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Homework Help: Continuous and Disc. functions

  1. Nov 19, 2008 #1
    I am trying to understand continuous and discontinuous. These are two assignments I have for a class. I am just looking for some feedback...

    Let A= {1/n : n is natural}

    Then, f(x)= (x , if x in A)
    (0 , if x not in A)

    This is discontinuous on A but continuous on A complement?

    Let B = { x : 0 [tex]\leq[/tex] x < 1 }

    Then, g(x) = (1-x , 0[tex]\leq[/tex]x<1 and x is rational)
    (0 , otherwise)

    And this is discontinuous on B but continuous on B complement?
  2. jcsd
  3. Nov 19, 2008 #2


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    Are you just guessing? For the first one, 0 is in A complement. Is f continuous at 0? For the second one, maybe. But why do you think so? You should give reasons as well as just a guess at the answer.
  4. Nov 20, 2008 #3

    I think that the problem point for set A is 0. I believe that f(x) is continuous there because I drew an epsilon in my mind around f(x) on the y-axis and imagined that I could find a delta such that not only the (x not in A) values would map into it (obviously) but that I could choose delta such that the 1/n values were far enough down, and thus in the epsilon N. I hope I am right on this. The problem points are hard to figure out.

    With B I am using the same reasoning. This time though we have discontinuous at x=0 because the f(x) values are at value 1 while there are always values in your delta N such that f(x)=0. But at x=1, f(x) is getting as close as we want it to get to f(x)=0.
  5. Nov 20, 2008 #4


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    Now that you've explained your reasoning, I feel better better about agreeing with you. I think that's right.
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