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Homework Help: Checking a proof of a basic property of prime numbers

  1. Dec 3, 2016 #1
    1. The problem statement, all variables and given/known data
    Prove: If p is prime and m, n are positive integers such that p divides mn, then either p divides n or p divides m.

    Is anyone willing to look through this proof and give me comments on the following: a) my reasoning within the strategy I chose (validity, any constraints or cases I might have missed), b) the strategy I chose (was it a good one, is there a better one, some other angle I might have taken), and c) my presentation (messiness, inelegance)?

    2. Relevant equations n/a

    3. The attempt at a solution
    Here's my proof.

    If p divides mn, then there must be some positive integer q such that mn = pq. Then (mn)/q = p is prime. The factors of p are m/q and n. Since p is prime, one of these factors must be equal to 1. Therefore either

    a) n = 1


    b) m/q = 1.

    If a) holds, then mn = m = pq and we see that p divides m.

    If b) holds, then m = q, and so mn = qn = pq, which means p = n and so p divides n (with quotient of 1).

    Therefore, if p divides mn, then either p divides m or p divides n, as desired.
  2. jcsd
  3. Dec 3, 2016 #2


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    or p divides both m and n.
  4. Dec 3, 2016 #3


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    That's not nessecary in the proof. If p divides both m and n, p divides obviously m or n.
  5. Dec 3, 2016 #4


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    You can't conclude that ##m/q## and ##n## are integer factors of ##mn/q##.

    ##q## might divide neither ##m## nor ##n##.
  6. Dec 3, 2016 #5

    Ray Vickson

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    What results do you have available already? What properties are you allowed to use?
  7. Dec 3, 2016 #6
    The text I'm using has so far defined the greatest common divisor and gives the theorem (with proof) that two nonzero integers a and b have a unique positive greatest common divisor.

    I'll try to use these to make the proof. Thanks!
  8. Dec 3, 2016 #7
    I see! Thanks for the heads up!
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