We work on a bounded smooth domain. Let $C_0(\bar\Omega)$ be the set of continuous functions with compact support and let $s \in (0,1)$.

Is $H^s(\Omega) \cap C_0(\bar\Omega) \subset H^s(\Omega)$ dense?

I cannot find any reference..

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    $\begingroup$ ok i probably misunderstood what is meant by $C_0(\bar\Omega)$ and deleted my answer $\endgroup$ – supinf Aug 23 '17 at 12:08
  • $\begingroup$ How is $H^s(\Omega)$ defined in this case? $\endgroup$ – felipeh Aug 29 '17 at 1:32
  • $\begingroup$ @felipeh we do it through the double integral definition, see Sobolev–Slobodeckij spaces in the Wikipedia article en.wikipedia.org/wiki/… $\endgroup$ – PostName Aug 30 '17 at 14:18
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    $\begingroup$ I see. Since you do not assume anything about the behavior on the boundary, what I can already say is that $C_0(\bar{\Omega})$ is not dense when $s>\frac{1}{2}$. This is related to the fact that functions with jump discontinuities fail to be in $H^{1/2}$ and also to the continuity of the trace operator $H^s(\Omega)\to L^2(\partial\Omega)$ when $s>\frac{1}{2}$. I am not sure what happens at $s=\frac{1}{2}$ and below for your question, though I will try to answer. $\endgroup$ – felipeh Aug 30 '17 at 14:30

Did you check Partial Differential Equations of Evans ? It is one of the best reference I've read on the subject.

  • $\begingroup$ I think this result cannot be find in Evans. $\endgroup$ – Pedro Aug 29 '17 at 1:15

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