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Jun 13, 2020 at 22:11 comment added user69642 I think it was stated by Zygmund in this paper projecteuclid.org/euclid.aoms/1177730443 but without proof. For a proof, you should have a look at his fundamental work on trigonometric series.
Jun 13, 2020 at 15:57 vote accept trisct
Jun 13, 2020 at 15:56 comment added trisct Thanks. I saw the example. But it was provided without proof that it is differentiable. Did Zygmund prove it? Can you give a reference to the proof of differentiability?
Jun 13, 2020 at 15:36 comment added user69642 To complement my post and to counter the down vote: in the same chapter there is an example due to Zygmund of a characteristic function with a continuous first derivative but for which the associated law has an infinite first moment...
Jun 13, 2020 at 12:47 history answered user69642 CC BY-SA 4.0