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Math Help - Open sets

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    Open sets

    Prove that (0,1] is open in [-1,1] but not in R?
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    Super Member Matt Westwood's Avatar
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    Quote Originally Posted by bigdoggy View Post
    Prove that (0,1] is open in [-1,1] but not in R?
    The point 1 does not have a neighbourhood in R that does not include elements that are not in (0,1].

    Too many negatives. Try again.

    All neighbourhoods of 1 in R contain elements that are not in (0,1]. However, in [-1,1] this is not the case as there are no elements of [-1,1] that are not in a neighbourhood of 1.

    At the other end of the interval (0,1], the end is open. Every point near 0 has a neighbourhood completely contained in (0,1].

    So every point of (0,1] has a neighbourhood entirely within (0,1] except 1, for which see above.
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    Quote Originally Posted by bigdoggy View Post
    Prove that (0,1] is open in [-1,1] but not in R?

    Lemma: if X is a topological space and Y is a subset of Y with the inherited topology from X, then a subset A of Y is opne in Y iff A = Y /\ U, with U open in X.

    This solves the problem.

    Tonio
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    Super Member Matt Westwood's Avatar
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    Quote Originally Posted by tonio View Post
    Lemma: if X is a topological space and Y is a subset of Y with the inherited topology from X, then a subset A of Y is opne in Y iff A = Y /\ U, with U open in X.

    This solves the problem.

    Tonio
    What's Y /\ U?
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    I guess that's the intersection.. he means of course that if you use the subspace topology then a set is open if it is the intersection of he subset and an open set in the base set.

    eg (0,1] is open in [1,1] because (0,2) is open in R, and (0,2) intersected with [0,1] = (0,1]..
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    Quote Originally Posted by Matt Westwood View Post
    What's Y /\ U?

    Intersection....

    Tonio
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