Almost sharp descriptions of traces of Sobolev $W_{p}^{1}(\mathbb{R}^{n})$-spaces to arbitrary compact subsets of $\mathbb{R}^{n}$. The case $p \in (1,n]$

TL;DR

This paper provides an almost sharp intrinsic description of the trace space of Sobolev spaces on arbitrary compact sets with positive Hausdorff content and constructs bounded linear extension operators into slightly lower order Sobolev spaces.

Contribution

It offers a new almost sharp characterization of Sobolev trace spaces on arbitrary compact sets and constructs bounded extension operators into lower order Sobolev spaces.

Findings

Explicit description of Sobolev trace spaces on arbitrary compact sets.

Construction of bounded linear extension operators into lower order Sobolev spaces.

Extension operators are valid for a range of p and epsilon values.

Abstract

Let $S \subset \mathbb{R}^{n}$ be an arbitrary nonempty compact set such that the $d$-Hausdorff content $\mathcal{H}^{d}_{\infty}(S) > 0$ for some $d \in (0,n]$. For each $p \in (\max\{1,n-d\},n]$, an almost sharp intrinsic description of the trace space $W_{p}^{1}(\mathbb{R}^{n})|_{S}$ of the Sobolev space $W_{p}^{1}(\mathbb{R}^{n})$ to the set $S$ is obtained. Furthermore, for each $p \in (\max\{1,n-d\},n]$ and $\varepsilon \in (0, \min\{p-(n-d),p-1\})$, new bounded linear extension operators from the trace space $W_{p}^{1}(\mathbb{R}^{n})|_{S}$ into the space $W_{p-\varepsilon}^{1}(\mathbb{R}^{n})$ are constructed.