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Nov 22 at 10:08 comment added Zygmund @IosifPinelis yes. It is. I have added that in the question.
Nov 22 at 10:07 history edited Zygmund CC BY-SA 4.0
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Nov 21 at 12:54 comment added Iosif Pinelis Is $\chi$ smooth?
Nov 21 at 2:42 comment added Zygmund @IosifPinelis. I believe one can show that $\tilde{\sigma} \ast \sigma$ is absolutely continuous with respect to the Lebesgue measure, and we are talking about its Radon--Nikodym derivative.
Nov 21 at 2:33 comment added Zygmund @AlekseiKulikov, sorry about the confusion. $d\theta$ is the standard spherical measure on $\mathbb{S}^{d-1}$. The purpose of $\chi$ is to localise the measure. If it is supported on a ball centered at the north pole with radius $\leq 1/2$, then we can see that $\sigma$ is supported on the half sphere. Does this clarify your doubt?
Nov 19 at 13:02 comment added Iosif Pinelis What is the meaning of $\partial^{\gamma} (\widetilde{\sigma} \ast \sigma)(x)$, given that $\widetilde{\sigma} \ast \sigma$ is a measure?
Nov 19 at 7:46 comment added Aleksei Kulikov To be honest, now I am even more confused, as I do not seem to understand what is $\sigma$ (you say it is a measure on a sphere, but then talk about a function $\chi$ with a support in a ball of radius $\frac{1}{2}$).
Nov 19 at 6:04 comment added Zygmund @AlekseiKulikov, I have edited the question to include the definitions.
Nov 19 at 6:03 history edited Zygmund CC BY-SA 4.0
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Nov 19 at 5:50 comment added Aleksei Kulikov Could you please explain what do you mean by a reflection of a measure?
Nov 19 at 5:42 history edited Zygmund
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Nov 19 at 6:46
S Nov 19 at 5:31 history asked Zygmund CC BY-SA 4.0