Contents

# Contents

## Definition

Let $n \in \mathbb{N}$ and consider $\mathbb{R}^n$ the Cartesian space of dimension $n$.

###### Definition

(product of a distribution with a smooth function)

For $u \in \mathcal{D}'(\mathbb{R}^n)$ a distribution, and $f \in C^\infty(\mathbb{R}^n)$ a smooth function, their product

$f \cdot u \;\in\; C^\infty(\mathbb{R}^n)$

is the distribution given on a compactly supported smooth function $g \in C^\infty_{cp}(\mathbb{R}^n) = \mathcal{D}(\mathbb{R}^n)$ by

$(f\cdot u)(g) \;\coloneqq\; u\left( f \cdot g\right) \,,$

where on the right we have the application of $u$ regarded as a continuous linear functional $u \colon \mathcal{D}(\mathbb{R}^n) \to \mathbb{C}$ to the ordinary pointwise product of smooth functions $f \cdot g$.

## Properties

###### Definition

(product of a distribution with a non-singular distributions is product of distribution with a smooth function)

The wave front set of a non-singular distribution $u_f$ corresponding to a smooth function $f \in C^\infty(\mathbb{R}^n)$, is empty (this prop.). Therefore the product of distributions (def. ) of a non-singular distribution with any distribution $u$ is defined, and given by the product of distributions with smooth functions according to def. :

$u_f \cdot u = f \cdot u = u(f\cdot (-)) \,.$