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Abelian Groups - László Fuchs [Springer]

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630 16 Endomorphism Rings<br />

(b) For each non-zero a 2 R choose a pair a ; a of p-adic integers such that the set<br />

f a ; a j 0 ¤ a 2 Rg is algebraically independent over S. This is possible, for S<br />

is countable, and the transcendence degree of J p over S is the continuum. Set<br />

and define the group A as the pure subgroup<br />

e a D a 1 C a a 2 Q R; (16.3)<br />

A DhR; Re a 8a 2 Ri Q R: (16.4)<br />

Obviously, A is countable, reduced, and torsion-free.<br />

(c) It is evident from the definition that A is a left R-module. It is faithful, as<br />

different elements of R act differently on 1 2 A. Consequently, R is isomorphic<br />

to a subring of End A.<br />

(d) In order to show that it is not a proper subring, select an 2 End A. SinceR is<br />

pure and dense in A (which is pure and dense in Q R), it follows that QA D Q R.By<br />

Proposition 2.10 in Chapter 6, extends uniquely to a J p -endomorphism Q of<br />

QR.Then<br />

e a DQ. a 1 C a a/ D a . Q1/ C a . Qa/ D a .1/ C a .a/<br />

for any a 2 A. More explicitly, write<br />

p k .e a / D b 0 C<br />

nX<br />

b i e ai ; p k .1/ D c 0 C<br />

iD1<br />

nX<br />

c i e ai ; p k .a/ D d 0 C<br />

for a i ; b i ; c i ; d i 2 R, and for some k; n 2 N. Substitution yields<br />

b 0 C<br />

nX<br />

b i . ai 1 C ai a i /<br />

iD1<br />

D a Œc 0 C<br />

iD1<br />

nX<br />

c i . ai 1 C ai a i / C a<br />

"d 0 C<br />

iD1<br />

nX<br />

d i e ai<br />

iD1<br />

#<br />

nX<br />

d i . ai 1 C ai a i /<br />

where we may assume that a 1 D a. Comparing the corresponding coefficients<br />

on both sides, we use algebraic independence to argue that b 1 D c 0 , b 1 a D d 0 ,<br />

while all other b i ; c i ; d i vanish. This means p k .1/ D c 0 ; p k .a/ D c 0 a which<br />

thus holds for all a 2 R. Therefore, with the notation 1 D c 2 R, wehave<br />

a D ca for all a 2 R, showing that acts on R as left multiplication by c 2 R.<br />

The same holds for Q and for DQ A. This completes the proof of the local<br />

case.<br />

iD1

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