TITLE: Prove divisibility: If $j_1\mid j_2$ and $j_2\mid j_1$, then $j_1 = \pm j_2$
QUESTION [2 upvotes]: I have this math question. I'm not 100% sure how to answer it.

If $j_1\mid j_2$ and $j_2\mid j_1$, then $j_1 = \pm j_2$

I know that by definition $j_1\mid j_2\implies j_2 = j_1\cdot n$ for some $n\in \mathbb{Z}$. Also that $j_2\mid j_1\implies j_1 = j_2\cdot k$ for some $k\in \mathbb{Z}$. However, I'm not sure how to use this fact to prove that $j_1 = \pm j_2$. Thanks

REPLY [4 votes]: $j_1\mid j_2\implies $ either $j_2=0$ or $|j_1|\le |j_2|$.
$j_2\mid j_1\implies $ either $j_1=0$ or $|j_2|\le |j_1|$.
If $j_2=0$, then $0\mid j_1$, so $j_1=0$, so $j_1=j_2$.
If $j_1=0$, then $0\mid j_2$, so $j_2=0$, so $j_1=j_2$.
If $j_1j_2\neq 0$, then $|j_1|\le |j_2|$ and $|j_2|\le |j_1|$, so $|j_1|=|j_2|$, so $j_1=\pm j_2$.

$j_1\mid j_2\iff j_2=kj_1$ for some $k\in\Bbb Z$.
$j_2\mid j_1\iff j_1=nj_2$ for some $n\in\Bbb Z$.
$j_1=nkj_1$. If $j_1=0$, then $0\mid j_2$, so $j_2=0$, so $j_1=j_2$.
If $j_1\neq 0$, then $1=nk$, so $n,k\in\{-1,1\}$.
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