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Homework Help: Discontinuous functions

  1. Jan 23, 2010 #1
    1. The problem statement, all variables and given/known data

    I have to find f: R [tex]\rightarrow[/tex] R which is discontinuous at the points of the set {1/n : n a positive integer}[tex]\cup[/tex] {0} but continuous everywhere else.

    Also find g: R [tex]\rightarrow[/tex] R which is discontinuous at the points of the set {1/n : n a positive integer}but continuous everywhere else.

    2. Relevant equations



    3. The attempt at a solution

    Could I define f as f(x) = 1/(integer(x) -1) for x [tex]\in[/tex] [0,1) (where integer means round up to next integer), f(x) = x otherwise.

    Similarly for g, can I say g(x) = 1/(integer(x) -1) for x [tex]\in[/tex] (0,1) (where integer means round up to next integer), g(x) = x otherwise.

    I'm not too sure about these functions but cannot think of any more 'normal' ones that would satisfy the criteria.
     
  2. jcsd
  3. Jan 24, 2010 #2

    CompuChip

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    If you are looking for a 'normal' function, you probably want f(x) = 1/g(x), where g(x) has a zero at every integer n and at 0. In that case, I can think of a suitable g(x) off the top of my head.

    By the way, note that restricting the domain to [0, 1) or (0, 1) does not define a function on R.
     
  4. Jan 24, 2010 #3
    Can I say f(x) = 1/g(x) where:
    g(x) = 0 if x is in (0,1)
    g(x) = 1 otherwise
     
  5. Jan 24, 2010 #4
    Well no, since then you are having it be discontinuous on (0,1). Perhaps try 1/g, where g is a step function with an appropriate argument so that you get 1/x back when x is of the form 1/n, but the left and right limits as you approach an x of this form are different fractions altogether.
     
  6. Jan 24, 2010 #5
    I think I may be confused about what 'at the points of the set' means, does it just mean the end points, I thought it meant all points in the set?

    So for {1/n: n a positive integer} U {0} could I say:
    f(x) = -1, x<0
    f(x) = -1/2, x=0
    f(x) = 1/x, 0<x<1
    f(x) = 2, x>1

    For {1/n: n a positive integer} I could change this to:
    f(x) = -1, x[tex]\leq[/tex]0
    f(x) = 1/x, 0<x<1
    f(x) = 2, x>1
     
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