# Contents

## Idea

The Paley-Wiener-Schwartz theorem characterizes compactly supported smooth functions (bump functions) and compactly supported distributions in terms of the decay property of their Fourier transform (of distributions).

Conversely this means that for a general distribution those covectors along which its Fourier transform does not suitably decay detect the singular nature of the distribution (its failure to be represented by a smooth function) in a refined form which records not only the singular support (the point at which that covector is based), but also the “direction of propagation of singularities”. The collection of these covectors is called the wave front set of the distribution. The study of functional analysis with attention to the wave front sets is called microlocal analysis. This plays a central role in the definition of various operations on distributions, such as the pullback of distributions and the product of distributions.

## Statement

###### Theorem

(Paley-Wiener-Schwartz theorem)

For $n \in \mathbb{N}$ the vector space $C^\infty_c(\mathbb{R}^n)$ of compactly supported smooth functions (bump functions) on Euclidean space $\mathbb{R}^n$ is (algebraically and topologically) isomorphic, via the Fourier transform, to the space of entire functions $F$ on $\mathbb{C}^n$ which satisfy the following estimate: there is a positive real number $B$ such that for every integer $N \gt 0$ there is a real number $C_N$ such that:

$\underset{\xi \in \mathbb{C}^n}{\forall} \left( {\Vert F(\xi) \Vert} \le C_N (1 + {\vert \xi\vert })^{-N} \exp{ (B \; |Im(\xi)|)} \right) \,.$

More generally, the space of compactly supported distributions on $\mathbb{R}^n$ of order $N$ is isomorphic via Fourier transform of distributions to those entire functions on $\mathbb{C}^n$ for which there exists positive real numbers $C$ and $B$ such that

$\underset{\xi \in \mathbb{C}^n}{\forall} \left( {\Vert F(\xi) \Vert} \le C_N (1 + {\vert \xi\vert })^{N} \exp{ (B \; |Im(\xi)|)} \right) \,.$

(Notice that the Fourier transform of a compactly supported distribution is guaranteed to be a smooth function, by this prop..)

## Refereces

• Lars Hörmander, section 7.3 of The analysis of linear partial differential operators, vol. I, Springer 1983, 1990