Thursday 18 August 2016

Solving a basic linear congruence



I have been tasked with solving a linear congruence:



$$-12x\equiv-3\pmod{26}$$







How do I do this? I've never done linear congruences with minus signs so I'm quite confused.



Usually I would find the inverse of the LHS and multiply the RHS by the inverse however obviously since we have a negative number, the number isn't in $\mathbb{Z}_{26}$ so we can't find an inverse.


Answer



For some integer $k$,
$$\begin{align*}
-12x&\equiv -3 \pmod{26}\\

-12x &= -3 + 26k
\end{align*}$$
The left hand side is divisible by $2$, but the right hand side is not, so there is no solution for $x$.


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