<s>[INST] Prove: $2^b-1\mid{}2^a-1 \iff b\mid{}a$ I figured we'd have to show $a=qb$ from $2^a-1=r(2^b-1)$ for $q,r\in\mathbb{Z}$ for the first implication and vice versa, how should I proceed from here?
 [/INST] Hint $:$ $2^{qb}-1  = (2^b -1) (2^{b(q-1)} + 2^{b(q-2)} + \cdots + 1).$
The above hint is enough to prove that $b \mid a \implies 2^b-1 \mid 2^a - 1.$ 
To prove that $2^b - 1 \mid 2^a - 1 \implies b \mid a$ use division algorithm to write $a = bq + r,$ where $0 \leq r < b.$ Then we have
$$\begin{align}
2^a - 1 & = 2^{bq+r} - 1. \\ & = 2^{bq+r} - 2^r + 2^r -1. \\ & = 2^r (2^{bq} -1) + 2^r - 1. \end{align}$$
Now if $2^b-1 \mid 2^a-1$ then since $2^b - 1\mid 2^r(2^{bq} -1)$ so we have $2^b - 1 \mid 2^r - 1,$ which is absurd if $0<r < b.$ Therefore we must have $r=0$ and hence we have $a=bq.$ Or in other words $b \mid a,$ as claimed.
QED
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