Tuesday, 11 June 2013

modular arithmetic - A small question about multiplicative inverses



I wan't to find the multiplivative inverse of [22]112 in Z12




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]112 in Z12


Answer



Suppose that there was a multiplicative inverse of 22 in Z12, 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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