groups</strong><br />

6 Cotorsion <strong>Groups</strong><br />

In this section we get acquainted with a remarkable class of groups which fits<br />

perfectly into the structure theory of the groups of extensions. They were introduced<br />

independently and almost simultaneously by Harrison [1], Nunke [1], and <strong>Fuchs</strong><br />

[11]. The name was coined by Harrison who pointed out their dual behavior to<br />

torsion groups (Theorem 7.4).<br />

Cotorsion <strong>Groups</strong> Here is the definition: a group G is called cotorsion if it<br />

satisfies<br />

Ext.A; G/ D 0 for all torsion-free groups A:<br />

Visibly, cotorsion generalizes the concept of algebraic compactness.<br />

Every torsion-free group A can be embedded in a direct sum of copies of Q.The<br />

inclusion map A !˚Q implies that the sequence Ext.˚ Q; G/ D Q Ext.Q; G/ !<br />

Ext.A; G/ ! 0 is exact. Consequently, the single equality<br />

Ext.Q; G/ D 0<br />

suffices to guarantee that the group G is cotorsion. This is a handy criterion for<br />

cotorsionness.<br />

Example 6.1. From Theorem 3.8 we can deduce that every Ext is cotorsion. Indeed, we have<br />

Ext.Q; Ext.C; A// Š Ext.Tor.Q; C/; A/ D 0, since the Tor vanishes. (We will give another proof<br />

in Theorem 6.5 that is independent of Theorem 3.8 whose proof has been omitted.)<br />

As usual, we start with a list of elementary consequences of the definition. The<br />

immediate goal is to show that the class of cotorsion groups is closed under direct<br />

products, extensions and epic images.<br />

(A) A direct product Q i2I G i is cotorsion if and only if every summand G i is<br />

cotorsion. This is a straightforward consequence of the natural isomorphism<br />

Ext.Q; Q i G i/ Š Q i Ext.Q; G i/.<br />

(B) A group G is cotorsion if a subgroup H and the factor group G=H are<br />

cotorsion. The exact sequence 0 ! H ! G ! G=H ! 0 implies the<br />

exactness of Ext.Q; H/ ! Ext.Q; G/ ! Ext.Q; G=H/.<br />

(C) Epimorphic images of cotorsion groups are cotorsion. If G is cotorsion, and<br />

H is an epic image of G, then the sequence Ext.Q; G/ ! Ext.Q; H/ ! 0 is<br />

exact, whence the claim is evident.<br />

(D) Assume G is reduced and cotorsion. A subgroup H of G is cotorsion<br />

exactly if the factor group G=H is reduced. The exact sequence<br />

0 ! H ! G ! G=H ! 0 leads to the exact sequence<br />

0 D Hom.Q; G/ ! Hom.Q; G=H/ ! Ext.Q; H/ ! Ext.Q; G/ D 0:

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