Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Bounded function math help

  1. Nov 14, 2006 #1
    Hi, I would like to know if the function f(x) = (x+2)^-1 is bounded on the open interval (-2,2)? The interval doesn't include the point x = -2 but I'm not sure if I can say that there is a K>=0 such that |f(x)| < K for all x in (-2,2).

    The function is defined everywhere in that interval but still approaches positive infinity as x tends to x = - 2 from above so I'm not sure what to conclude here. Any help would be great thanks.
  2. jcsd
  3. Nov 14, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Instead of trying to find such a K, i.e. instead of trying to proove that f is bounded, try to prove that it is not bounded. (And it obviously is not bounded since, as you noted yourself, it is intuitively evident that the function goes to infinity as x gets nearer and nearer to -2)

    A function is not bounded on its domain if given any number M, you can find an element 'a' of the domain such that f(a)>M.

    So somewhat like in the epsilon-delta proofs, if you can find a relation a(M) that associated to every M a number 'a' in (-2,2) such that f(a)>M, then you will have proven the unboundedness of f.
    Last edited: Nov 14, 2006
  4. Nov 14, 2006 #3


    User Avatar
    Homework Helper
    Gold Member

    Do you know anything about finding the maximum of a function using derivatives?

    If yes, find the derivative of the function and try to find a maximum/minimum point. You shouldn't be able to find one, which means the function itself isn't bounded in the open interval.

    Otherwise, assume there exists some K that does satisfy your equation. Now, find an x that does not satisfy the equation, which is a contradiction so that such a K can not exist.

    Therefore, simply solve |f(x)|<K, where K is a constant and be sure to pick an x that is in the open interval (-2,2).

    Note: The answer for x should be dependent on K.
  5. Nov 15, 2006 #4
    Ok thanks. I probably made it more complicated than it was.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook