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A Whitney extension theorem for functions taking values in scales of Banach spaces

We introduce a modified version of the Whitney extension operators for collections of functions from a closed subset of $\mathbb{R}^n$ into scales of Banach spaces with smoothing operators. We prove an extension theorem for collections whose elements take values in different spaces of the scale. A motivation for considering this kind of collections comes from very basic observations on the composition of functions of more than one real variable. The idea at the base of the proof is rather natural in the context of scales of Banach spaces, and consists in introducing smoothing operators in the construction of the extension, with smoothing parameters related to the diameter of each Whitney dyadic cube. Classical examples of scales of Banach spaces with smoothing operators are also given, and some new related observations are proved.