The Structure of Sobolev Extension Operators

@article{Fefferman2012TheSO,
  title={The Structure of Sobolev Extension Operators},
  author={Charles Fefferman and Arie Israel and Garving K Luli},
  journal={arXiv: Classical Analysis and ODEs},
  year={2012}
}
Let $L^{m,p}(\R^n)$ denote the Sobolev space of functions whose $m$-th derivatives lie in $L^p(\R^n)$, and assume that $p>n$. For $E \subset \R^n$, denote by $L^{m,p}(E)$ the space of restrictions to $E$ of functions $F \in L^{m,p}(\R^n)$. It is known that there exist bounded linear maps $T : L^{m,p}(E) \rightarrow L^{m,p}(\R^n)$ such that $Tf = f$ on $E$ for any $f \in L^{m,p}(E)$. We show that $T$ cannot have a simple form called "bounded depth." 
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