Sunday, 23 August 2015

measure theory - Example of a Lebesgue measurable set that can't be constructed from Borel sets and projections?

The Borel sigma algebra on Rn is obtained by starting with open sets and repeatedly applying the operations of complement, countable union, countable intersection. Now Henri Lebesgue famously made the mistake of thinking that the projection of a Borel set is always a Borel set. In reality, the projection of a Borel set need not be a Borel set, although it is still Lebesgue measurable. So that is a way of constructing a Lebesgue measurable set that is not a Borel set.




But my question is, what is an example of a Lebesgue measurable set that cannot be constructed in this way? That is, what is a Lebesgue measurable set that cannot be constructed by starting with open sets in Rn and repeatedly applying the operations of complement, countable union, countable intersection, and projection?

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