Monday, June 3, 2019

elementary number theory - How to solve this congruence $17x equiv 1 pmod{23}$?


Given $17x \equiv 1 \pmod{23}$


How to solve this linear congruence? All hints are welcome.


edit: I know the Euclidean Algorithm and know how to solve the equation $17m+23n=1$ but I don't know how to compute x with the use of m or n.


Answer




To do modular division I do this:


an - bm = c where c is dividend, b is modulo and a is divisor, then n is quotient


17n - 23m = 1


Then using euclidean algorithm, reduce to gcd(a,b) and record each calculation


As described by http://mathworld.wolfram.com/DiophantineEquation.html


17 23 $\quad$ 14 19


17 6 $\quad\;\;$ 14 5


11 6 $\quad\;\;\;\;$ 9 5


5 6 $\quad\;\;\;\;\;$ 4 5


5 1 $\quad\;\;\;\;\;$ 4 1



1 1 $\quad\;\;\;\;\;$ 0 1


Left column is euclidean algorithm, Right column is reverse procedure


Therefore $ 17*19 - 23*14 = 1$, i.e. n=19 and m=14.


The result is that 1/17 ≡ 19 mod 23


this method might not be as quick as the other posts, but this is what I have implemented in code. The others could also be, but I thought I would share my method.


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