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I'm given three values $s_0,s_1,s_2 \in \mathbb{Z}_m$ that follows the recursion formular $s_{n+1} = as_n+b \mod m$.

I know $m$ and want to calculate $a,b$. It's easy to see that $a=\frac{s_1-s_2}{s_0-s_1}$ however in my case $gcd(s_0-s_1,m)\neq 1$ thus I'm unable calculate the multiplicative inverse of $s_0-s_1 \mod m$.

Is there any other way of doing this without bruteforcing?

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  • $\begingroup$ Hint: be rigorous in establishing a relation between $m$, $a$, $s_0-s_1$, and $s_1-s_2$, in order to come with one that holds without assuming $\gcd(s_0-s_1,m)=1$. Then use that if $v$ divides $u$ and $w$ divides $v$, then $v/w$ divides $u$. $\endgroup$
    – fgrieu
    Sep 14, 2022 at 13:00

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