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Abelian Groups - László Fuchs [Springer]

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162 5 Purity and Basic Subgroups<br />

Proof. The proof is immediate, all that we have to observe is that both Z.n/ ˝ G Š<br />

G=nG and Hom.Z.n/; G/ Š GŒn are natural isomorphisms for every group G; see<br />

Sect. 1(B) in Chapter 8 and Example 1.2 in Chapter 7.<br />

ut<br />

The next corollary is an important result on purity, it is intimately related to the<br />

foregoing discussions.<br />

Corollary 3.7. An exact sequence (5.3) is pure-exact if and only if, for every group<br />

G, the induced sequence<br />

0 ! G ˝ A 1 G˝˛<br />

! G ˝ B 1 G˝ˇ<br />

! G ˝ C ! 0<br />

is exact. It is even pure-exact, if so is (5.3).<br />

Proof. Suppose (5.3) is an exact sequence. The exactness of the tensored sequence<br />

holds for finitely generated groups G, as is shown in Corollary 3.6(a). By Theorem<br />

3.3, exactness is preserved under taking direct limits. Since tensor product<br />

commutes with direct limits, the claim also holds for direct limits of finitely<br />

generated groups, so also for any G. The converse is a consequence of Corollary 3.6<br />

if we choose G as finite cyclic groups. Purity follows from Corollary 3.5 at<br />

once.<br />

ut<br />

F Notes. The results above show that purity has several remarkable characterizations.<br />

Several mathematicians noticed almost simultaneously that pure-exact sequences are exactly the<br />

direct limits of splitting exact sequences, this being true also in the module-theoretic version.<br />

Interestingly, for the generalization of purity to modules, the results of this section continue to<br />

hold mutatis mutandis. For integral domains, see, e.g., <strong>Fuchs</strong>–Salce, Modules over non-Noetherian<br />

Domains (2001).<br />

In general, pure submodules of injective left R-modules need not be injective; they are always<br />

injective exactly if R is left noetherian. The von Neumann regular rings can be characterized as<br />

rings over which all exact sequences are pure-exact.<br />

Exercises<br />

(1) Show that (5.3) is pure-exact if and only if the induced sequence<br />

0 ! Hom.A; Z.n// ! Hom.B; Z.n// ! Hom.C; Z.n// ! 0<br />

is exact for every n 2 N. [Hint: Hom.G; Z.n//.]<br />

(2) If C is a pure subgroup of the p-group A, thenAŒp=CŒp Š .A=C/Œp.<br />

(3) If (5.3) is a pure-exact sequence, then the sequence 0 ! tA ! tB ! tC ! 0<br />

of torsion subgroups is exact.<br />

(4) Suppose (5.3) is pure-exact. The sequences 0 ! A 1 ! B 1 ! C 1 ! 0 of first<br />

Ulm subgroups and 0 ! A=A 1 ! B=B 1 ! C=C 1 ! 0 of first Ulm factors<br />

need not be exact; but if one is exact, then so is the other.

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