# Linear Congruence

• Sep 27th 2007, 05:03 PM
clockingly
Linear Congruence
The following was confusing me, so any help/explanations would be greatly appreciated!

Find all incongruent solutions to the following linear congruence:

21x ≡ 14 (mod 91)
• Sep 29th 2007, 05:12 PM
ThePerfectHacker
Quote:

Originally Posted by clockingly
The following was confusing me, so any help/explanations would be greatly appreciated!

Find all incongruent solutions to the following linear congruence:

21x ≡ 14 (mod 91)

$\gcd(21,91)=7$ and $7|14$ so there shall be $7$ solutions in each congruence class. By inspection we see that $x\equiv 5$ is one equivalence class which solves this. By theorem it means $5 + \frac{91}{7}t$ for $t=0,1,...,6$ shall all be solutions.
• Sep 30th 2007, 08:07 AM
topsquark
Quote:

Originally Posted by ThePerfectHacker
$\gcd(21,91)=7$ and $7|14$ so there shall be $7$ solutions in each congruence class. By inspection we see that $x\equiv 5$ is one equivalence class which solves this. By theorem it means $5 + \frac{91}{7}t$ for $t=0,1,...,6$ shall all be solutions.

By "incongruent" solutions the question is asking for all distinct residue classes that solve the equation?

-Dan
• Sep 30th 2007, 09:05 AM
ThePerfectHacker
Quote:

Originally Posted by topsquark
By "incongruent" solutions the question is asking for all distinct residue classes that solve the equation?

-Dan

Yes.

For example $x\equiv 2 (\bmod 4)$ the solutions $x=2,6$ are really the same ("congruent") solutions because they are contained in the same equivalence class (equivalence class mod $4$*)

*)The congruence classes are $\{...,-4,0,4,...\} , \{...,-3,1,5,...\}, \{...,-2,2,6,...\}, \{...,-1,3,7,...\}$. So $x=2,6$ are in the same equivalence class.
• Jun 3rd 2008, 11:04 AM
duggaboy
hmm
so if showing the incongruences would it be x=-12=14 (mod13)??