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Is this contains an open set?

  1. Apr 9, 2010 #1
    On the number line R, does [-1,[tex]\infty[/tex]) contain an open set?

    because it includes -1, don't think it is an open set.
  2. jcsd
  3. Apr 9, 2010 #2
    It's a half open interval that you've shown. If you define a set {[tex][-1,\infty)[/tex]} then it contains at least one half open subset.
    Last edited: Apr 9, 2010
  4. Apr 9, 2010 #3


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    Is the question: Is [tex] [-1, \infty)[/tex] an open set?

    Or is the question: Does [tex] [-1, \infty)[/tex] contain an open set?
  5. Apr 10, 2010 #4
    What's the difference?
  6. Apr 10, 2010 #5
    It is not an open set. But it contains the open set (4, 7) for example.
  7. Apr 10, 2010 #6
    I guess I'm not understanding the OP's question. Any non zero interval on the reals "contains" every possible combination: [a,b],(a,b),(a,b],[a,b). Any such interval has a bijective mapping to the entire set of reals, so of course the interval [-1,[tex]\infty)[/tex] "contains" open sets.

    EDIT: Perhaps I'm mistaken, but in terms of open and closed sets or subsets, I'm considering the actual membership of a given set to be dependent on the specification (choice) of that set. Therefore I could specify that every subset of C:C subset of R be closed.
    Last edited: Apr 10, 2010
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