Sunday, 10 February 2013

real analysis - If f measurable and B a Borel set, then f1(B) measurable




Let f be a Lebesgue measurable function and B be a Borel set. Show that f1(B) is also measurable.




Attempt at the proof:




Suppose f is Lebesgue measurable. Then f1((α,)) is measurable as well (by definition of a measurable function), αR. We note that (α,)B, where B is the Borel σ-algebra, since:




  • (α,)

  • (α,)c=R(α,)B

  • the infinite union of open sets is once again an open set



So, (α,) is a Borel set. So let B=(α,). Since f is measurable, f1((α,)) is measurable and so f1(B) is measurable.




However I'm doubtful that this is correct, since I didn't choose an arbitrary Borel set B. Can anyone nudge me in the right direction? Thanks.


Answer



{A|f1(A)is Borel} is a σ-algebra containing all open intervals. What can you say now?


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