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May 28, 2016 at 9:58 comment added Anton Petrunin @Eric Oh, right, try to apply Lebegues density theorem --- hope it works.
May 27, 2016 at 23:41 comment added user92157 @Anton Yeah I see how to do it if the set $A$ is dense somewhere but sets of positive measure can be nowhere dense and that's what I'm not sure about.
May 27, 2016 at 11:17 comment added Anton Petrunin @Eric I think so. You have to start with an interval in which most of the points have undefined derivative.
May 27, 2016 at 3:34 comment added user92157 Could we modify this to show that if the derivative of $f$ fails to exist on some set of positive measure, then it fails to have the Luzin N property?
May 26, 2016 at 13:48 history edited Anton Petrunin CC BY-SA 3.0
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May 26, 2016 at 13:22 vote accept CommunityBot
May 26, 2016 at 12:55 history edited Anton Petrunin CC BY-SA 3.0
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May 26, 2016 at 12:50 comment added Anton Petrunin @FedorPetrov I made an update.
May 26, 2016 at 12:45 history edited Anton Petrunin CC BY-SA 3.0
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May 26, 2016 at 12:29 comment added Fedor Petrov Hm, how do you apply Vitali theorem? Intervals $I$ with image $f(I)$ much greater than $I$ should be everywhere dense, but they should not cover the whole segment or even most of it.
May 26, 2016 at 8:36 history answered Anton Petrunin CC BY-SA 3.0