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Abelian Groups - László Fuchs [Springer]

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232 8 Tensor and Torsion Products<br />

Here again, W c 7! N1 ˝ c is an epimorphism C ! Z.m/ ˝ C where N1 D<br />

1 C mZ. WehavemC Ker , since1 ˝ mc D m ˝ c D 0 ˝ c D 0. Now<br />

.n; c/ 7! nc C mC is a bilinear map Z.m/ C ! C=mC, so by the universal<br />

property, there is a homomorphism W Z.m/ ˝ C ! C=mC such that is<br />

the canonical map C ! C=mC. Thus Ker D mC.<br />

(C) The heights satisfy h p .a ˝ c/ h p .a/ C h p .c/ .a 2 A; c 2 C/. Thus if either A<br />

or C is p-divisible (resp. divisible), then so is A ˝ C.<br />

(D) If h p .a/ ! for a 2 A and C is a p-group, then a ˝ c D 0 for every c 2 C.<br />

Thus if A is p-divisible and C is a p-group, then A ˝ C D 0.<br />

(E) If a 2 mA and c 2 CŒm for some m 2 Z, thena˝ c D 0.<br />

(F) If either A or C is a p-group .torsion group/,thensoisA˝ C.<br />

(G) If A is a p-group and C is a q-group for different primes p; q, then A ˝ C D 0.<br />

Example 1.3. We have the following natural isomorphisms: Z ˝ Z.n/ Š Z.n/, Z.p n / ˝ Z.p m / Š<br />

Z.p k / with k D minfm; ng, andZ.n/ ˝ Z.m/ Š Z.d/ where d D gcdfm; ng.<br />

Example 1.4. Suppose A is a rational group. Then every x 2 A ˝ C can be written in the form<br />

x D a˝c with some a 2 A; c 2 C. In fact, as always, x D P n<br />

iD1 .a i˝c i / holds with a i 2 A; c i 2 C,<br />

but in the present case A is locally cyclic, i.e. there exists an a 2 A such that each a i is an integral<br />

multiple of a, saya i D m i a .m i 2 Z/. Thus<br />

x D X .m i a ˝ c i / D X .a ˝ m i c i / D a ˝ . X m i c i /<br />

where P m i c i D c 2 C. This form is not unique: if a D ma 0 ,thenalsox D a 0 ˝ mc.<br />

(H) It is very important to keep in mind that if B is a subgroup of A, then B ˝ C<br />

need not be a subgroup of A ˝ C. For instance, Z < Q,butZ ˝ Z.p/ Š Z.p/ is<br />

not a subgroup of Q ˝ Z.p/ D 0;or,Z.p/

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