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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The answer is yes, and is given in this link.
Last edited by girdav; Aug 3rd 2011 at 07:27 AM.
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.)
Originally Posted by pece 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 is a Borel set of ?").
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