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Homework Help: Proving that a set is not bounded from above.

  1. Jan 12, 2013 #1
    1. The problem statement, all variables and given/known data
    Prove that if a is a real number, a > 1, then the set {a, a^2, a^3, ...} is not bounded from above. Hint: First find a positive integer n such that a > 1 + 1/n and prove that a^n > (1 + 1/n)^n >/= 2.

    2. Relevant equations

    3. The attempt at a solution

    Showing that there exists a positive integer n such that a > 1 + 1/n is not difficult. Since a > 1, a-1 is a positive real number so there exists an integer 1/n such that a-1 > 1/n and thus a > 1 + 1/n. Proving the second set of inequalities is not difficult either. I'm at a complete loss as to how the hint relates to the problem.
  2. jcsd
  3. Jan 12, 2013 #2
    Look at the sequence ##(a^{nk})## as ##k## varies.
  4. Jan 12, 2013 #3
    Can you prove that {2. 22, 23, 24, ...} is not bounded above?
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