The infinity of primes

  1. Jan 21, 2010 #1
    Euclid's proof:

    1) Assume there is a finite number of primes.
    2) Let Pn be the largest prime.
    3) Let X be the P1 * P2 ... * Pn + 1

    At this point the statement is that "X cannot be divided by P1 through Pn", but why is that? This is not self-obvious to me. How can I know this?

    k
     
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  3. Jan 21, 2010 #2

    HallsofIvy

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    Because dividing by any of those primes gives a remainder of 1, not 0:

    [tex]\frac{P_1P_2...P_{i-1}P_iP_{i+1}...Pn+ 1}{Pi}= P_1P_2...P_{i-1}P_{i+1}...Pn+ \frac{1}{P_i}[/tex]
    The first term is an integer but the second is not.
     
  4. Jan 21, 2010 #3

    Hurkyl

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    Minor aside: "the infinity of primes" is bad grammar. The noun form is wrong here -- you want the adjective "infinite", such as in "the infinite set of primes".
     
  5. Jan 21, 2010 #4

    Hurkyl

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    Try dividing.
     
  6. Jan 22, 2010 #5
    That made perfect sense, thank you!

    k
     
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