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Math Help - a question about the borel set

  1. #1
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    a question about the borel set

    Is there a borel set A in R2 such that
    {x,x belongs to R1 and there is y such that (x,y) belongs to A} is not a borel set in R1.
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  2. #2
    Super Member girdav's Avatar
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    Re: a question about the borel set

    The answer is yes, and is given in this link.
    Last edited by girdav; August 3rd 2011 at 08:27 AM.
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  3. #3
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    Re: a question about the borel set

    Your link says the answer is yes.

    (It's amusing, by responding, I almost did the same mistake than Lebesgue. I stop just hitting the 'submit' button.)
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  4. #4
    Super Member girdav's Avatar
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    Re: a question about the borel set

    Quote Originally Posted by pece View Post
    Your link says the answer is yes.
    I red too fast the question, indeed the answer is yes (I red the question as "Is it true that the projection of every Borel set of \mathbb{R}^2 is a Borel set of \mathbb{R}?").
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