Tuesday 11 June 2013

modular arithmetic - A small question about multiplicative inverses



I wan't to find the multiplivative inverse of $[22]^{-1}_{12} $ in $Z_{12}$




But when I do the euclidean algorithm on 22 and 12 I get : gcd(22, 12) = 2
Does that mean there are no multiplicative inverses? I only know how to get the multiplicative inverse if gcd(22, 12) = 1.



Are there any other ways to get the multiplicative inverse of $[22]^{-1}_{12} $ in $Z_{12}$


Answer



Suppose that there was a multiplicative inverse of $22$ in $\Bbb Z_{12}$, call it $k$. Then $22k\equiv 1\pmod{12}$, implying that there exists an integer $n$ such that $22k=1+12n$, but this is an impossibility since the number on the left is even but the number on the right is odd for every integer values of $k$ and $n$



Therefore, $22$ does not have a multiplicative inverse in $\Bbb Z_{12}$.



In general, for $x$ to have a multiplicative inverse in $\Bbb Z_n$ one requires that $\gcd(x,n)=1$



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