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Homework Help: Can someone explain this to me?

  1. Apr 11, 2013 #1
    1. The problem statement, all variables and given/known data
    If gcd(a,m) > 1, then ax [itex]\equiv[/itex] 1 (mod m) is impossible.

    2. Relevant equations

    3. The attempt at a solution
    There is no solution per se, only an explanation. I know that m would have to divide ax and 1. Since only 1 divides 1, the statement is impossible. But that doesn't explain how the condition, gcd(a,m) > 1, is relevant. Furthermore, if the gcd(a,m) were 1, the statement would be true. Why is it that when the gcd > 1 that the congruence is impossible?
  2. jcsd
  3. Apr 11, 2013 #2


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    No, m doesn't have to divide ax and 1. m would have to divide ax-1. If gcd(a,m)=k then k would divide m and a. Think this through again.
  4. Apr 11, 2013 #3
    Yes, m most certainly does divide ax and 1. It's a modular congruence relation. Because m divides the difference it must also divide the minuend and the subtrahend. At least, that should be true. But for some reason, the gcd's being greater than 1 prevents that because the congruence relationship is impossible.

    Can no one explain this?
  5. Apr 11, 2013 #4


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    7 divides 10-3. That doesn't mean 7 divides 10 and 3.
    m has a factor in common with a, but bit need not divide a.
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