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The additive groups of integers and rational numbers are not isomorphic


Prove that the additive groups $\mathbb{Z}$ and $\mathbb{Q}$ are not isomorphic.


Solution: First we prove a lemma.

Lemma. If $\varphi : G \rightarrow H$ is a surjective group homomorphism and $G$ is generated by one element, then $H$ is generated by one element.

Proof: Let $h \in H$. Since $\varphi$ is surjective, there exists $g \in G$ with $\varphi(g) = h$. We can write $g = s^k$ for some $k \in \mathbb{Z}$, where $s$ is a generator of $G$. Now $$h = \varphi(g) = \varphi(s^k) = \varphi(s)^k.$$ Note that $\varphi(s)$ is a fixed element of $H$; thus $\varphi(s)$ is a generator of $H$.


Now suppose an isomorphism $\varphi : \mathbb{Z} \rightarrow \mathbb{Q} $ exists. We know that $\mathbb{Z}$ is generated by one element, so that by the lemma $\mathbb{Q}$ is also generated by one element; in particular, $\varphi(1)$. Suppose $\varphi(1) = \dfrac{m}{n}$. Now choose $\dfrac{1}{q} \in \mathbb{Q}$ such that $q$ does not divide $n$. Then we have $\dfrac{1}{q} = k \cdot \dfrac{m}{n}$ for some integer $k$, hence $n = kqm$. But then $q|n$, a contradiction. So no isomorphism $\varphi$ exists.

Linearity

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