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Abelian Groups - László Fuchs [Springer]

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4 Direct Limits 59<br />

We now move to three direct systems: A, B as above, and a third one, C D<br />

fC i .i 2 I/I j i<br />

g, all with the same directed index set I.IfˆW A ! B and ‰ W B ! C<br />

are homomorphisms between them such that the sequence 0 ! A i<br />

i<br />

!B i<br />

i<br />

!C i ! 0<br />

is exact for each i 2 I, then we say that the sequence<br />

0 ! A ˆ!B ‰ !C ! 0 (2.3)<br />

is exact. It is an important fact that direct limits of exact sequences is exact. More<br />

precisely,<br />

Theorem 4.6. Let A; B; C be direct systems over the same index set I, and ˆW A !<br />

B and ‰ W B ! C homomorphisms between them. If the sequence (2.3) is exact,<br />

then the sequence<br />

ˆ<br />

‰ <br />

0 ! A D lim A i !B D lim B i !C D lim C i ! 0<br />

!i !i !i<br />

of direct limits is likewise exact.<br />

Proof. Exactness at A and C is guaranteed by Lemma 4.5, so we prove exactness<br />

only at B . By Lemma 4.5, the diagram<br />

φ i<br />

0 −−−−→ A i −−−−→ Bi<br />

ψ i<br />

−−−−→ Ci −−−−→ 0<br />

π i<br />

⏐ ⏐↓<br />

ρ i<br />

⏐ ⏐↓<br />

⏐ ⏐↓<br />

σ i<br />

Φ<br />

0 −−−−→ A<br />

∗<br />

∗ −−−−→ B∗<br />

Ψ ∗<br />

−−−−→ C∗ −−−−→ 0<br />

is commutative for all i 2 I.Ifa 2 A ,then i a i D a for some a i 2 A i ,so‰ ˆa D<br />

‰ ˆ i a i D ‰ i i a i D i i i a i D 0. Nextletb 2 Ker ‰ .Forsomeb i 2 B i ,<br />

we have i b i D b, whence i i b i D ‰ i b i D ‰ b D 0. There exists j 2 I with<br />

j i ib i D 0, thus j b j D j j i b i D 0. Since the top row in the diagram is exact,<br />

there is an a j 2 A j with j a j D b j . Setting a D j a j , we arrive at ˆa D ˆ j a j D<br />

j j a j D j j i b i D i b i D b,i.e.b 2 Im ˆ, and the bottom row is exact at B . ut<br />

Exercises<br />

(1) Show that lim Z.p n / D Z.p 1 /, using inclusion maps.<br />

!n<br />

(2) Let A n Š Z .n

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