Can a smooth function $f : D\to\mathbb{R}^{n}$ where $D \subset \mathbb{R}^n$ is a closed perfect subset be extended to $g:\mathbb{R}^n\to \mathbb{R}^n$ where $g$ is a smooth function and $g|_D = f$. Smooth function mean $C^{\infty}$ functions.
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$delimiters. For example, instead of*f :D$\rightarrow$$R^{n}$*, try$f:D \rightarrow R^n$or$f: D \to \Bbb R^n$. – Ben Grossmann Oct 02 '20 at 22:19