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 $C_C^\infty(\Omega)$ in the Musielak-Orlicz-Sobolev space $W^{1,\Phi}(\Omega)$. It is a continuation of article \cite{KamZyl3}, where we have studied density of $C_C^\infty(\mathbb{R}^d)$ in $W^{k, \Phi}(\mathbb{R}^d)$ for $k\in\mathbb{N}$. The main theorem states that for an open subset $\Omega\subset \mathbb{R}^d$ with its boundary of class $C^1$, and Musielak-Orlicz function $\Phi$ satisfying {\rm condition (A1)} which is a sort of log-H\"older continuity and the growth condition $\Delta_2$, the set of restrictions of functions from $C_C^\infty(\mathbb{R}^d) $ to $\Omega$ is dense in $W^{1,\Phi}(\Omega)$. We obtain a corresponding result in variable exponent Sobolev space $W^{1,p(\cdot)}(\Omega)$ under the assumption that the exponent $p(x)$ is essentially bounded on $\Omega$ and $\Phi(x,t) = t^{p(x)}$, $t\ge 0$, $x\in\Omega$, satisfies the log-H\"older condition.