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The additive group of rational numbers has no subgroups of finite index


Solution to Abstract Algebra by Dummit & Foote 3rd edition Chapter 3.2 Exercise 3.2.21

Prove that $\mathbb{Q}$ has no proper subgroups of finite index. Deduce that $\mathbb{Q}/\mathbb{Z}$ has no proper subgroups of finite index. [Recall Exercise 1.6.21 and Exercise 3.1.15.]


Solution: We begin with a lemma.

Lemma: If $D$ is a divisible abelian group, then no proper subgroup of $D$ has finite index.

Proof: We saw previously that no finite group is divisible and that every proper quotient $D/A$ of a divisible group is divisible; thus no proper quotient of a divisible group is finite. Equivalently, $[D:A]$ is not finite. $\square$

Because $\mathbb{Q}$ and $\mathbb{Q}/\mathbb{Z}$ are divisible, the conclusion follows.


Linearity

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