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Abelian Groups - László Fuchs [Springer]

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2 Exact Sequences for Hom and Ext 265<br />

Here the rows are exact, denotes the injection map, and exists, due to the purity<br />

of the given exact sequence. The composite map ˇW CŒn ! C will be the natural<br />

injection. If the last but one row belongs to n Ext.B; G/, then Theorem 5.2 below<br />

shows that the second row splits, and hence so does the top row. This means that the<br />

bottom exact sequence belongs to n Ext.C; G/, so the last but one row is in n Im ˇ.<br />

This holds for every n 2 N, thus Im ˇ is pure in Ext.B; G/.<br />

ut<br />

Ext and Direct Sums, Products We have to find out how Ext behaves towards<br />

direct sums and products. In view of Theorem 2.3, it should not come as a surprise<br />

that Ext imitates Hom in this respect.<br />

Theorem 2.5. For all groups A; A i ; C; C i , there exist natural isomorphisms<br />

Ext.˚i2I C i ; A/ Š Y i2I<br />

Ext.C i ; A/; (9.9)<br />

Ext<br />

C; Y !<br />

A i Š Y Ext.C; A i /: (9.10)<br />

i2I i2I<br />

Proof. We prove (9.9), the proof of (9.10) runs dually. We start with free resolutions<br />

of the C i , 0 ! H i ! F i ! C i ! 0 with F i free, to obtain the exact sequences<br />

Hom.F i ; A/ ! Hom.H i ; A/ ! Ext.C i ; A/ ! Ext.F i ; A/ D 0. The exact sequence<br />

0 ! ˚iH i ! ˚iF i ! ˚iC i ! 0 induces the top exact sequence in the<br />

commutative diagram<br />

Hom(⊕ i F i ,A) −−−−−→ Hom(⊕ i H i ,A) −−−−−→ Ext(⊕ i C i ,A) −−−−−→ 0<br />

⏐<br />

⏐<br />

⏐<br />

↓<br />

↓<br />

↓<br />

∏<br />

i Hom(F i,A) −−−−−→ ∏ i Hom(H i,A) −−−−−→ ∏ i Ext(C i,A) −−−−−→ 0<br />

We know from Theorem 1.7 in Chapter 7 that the first two vertical maps represent<br />

natural isomorphisms, whence we can conclude that there is a natural isomorphism<br />

between the two Exts in the diagram.<br />

ut<br />

F Notes. The long exact sequence for Hom-Ext generalizes for modules over arbitrary rings.<br />

However, the 0 at the right end is then replaced by sequences of higher Exts.<br />

The functor Ext does not behave in the same way towards direct and inverse limits as Hom. If<br />

fC i .i 2 I/j j i g is a direct system of groups with direct limit C,thenfExt.C i; A/.i 2 I/j Ext. j i ; 1 A/g<br />

is an inverse system. The most we can say in general is that there is a natural homomorphism<br />

W Ext.C; A/ ! lim Ext.C i ; A/. For the special case when the direct system is indexed by the<br />

natural numbers, see Lemma 5.9. In general, the so-called Mittag-Leffler condition guarantees the<br />

surjectivity of .<br />

Göbel–Prelle [1] ask for groups G with the properties like Ext. Q A i ; G/ Š Q i Ext.A i ; G/ for<br />

all choices of the A i , and show that “only G Š Q” is the answer.

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