# Revista Matemática Iberoamericana

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**Volume 23, Issue 1, 2007, pp. 269–280**

**DOI: 10.4171/RMI/495**

Published online: 2007-04-30

The Structure of Linear Extension Operators for $C^m$

Charles Fefferman^{[1]}(1) Princeton University, United States

For any subset $E \subset \mathbb{R}^n$, let $C^m (E)$ denote the Banach space of restrictions to $E$ of functions $F \in C^m (\mathbb{R}^n)$. It is known that there exist bounded linear maps $T:C^m(E)\longrightarrow C^m(\mathbb{R}^n)$ such that $Tf = f$ on $E$ for any $f \in C^m (E)$. We show that $T$ can be taken to have a simple form, but cannot be taken to have an even simpler form.

*Keywords: *Extension operators, Whitney’s extension problem

Fefferman Charles: The Structure of Linear Extension Operators for $C^m$. *Rev. Mat. Iberoam.* 23 (2007), 269-280. doi: 10.4171/RMI/495