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Homework Help: Limit problem

  1. Jul 5, 2012 #1
    1. The problem statement, all variables and given/known data

    Imagine a function: f(x) = x + sin(x)/K where K is pi cubed.
    Suppose f(x)^n means f(f(f(f(.....f(x))))) n times.
    Find the value of lim n->infinity f(x)^n

    2. Relevant equations

    3. The attempt at a solution

    I have no idea on where to start. Can anyone give a hint, please?
  2. jcsd
  3. Jul 5, 2012 #2


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    Homework Helper

    Let the limit be y. Can you see that [itex]f(y) = y[/itex]? If the limit exists, then the sequence will converge to that value. Applying the function one more time to the limit will not change the value.

    BTW, there's ambiguity in your question. Did you mean [itex]f(x) = x + \frac{\sin x}{K}[/itex] or [itex]f(x) = \frac{x + \sin x}{K}[/itex]?
  4. Jul 5, 2012 #3
    Yes, I understand that much.

    Oh, sorry! I meant the former.
  5. Jul 5, 2012 #4


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    Homework Helper

    This is actually a fairly tricky problem. The answer is that there's no single limit. There are, however, an infinite number of "attractors" to which the infinitely iterated function will converge.

    Why don't you start by applying the hint in my first post, then we'll take it from there.

    Another very important hint: sketch the curve of [itex]y = \sin{\frac{x}{\pi^3}}[/itex].
    Last edited: Jul 5, 2012
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