Linear extension operators for restrictions of function spaces to irregular open sets

V. S. Rychkov¹

¹

Studia Mathematica, Tome 140 (2000) no. 2, pp. 141-162

Résumé

Let an open set $Ω ⊂ ℝ^n$ satisfy for some 0≤d≤n and ε > 0 the condition: the $d$-Hausdorff content $H_d(Ω∩B) ≥ ε|B|^{d/n}$ for any ball B centered in Ω of volume |B|≤1. Let $H_p^s$ denote the Bessel potential space on $ℝ^n$ 1 p ∞,s > 0, and let $H_p^s[Ω]$ be the linear space of restrictions of elements of $H_p^s$ to Ω endowed with the quotient space norm. We find sufficient conditions for the existence of a linear extension operator for $H_p^s[Ω]$, i.e., a bounded linear operator $H_p^s[Ω]→H_p^s$ such that $ext⨍|_Ω}=⨍$ for all ⨍. The main result is that such an operator exists if (i) d > n-1 and s > (n-d)/min(p,2), or (ii) d≤n-1 and s-[s] > (n-d)/min(p,2). It is an open problem whether these assumptions are sharp.

Détail

DOI : 10.4064/sm-140-2-141-162

Keywords: Sobolev spaces, Besov-Triebel-Lizorkin spaces, restrictions, extension operators, irregular domains, Hausdorff content, local polynomial approximation, complemented subspaces