Density of compactly supported smooth functions $C_C^\infty(\mathbb{R}^d)$ in Musielak-Orlicz-Sobolev spaces $W^{1,Φ}(Ω)$

Abstract

We investigate here the density of the set of the restrictions from $C_C^\infty(\mathbb{R}^d)$ to $C_C^\infty(Ω)$ in the Musielak-Orlicz-Sobolev space $W^{1,Φ}(Ω)$. It is a continuation of article \cite{KamZyl3}, where we have studied density of $C_C^\infty(\mathbb{R}^d)$ in $W^{k, Φ}(\mathbb{R}^d)$ for $k\in\mathbb{N}$. The main theorem states that for an open subset $Ω\subset \mathbb{R}^d$ with its boundary of class $C^1$, and Musielak-Orlicz function $Φ$ satisfying {\rm condition (A1)} which is a sort of log-Hölder continuity and the growth condition $Δ_2$, the set of restrictions of functions from $C_C^\infty(\mathbb{R}^d) $ to $Ω$ is dense in $W^{1,Φ}(Ω)$. We obtain a corresponding result in variable exponent Sobolev space $W^{1,p(\cdot)}(Ω)$ under the assumption that the exponent $p(x)$ is essentially bounded on $Ω$ and $Φ(x,t) = t^{p(x)}$, $t\ge 0$, $x\inΩ$, satisfies the log-Hölder condition.