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Abelian Groups - László Fuchs [Springer]

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6 Direct Products vs. Direct Sums 65<br />

(2) Let C n Dhc n i be cyclic of order n, andfornjm let n m W C m ! C n be the<br />

homomorphism induced by c m 7! c n .ThenC D fC n .n 2 N/I n m g is an<br />

inverse system where N is partially ordered by the divisibility relation. Show<br />

that lim C Š Q n p J p:<br />

(3) Let A n Š Z.p 1 /,andletn<br />

nC1 W Z.p 1 / ! Z.p 1 / be the multiplication by<br />

p. Then the inverse limit of the inverse system A DfA n .n 2 N/I n<br />

nC1 g is<br />

isomorphic to the group of all p-adic numbers.<br />

(4) The inverse limit of torsion-free groups is torsion-free, but the inverse limit of<br />

torsion groups need not be torsion.<br />

(5) Let B n Dhb n iŠZ and n m W b m 7! b n for all n m in N. LetC n Dhc n iŠ<br />

Z.p n / and n m W c m 7! c n for n m. Show that<br />

(a) B DfB n .n 2 N/I n mg and C DfC n .n 2 N/I n m g are inverse systems, and<br />

the epimorphisms n W b n ! c n .n 2 N/ define a map ˆW B ! C.<br />

(b) The induced homomorphism ˆ W B ! C between the inverse limits is<br />

not epic. [Hint: Z ! J p .]<br />

(6) The inverse limit of splitting exact sequences need not be exact.<br />

(7) Let A DfA n .n

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