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Continuous function on intervals

  1. Dec 11, 2008 #1
    1. The problem statement, all variables and given/known data
    f:(0,1]->R be a continuous function. Is it possible that f does not have an absolute min or max. Give counter examples


    2. Relevant equations



    3. The attempt at a solution
    Since f is partially bounded, if I break the interval down into smaller sub intervals, each will have an abs min and max. So I will have at least one of the extremes.
     
  2. jcsd
  3. Dec 11, 2008 #2

    Dick

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    The problem says 'give counterexamples'. Do that. They aren't hard to find.
     
  4. Dec 11, 2008 #3
    "if I break the interval down into smaller sub intervals, each will have an abs min and max."

    If you break the interval into a union smaller intervals, you will always have one interval of the form (0, a) where a is some real in (0,1]. What is the difference between [0,1] and (0,1]? Does a continuous function on [0,1] attain a min/max?

    Think of some functions on (0,1] that may not have a min or max (you might want to think of f(x) as x approaches 0)
     
  5. Dec 12, 2008 #4
    How about f(x) = 1 if x=1 and f(x) = 1/(x-1) if x Not= 1. Then as x->0 , f(x) is increasing and as x-> 1, f(x) is decreasing. Since it is 1 at one, it is not the minimum.
     
  6. Dec 12, 2008 #5

    Dick

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    That f is also not continuous on (0,1]. Keep it simple. How about f(x)=1/x?
     
  7. Dec 12, 2008 #6
    Then f has an abs min at x=1. even though it is continuous
     
  8. Dec 12, 2008 #7

    Dick

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    Oh, I thought you wanted no max or no min. You want neither max NOR min. How about a function that oscillates with increasing increasing frequency and magnitude as x->0? Can you give an example of one of those?
     
  9. Dec 12, 2008 #8
    How about sin(1/x)/x. But I don't think it is continuous because of the sharp peaks. Is it a form of sin function?
     
  10. Dec 12, 2008 #9

    Dick

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    That's perfect. It's continuous on (0,1]. The only place it has a problem is at x=0. But x=0 is not in (0,1].
     
  11. Dec 12, 2008 #10
    Thank you. You seem to have a way of making it happen!
     
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