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Homework Help: Topology, defn of a nowhere dense set in a metric space

  1. Feb 20, 2007 #1
    1. The problem statement, all variables and given/known data
    Defn: A subset A of a metric space (X, d) is NOWHERE DENSE if its closure has empty interior.

    Now I am told that this implies 1. A is nowhere dense iff closure of A does not contain any non-empty open set and 2. A is nowhere dense iff each non-empty open set has a non-empty open subset disjoint from A.

    I have no problem with statement 1, but with regards to statement 2, my query is, is it implicit that each non-empty open set is a non-empty open set in X?

    2. Relevant equations
    The closure of A is the union of A and its limit pts

    3. The attempt at a solution
    I ask the question because from statement 1, there can be no open set in the closure of A, and thus in A or the set of its limit pts. The only other set where an open set could be seems to be X.
  2. jcsd
  3. Feb 20, 2007 #2


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    Yes. I would take 'open' to mean 'open in X'. After all, a nowhere dense set has to have someplace to be nowhere dense in.
  4. Feb 20, 2007 #3
    Thanks Dick, that's the second time you've helped me out this week.
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