Sunday, 4 January 2015

elementary number theory - Difficulty in finding modulus of fraction

It is quite easy to evaluate abmodm when a, b and m are integers and gcd(b,m)=1 by replacing 1b with an inverse of b modulo m.



But, is it possible to evaluate that when gcd(b,m)1? For example I tried to find 167mod7 following way:




We are actually to find some x such that \frac{16}{7}\equiv x \pmod 7
As modular arithmetic permits multiplication16\equiv 7x \pmod 7
But, 7x\equiv0 \pmod 7
Hence 16\equiv0 \pmod 7 which is absolutely wrong.



What wrong am I doing or should I conclude no such x exists?

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