Hw10 Problem 4

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### Problem 4

Suppose that $k$ is a divisor of $n$. Prove that $\mathbb{Z}_n / \langle k \rangle \simeq \mathbb{Z}_k$.

Solution

Suppose that $k$ is a divisor of $n$, with $\mathbb{Z}_n / \langle k \rangle$.
Then $\mid k \mid = \dfrac{n}{gcd(n,k)}$.
$\hspace{40pt} =\dfrac{n}{k}$, since $k$ divides $n$ and $k$ is the greatest divisor of itself.
Consequently, $\mid \mathbb{Z}_n / \langle k \rangle \mid = \dfrac{n}{\bigg( \dfrac{n}{k} \bigg) } = k$.
Now, by Theorem 15.9, $\mathbb{Z}_n / \langle k \rangle$ is cyclic.
Furthermore, $\mathbb{Z}_n / \langle k \rangle$ is abelian, since every cyclic group is abelian.
Now by the Fundamental Theorem of Finitely Generated Abelian Groups, any cyclic group of order $k$ must be isomorphic to $\mathbb{Z}_k$.
Therefore, $\mathbb{Z}_n / \langle k \rangle \simeq \mathbb{Z}_k$.
$\hspace{100 pt} \clubsuit$

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