Let $\Omega$ be a bounded domain in $\mathbb{R}^n$ with a Sobolev extension property around the complement of a closed part $D$ of its boundary. We prove that a function $u \in {\rm{W}}^{1,p}(\Omega)$ vanishes on $D$ in the sense of an interior trace if and only if it can be approximated within ${\rm{W}}^{1,p}(\Omega)$ by smooth functions with support away from $D$. We also review several other equivalent characterizations, so to draw a rather complete picture of these Sobolev functions vanishing on a part of the boundary.
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Figure 1. The dyadic 'skeleton' of $\Omega$ is obtained from the square $[0,1] \times [1,2]$ by iteratively attaching a total number of $2^j$ disjoint squares of side length $2^{-j}$ at the bottom of the existing construction. The domain $\Omega$ is then constructed by blowing up the line segments to appropriately sized open rectangles
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The dyadic 'skeleton' of