24.11.2017 Views

Abelian Groups - László Fuchs [Springer]

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

280 9 <strong>Groups</strong> of Extensions and Cotorsion <strong>Groups</strong><br />

is a pure subgroup in QA with divisible QA=A. Now QA=A cannot contain any copy of<br />

Q, sinceExt.E; A/ D 0 for all torsion-free divisible E, and neither can it contain<br />

any torsion divisible subgroup because of Hom.Q=Z; QA=A/ Š Pext.Q=Z; A/ D 0.<br />

Consequently, QA=A D 0, establishing the claim.<br />

ut<br />

More on Pext In general, neither Ext nor Pext needs to convert a direct limit to<br />

an inverse limit of Exts or Pexts, but in the countable case something definite can be<br />

stated.<br />

Lemma 5.9 (Nunke [1]). Let C n .n

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

Saved successfully!

Ooh no, something went wrong!