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About limit point

  1. Mar 23, 2012 #1
    the definition of limit point:
    a point p is a limit point of E(subset of metric space X) if every neighborhood of p contains a point q≠p which is in E.

    My question is that is there a limit point p which is not in E?
  2. jcsd
  3. Mar 23, 2012 #2
    Take the open unit interval E = (0,1) as a subset of the real numbers. Can you think of a real number that's not in (0,1) but that satisfies the definition of limit point of E? (Hint: Can you think of TWO such points?)
  4. Mar 23, 2012 #3
    thanks but i have another question

    can you give me an example of a set that is perfect?
    def: E is perfect if E is closed and if every point of E is a limit point of E
  5. Mar 23, 2012 #4


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    So you refuse to answer SteveL27's question?

    But I will answer your question: [0, 1].

    It's actually harder to give an example of a closed set that is NOT perfect. Can you?
  6. Mar 25, 2012 #5
    What stops me from adding {0} to usual topology of real line, so that is´s open set? Then (0, 1] would be closed and not perfect. Certainly not easy, I can´t think of any more standard example.
  7. Mar 25, 2012 #6
    (0, 1] is not closed; it's just also not open. A good example of a closed non-perfect set is one with an isolated point, like {2}, or [0,1][itex]\cup[/itex]{2}.
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