Uniqueness of the approximative trace

Abstract

We study the approximative trace for individual elements in the Sobolev space $W^{1,p}(\Omega)$ for $1\le p\le\infty$. This notion of a trace was introduced for $p=2$ in [AtE11] in the setting of general open sets $\Omega\subset\mathbb{R}^d$. The approximative trace exhibits a curious nonuniqueness phenomenon. We provide a detailed analysis of this phenomenon based on methods of geometric measure theory and are able to give very weak geometric conditions that are sufficient for the uniqueness of the approximative trace. In particular, we prove that the approximative trace is unique on open sets with continuous boundary and on arbitrary connected domains in $\mathbb{R}^2$. Furthermore, we provide an example which shows that the uniqueness of the approximative trace depends on $p$. These results answer several open questions.