Thursday, 14 November 2013

elementary number theory - More than one modular multiplicative inverse possible?

I am redoing exams as a preparation and I found this weird particular exercise to me.




"Does 32 have a multiplicative inverse in modulo 77? If yes, calculate the inverse."



Since the gcd is 1, it has an inverse.
However, when I calculated it using the extended euclidean algorithm, I ended up with



1 = (-12)32 + (5)77, which means my inverse of 32 in mod 77 is -12?
When I used an online calculator to check my answer I always got 65, though.



I'm not quite sure I understand why or how it is 65 and not -12...
I have redone my method multiple times but I always end up with -12




Thank you for your time in advance.

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