Approximation of divergence-free vector fields vanishing on rough planar sets

Abstract

Given any divergence-free vector field of Sobolev class $W^{m,p}_0(Ω)$ in a bounded open subset $Ω\subset \mathbb{R}^2$, we are interested in approximating it in the $W^{m,p}$ norm with divergence-free smooth vector fields compactly supported in $Ω$. We show that this approximation property holds in the following cases: For $p>2$, this holds given that $\partial Ω$ has zero Lebesgue measure (a weaker but more technical condition is sufficient); For $p \leq 2$, this holds if $Ω^c$ can be decomposed into finitely many disjoint closed sets, each of which is connected or $d$-Ahlfors regular for some $d\in[0,2)$. This has links to the uniqueness of weak solutions to the Stokes equation in $Ω$. For Hölder spaces, we prove this approximation property in general bounded domains.