24.11.2017 Views

Abelian Groups - László Fuchs [Springer]

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

290 9 <strong>Groups</strong> of Extensions and Cotorsion <strong>Groups</strong><br />

on the objects. They act in the same way on the morphisms, as witnessed by the<br />

commutative diagrams<br />

Hom(Q/Z,∗)<br />

D<br />

⏐<br />

↓<br />

C<br />

δ<br />

−−−−→ D ′<br />

⏐<br />

↓Hom(Q/Z,∗)<br />

Hom(Q/Z,δ)<br />

−−−−−−−−→ C ′<br />

Tor(Q/Z,∗)<br />

C<br />

⏐<br />

↓<br />

D<br />

γ<br />

−−−−→ C ′<br />

⏐<br />

↓Tor(Q/Z,∗)<br />

Tor(Q/Z,γ)<br />

−−−−−−−→ D ′<br />

ut<br />

An especially convenient and handy way to capture the correspondence D $ C<br />

is via the exact sequence 0 ! C ! E ! D ! 0, where, we repeat, E is the<br />

divisible hull of C, a direct sum of copies of Q.<br />

Example 7.7. If D D Z.p 1 /,thenC D Hom.Q=Z; Z.p 1 // Š J p . There is an exact sequence<br />

0 ! J p !˚Q ! Z.p 1 / ! 0. IfD D Q=Z, thenC D Hom.Q=Z; Q=Z/ Š QZ, andthe<br />

sequence 0 ! QZ !˚Q ! Q=Z ! 0 is exact.<br />

F Notes. The dual behavior of Ext-Tor as well as Hom-tensor is most interesting and most<br />

important; there are other similar, less relevant, examples of duality in Homological Algebra.<br />

While the generalization of Theorem 7.6 has widespread applications in the theory of modules<br />

over integral domains (this is the Matlis category equivalence), so far Theorem 7.4 remains an<br />

isolated result, though it easily generalizes to modules over integral domains.<br />

Exercises<br />

(1) A non-zero cotorsion group has a summand isomorphic to one of the following<br />

groups: Q, Z.p k /.k 1/, J p , for some prime p.<br />

(2) Show that T \ C is the class of bounded groups.<br />

(3) Let D D Z.p 1 / .@ 0/ .FindHom.Q=Z; D/.<br />

(4) If G is adjusted cotorsion, then jGj jtGj @ 0<br />

.<br />

(5) In the category equivalence of Theorem 7.6, what subcategory of F corresponds<br />

to the category of divisible p-groups?<br />

(6) If G; H are adjusted cotorsion groups, then Hom.G; H/ Š Hom.tG; tH/<br />

naturally.<br />

(7) End G Š End.tG/ for an adjusted cotorsion G.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!