Optimal $C^\infty$-approximation of functions with exponentially or sub-exponentially integrable derivative

We discuss Meyers-Serrin's type results for smooth approximations of functions $b=b(t,x):\mathbb{R}\times\mathbb{R}^n\to\mathbb{R}^m$, with convergence of an energy of the form $\int_{\mathbb{R}}\int_{\mathbb{R}^n} w(t,x) φ\left(|Db(t,x)|\right)\mathrm{d} x \mathrm{d} t\,,$ where $w>0$ is a suitable weight function, and $φ:[0,\infty)\to [0,\infty)$ is a convex function with $φ(0)=0$ having exponential or sub-exponential growth.

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