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Abelian Groups - László Fuchs [Springer]

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652 16 Endomorphism Rings<br />

The Torsion Case The problem for torsion groups is a different ball game.<br />

The endomorphism rings are extremely restricted: they are built on algebraically<br />

compact groups, and admit unavoidable small endomorphisms. The big problem<br />

is that these small endomorphisms form an ideal that already totally characterizes<br />

the group itself. Thus the only hope is to have some control on the endomorphism<br />

rings modulo small endomorphisms. We know from Theorem 2.2 that End A is a<br />

split extension of the ideal End s A by an algebraically compact ring that faithfully<br />

reflects direct decompositions with unbounded summands. We have something to<br />

say about this ring.<br />

Theorem 7.3 (Corner [5], Dugas–Göbel [3]). Let R denote a ring whose additive<br />

group is the p-adic completion of a free group. There exists a separable p-group A<br />

such that<br />

End A Š R ˚ End s A:<br />

ut<br />

Moreover, if the ring R is given as stated, then for every infinite cardinal ,there<br />

exists a family fA j

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