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Jul 22, 2021 at 17:34 comment added Iosif Pinelis @Bogdan : I think this is impossible in general. Consider e.g. $\phi_0(x_1,x_2):=x_1$ for $(x_1,x_2)\in\overline\omega$, where $\omega$ is the open unit disk. Then $\sup_{\overline\omega}|\phi_0|=1$, but $f(x_1,0)>1$ for all $x_1>1$ close enough to $1$. Perhaps, you misunderstood the proposition.
Jul 22, 2021 at 17:09 comment added Bogdan Sorry. I mean in $|f|\leq sup_{x\in\overline{\omega}} |\phi_0|$ in $\Omega$. I found an article which says something like this but I do not understand exactly. Here it is: core.ac.uk/download/pdf/82133103.pdf (the proposition at page 326). Is it indeed true that $f$ can be chosen that way?
Jul 22, 2021 at 15:42 comment added Iosif Pinelis @Bogdan : Since $f$ and $\phi_0$ are continuous and $f=\phi_0$ on $\omega$, we have $f=\phi_0$ on $\overline\omega$.
Jul 22, 2021 at 15:14 comment added Bogdan Is it true that the Whitney Extension of $\phi_0$ satisfies the inequality $|f|\leq |\phi_0|$ on $\overline{\omega}$?
Jul 22, 2021 at 12:56 vote accept Bogdan
Jul 22, 2021 at 12:44 history edited Iosif Pinelis CC BY-SA 4.0
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Jul 22, 2021 at 12:35 history edited Iosif Pinelis CC BY-SA 4.0
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Jul 22, 2021 at 12:29 history edited Iosif Pinelis CC BY-SA 4.0
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Jul 22, 2021 at 12:13 history answered Iosif Pinelis CC BY-SA 4.0