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Outer Measure

  1. Feb 9, 2013 #1
    1. The problem statement, all variables and given/known data

    Let E have finite outer measure. Show that if E is not measurable, then there is an open set O containing E that has finite measure and for which

    m*(O~E) > m*(O) - m*(E)

    2. Relevant equations

    3. The attempt at a solution

    This is what I did...

    [itex]m^*(O) = m^*((O \cap E^c) \cup m^*((O \cap E)) \leq m^*(O \cap E^c) + m^*(E)[/itex]


    [itex]m^*(O) - m^*(E) \leq m^*(O \cap E^c)[/itex]

    But I'm confused about the equality, because the one in the question is a strict inequality
    Last edited: Feb 9, 2013
  2. jcsd
  3. Feb 9, 2013 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    What you have done is just to prove that if there's an open O that contains E, then the last inequality in your post is satisfied. You have done nothing to prove that such a set exists. Of course, the entire space is always an open set, so that's not really an issue, at least not when the space has finite measure. But this still suggests that you need to find a special kind of open set, not an arbitrary one. And you should expect to have to use the definition of "measurable" to find it.

    It would have been a good idea to include the definition of "measurable" under "relevant equations".
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