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Homework Help: Archimedian Property proof

  1. Oct 3, 2008 #1
    Statement to prove:
    If x > 0, show there exists n in N (the set of all natural numbers) such that 1/(2^n) < x.

    My work on the proof so far:
    Let x > 0. By the Archimedian Property, we know if ε > 0, there exists an n in N such that 1/n < ε.
    Take x = ε . So there exists an n in N such that 1/n < x.

    That is as far as I've gotten. I am stuck as to how I can algebraically manipulate the inequality to get the 2 in there somehow and to get the final form of 1/(2^n) < x.
     
  2. jcsd
  3. Oct 3, 2008 #2

    Dick

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    Can you show 2^n>n for n>1?
     
  4. Oct 3, 2008 #3
    That statement 2^n > n sounds familiar. I think we proved something like that using math induction before. So if I can state that, then it would be helpful in finishing the proof, right?

    Thanks for looking!
     
  5. Oct 3, 2008 #4

    Dick

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    Since that would mean 1/2^n<1/n, I think it would be extremely helpful. You're welcome.
     
  6. Oct 5, 2008 #5
    Thanks again for the help!

    I think I got it [I have a proof within a proof]:

    My proof: Let x > 0. By induction 2^n > n
    (Proof: Base case: for n = 1, 2^1 > 1. Check. Inductive step: Assume true for n = k, k a natural number. That is 2^k > k is true. To show this true for k + 1, we need to show 2^(k+1) > k +1 which implies 2*2^k > k +1. By assumption that 2^k > k, then 2*2^k > 2k. But for all k ≥ 1, 2k ≥ k + 1, so
    2^(k+1) > 2k ≥ k+1. Therefore 2^(k+1) > k + 1, and 2^n > n. QED)
    So since 2^n > n, then 1/2^n < 1/n. By the Archimedian Property, 1/n < x. Therefore
    1/2^n < 1/n < x which implies 1/2^n < x. QED.
     
  7. Oct 5, 2008 #6

    Dick

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    You nailed it.
     
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