# Distribution (mathematics)

https://en.wikipedia.org/wiki/Distribution_(mathematics)

Distributions, also known as Schwartz distributions or generalized functions, are objects that generalize the classical notion of functions in mathematical analysis. Distributions make it possible to differentiate functions whose derivatives do not exist in the classical sense. In particular, any locally integrable function has a distributional derivative. Distributions are widely used in the theory of partial differential equations, where it may be easier to establish the existence of distributional solutions than classical solutions, or appropriate classical solutions may not exist. Distributions are also important in physics and engineering where many problems naturally lead to differential equations whose solutions or initial conditions are distributions, such as the Dirac delta function.

A function ${\displaystyle f}$ is normally thought of as acting on the points in its domain by "sending" a point x in its domain to the point ${\displaystyle f(x).}$ Instead of acting on points, distribution theory reinterprets functions such as ${\displaystyle f}$ as acting on test functions in a certain way. Test functions are usually infinitely differentiable complex-valued (or sometimes real-valued) functions with compact support ( bump functions are examples of test functions). Many "standard functions" (meaning for example a function that is typically encountered in a Calculus course), say for instance a continuous map ${\displaystyle f:\mathbb {R} \to \mathbb {R} ,}$ can be canonically reinterpreted as acting on test functions (instead of their usual interpretation as acting on points of their domain) via the action known as " integration against a test function"; explicitly, this means that ${\displaystyle f}$ "acts on" a test function g by "sending" g to the number ${\displaystyle \textstyle \int _{\mathbb {R} }fg\,dx.}$ This new action of ${\displaystyle f}$ is thus a complex (or real)-valued map, denoted by ${\displaystyle D_{f},}$ whose domain is the space of test functions; this map turns out to have two additional properties [note 1] that make it into what is known as a distribution on ${\displaystyle \mathbb {R} .}$ Distributions that arise from "standard functions" in this way are the prototypical examples of a distributions. But there are many distributions that do not arise in this way and these distributions are known as "generalized functions." Examples include the Dirac delta function or some distributions that arise via the action of "integration of test functions against measures." However, by using various methods it is nevertheless still possible to reduce any arbitrary distribution down to a simpler family of related distributions that do arise via such actions of integration.

In applications to physics and engineering, the space of test functions usually consists of smooth functions with compact support that are defined on some given non-empty open subset ${\displaystyle U\subseteq \mathbb {R} ^{n}.}$ This space of test functions is denoted by ${\displaystyle C_{c}^{\infty }(U)}$ or ${\displaystyle {\mathcal {D}}(U)}$ and a distribution on U is by definition a linear functional on ${\displaystyle C_{c}^{\infty }(U)}$ that is continuous when ${\displaystyle C_{c}^{\infty }(U)}$ is given a topology called the canonical LF topology. This leads to the space of (all) distributions on U, usually denoted by ${\displaystyle {\mathcal {D}}'(U)}$ (note the prime), which by definition is the space of all distributions on ${\displaystyle U}$ (that is, it is the continuous dual space of ${\displaystyle C_{c}^{\infty }(U)}$); it is these distributions that are the main focus of this article.

There are other possible choices for the space of test functions, which lead to other different spaces of distributions. If ${\displaystyle U=\mathbb {R} ^{n}}$ then the use of Schwartz functions [note 2] as test functions gives rise to a certain subspace of ${\displaystyle {\mathcal {D}}'(U)}$ whose elements are called tempered distributions. These are important because they allow the Fourier transform to be extended from "standard functions" to tempered distributions. The set of tempered distributions forms a vector subspace of the space of distributions ${\displaystyle {\mathcal {D}}'(U)}$ and is thus one example of a space of distributions; there are many other spaces of distributions.

There also exist other major classes of test functions that are not subsets of ${\displaystyle C_{c}^{\infty }(U),}$ such as spaces of analytic test functions, which produce very different classes of distributions. The theory of such distributions has a different character from the previous one because there are no analytic functions with non-empty compact support. [note 3] Use of analytic test functions lead to Sato's theory of hyperfunctions.

## History

The practical use of distributions can be traced back to the use of Green functions in the 1830s to solve ordinary differential equations, but was not formalized until much later. According to Kolmogorov & Fomin (1957), generalized functions originated in the work of Sergei Sobolev ( 1936) on second-order hyperbolic partial differential equations, and the ideas were developed in somewhat extended form by Laurent Schwartz in the late 1940s. According to his autobiography, Schwartz introduced the term "distribution" by analogy with a distribution of electrical charge, possibly including not only point charges but also dipoles and so on. Gårding (1997) comments that although the ideas in the transformative book by Schwartz (1951) were not entirely new, it was Schwartz's broad attack and conviction that distributions would be useful almost everywhere in analysis that made the difference.

## Notation

• ${\displaystyle n}$ is a fixed positive integer and ${\displaystyle U}$ is a fixed non-empty open subset of Euclidean space ${\displaystyle \mathbb {R} ^{n}.}$
• ${\displaystyle \mathbb {N} =\{0,1,2,\ldots \}}$ denotes the natural numbers.
• ${\displaystyle k}$ will denote a non-negative integer or ${\displaystyle \infty .}$
• If ${\displaystyle f}$ is a function then ${\displaystyle \operatorname {Dom} (f)}$ will denote its domain and the support of ${\displaystyle f,}$ denoted by ${\displaystyle \operatorname {supp} (f),}$ is defined to be the closure of the set ${\displaystyle \{x\in \operatorname {Dom} (f):f(x)\neq 0\}}$ in ${\displaystyle \operatorname {Dom} (f).}$
• For two functions ${\displaystyle f,g:U\to \mathbb {C} }$, the following notation defines a canonical pairing:
${\displaystyle \langle f,g\rangle :=\int _{U}f(x)g(x)\,dx.}$
• A multi-index of size ${\displaystyle n}$ is an element in ${\displaystyle \mathbb {N} ^{n}}$ (given that ${\displaystyle n}$ is fixed, if the size of multi-indices is omitted then the size should be assumed to be ${\displaystyle n}$). The length of a multi-index ${\displaystyle \alpha =(\alpha _{1},\ldots ,\alpha _{n})\in \mathbb {N} ^{n}}$ is defined as ${\displaystyle \alpha _{1}+\cdots +\alpha _{n}}$ and denoted by ${\displaystyle |\alpha |.}$ Multi-indices are particularly useful when dealing with functions of several variables, in particular we introduce the following notations for a given multi-index ${\displaystyle \alpha =(\alpha _{1},\ldots ,\alpha _{n})\in \mathbb {N} ^{n}}$:
{\displaystyle {\begin{aligned}x^{\alpha }&=x_{1}^{\alpha _{1}}\cdots x_{n}^{\alpha _{n}}\\\partial ^{\alpha }&={\frac {\partial ^{|\alpha |}}{\partial x_{1}^{\alpha _{1}}\cdots \partial x_{n}^{\alpha _{n}}}}\end{aligned}}}
We also introduce a partial order of all multi-indices by ${\displaystyle \beta \geq \alpha }$ if and only if ${\displaystyle \beta _{i}\geq \alpha _{i}}$ for all ${\displaystyle 1\leq i\leq n.}$ When ${\displaystyle \beta \geq \alpha }$ we define their multi-index binomial coefficient as:
${\displaystyle {\binom {\beta }{\alpha }}:={\binom {\beta _{1}}{\alpha _{1}}}\cdots {\binom {\beta _{n}}{\alpha _{n}}}.}$
• ${\displaystyle \mathbb {K} }$ will denote a certain non-empty collection of compact subsets of ${\displaystyle U}$ (described in detail below).

## Definitions of test functions and distributions

In this section, we will formally define real-valued distributions on U. With minor modifications, one can also define complex-valued distributions, and one can replace ${\displaystyle \mathbb {R} ^{n}}$ with any ( paracompact) smooth manifold.

Notation: Suppose ${\displaystyle k\in \{0,1,2,\ldots ,\infty \}.}$
1. Let ${\displaystyle C^{k}(U)}$ denote the vector space of all k-times continuously differentiable real-valued functions on U.
2. For any compact subset ${\displaystyle K\subseteq U,}$ let ${\displaystyle C^{k}(K)}$ and ${\displaystyle C^{k}(K;U)}$ both denote the vector space of all those functions ${\displaystyle f\in C^{k}(U)}$ such that ${\displaystyle \operatorname {supp} (f)\subseteq K.}$
• Note that ${\displaystyle C^{k}(K)}$ depends on both K and U but we will only indicate K, where in particular, if ${\displaystyle f\in C^{k}(K)}$ then the domain of ${\displaystyle f}$ is U rather than K. We will use the notation ${\displaystyle C^{k}(K;U)}$ only when the notation ${\displaystyle C^{k}(K)}$ risks being ambiguous.
• Clearly, every ${\displaystyle C^{k}(K)}$ contains the constant 0 map, even if ${\displaystyle K=\varnothing .}$
3. Let ${\displaystyle C_{c}^{k}(U)}$ denote the set of all ${\displaystyle f\in C^{k}(U)}$ such that ${\displaystyle f\in C^{k}(K)}$ for some compact subset K of U.
• Equivalently, ${\displaystyle C_{c}^{k}(U)}$ is the set of all ${\displaystyle f\in C^{k}(U)}$ such that ${\displaystyle f}$ has compact support.
• ${\displaystyle C_{c}^{k}(U)}$ is equal to the union of all ${\displaystyle C^{k}(K)}$ as ${\displaystyle K}$ ranges over ${\displaystyle \mathbb {K} .}$
• If ${\displaystyle f}$ is a real-valued function on U, then ${\displaystyle f}$ is an element of ${\displaystyle C_{c}^{k}(U)}$ if and only if ${\displaystyle f}$ is a ${\displaystyle C^{k}}$ bump function. Every real-valued test function on ${\displaystyle U}$ is always also a complex-valued test function on ${\displaystyle U.}$
The graph of the bump function ${\displaystyle (x,y)\in \mathbf {R} ^{2}\mapsto \Psi (r),}$ where ${\displaystyle r=(x^{2}+y^{2})^{\frac {1}{2}}}$ and ${\displaystyle \Psi (r)=e^{-{\frac {1}{1-r^{2}}}}\cdot \mathbf {1} _{\{|r|<1\}}.}$ This function is a test function on ${\displaystyle \mathbb {R} ^{2}}$ and is an element of ${\displaystyle C_{c}^{\infty }\left(\mathbb {R} ^{2}\right).}$ The support of this function is the closed unit disk in ${\displaystyle \mathbb {R} ^{2}.}$ It is non-zero on the open unit disk and it is equal to 0 everywhere outside of it.

Note that for all ${\displaystyle j,k\in \{0,1,2,\ldots ,\infty \}}$ and any compact subsets K and L of U, we have:

{\displaystyle {\begin{aligned}C^{k}(K)&\subseteq C_{c}^{k}(U)\subseteq C^{k}(U)\\C^{k}(K)&\subseteq C^{k}(L)&&{\text{if }}K\subseteq L\\C^{k}(K)&\subseteq C^{j}(K)&&{\text{if }}j\leq k\\C_{c}^{k}(U)&\subseteq C_{c}^{j}(U)&&{\text{if }}j\leq k\\C^{k}(U)&\subseteq C^{j}(U)&&{\text{if }}j\leq k\\\end{aligned}}}
Definition: Elements of ${\displaystyle C_{c}^{\infty }(U)}$ are called test functions on U and ${\displaystyle C_{c}^{\infty }(U)}$ is called the space of test function on U. We will use both ${\displaystyle {\mathcal {D}}(U)}$ and ${\displaystyle C_{c}^{\infty }(U)}$ to denote this space.

Distributions on U are defined to be the continuous linear functionals on ${\displaystyle C_{c}^{\infty }(U)}$ when this vector space is endowed with a particular topology called the canonical LF-topology. This topology is unfortunately not easy to define but it is nevertheless still possible to characterize distributions in a way so that no mention of the canonical LF-topology is made.

Proposition: If T is a linear functional on ${\displaystyle C_{c}^{\infty }(U)}$ then the T is a distribution if and only if the following equivalent conditions are satisfied:

1. For every compact subset ${\displaystyle K\subseteq U}$ there exist constants ${\displaystyle C>0}$ and ${\displaystyle N\in \mathbb {N} }$ such that for all ${\displaystyle f\in C^{\infty }(K),}$ [1]
${\displaystyle |T(f)|\leq C\sup\{|\partial ^{\alpha }f(x)|:x\in U,|\alpha |\leq N\};}$
2. For every compact subset ${\displaystyle K\subseteq U}$ there exist constants ${\displaystyle C_{K}>0}$ and ${\displaystyle N_{K}\in \mathbb {N} }$ such that for all ${\displaystyle f\in C_{c}^{\infty }(U)}$ with support contained in ${\displaystyle K,}$ [2]
${\displaystyle |T(f)|\leq C_{K}\sup\{|\partial ^{\alpha }f(x)|:x\in K,|\alpha |\leq N_{K}\};}$
3. For any compact subset ${\displaystyle K\subseteq U}$ and any sequence ${\displaystyle \{f_{i}\}_{i=1}^{\infty }}$ in ${\displaystyle C^{\infty }(K),}$ if ${\displaystyle \{\partial ^{\alpha }f_{i}\}_{i=1}^{\infty }}$ converges uniformly to zero on ${\displaystyle K}$ for all multi-indices ${\displaystyle \alpha }$, then ${\displaystyle T(f_{i})\to 0.}$

The above characterizations can be used to determine whether or not a linear functional is a distribution, but more advanced uses of distributions and test functions (such as applications to differential equations) is limited if no topologies are placed on ${\displaystyle C_{c}^{\infty }(U)}$ and ${\displaystyle {\mathcal {D}}(U).}$ To define the space of distributions we must first define the canonical LF-topology, which in turn requires that several other locally convex topological vector spaces (TVSs) be defined first. First, a ( non-normable) topology on ${\displaystyle C^{\infty }(U)}$ will be defined, then every ${\displaystyle C^{\infty }(K)}$ will be endowed with the subspace topology induced on it by ${\displaystyle C^{\infty }(U),}$ and finally the ( non-metrizable) canonical LF-topology on ${\displaystyle C_{c}^{\infty }(U)}$ will be defined. The space of distributions, being defined as the continuous dual space of ${\displaystyle C_{c}^{\infty }(U),}$ is then endowed with the (non-metrizable) strong dual topology induced by ${\displaystyle C_{c}^{\infty }(U)}$ and the canonical LF-topology (this topology is a generalization of the usual operator norm induced topology that is placed on the continuous dual spaces of normed spaces). This finally permits consideration of more advanced notions such as convergence of distributions (both sequences and nets), various (sub)spaces of distributions, and operations on distributions, including extending differential equations to distributions.

Choice of compact sets ${\displaystyle \mathbb {K} }$

Throughout, ${\displaystyle \mathbb {K} }$ will be any collection of compact subsets of ${\displaystyle U}$ such that (1) ${\displaystyle U=\bigcup _{K\in \mathbb {K} }K,}$ and (2) for any compact ${\displaystyle K\subseteq U}$ there exists some ${\displaystyle K_{2}\in \mathbb {K} }$ such that ${\displaystyle K\subseteq K_{2}.}$ The most common choices for ${\displaystyle \mathbb {K} }$ are:

• The set of all compact subsets of ${\displaystyle U,}$ or
• A set ${\displaystyle \left\{{\overline {U_{1}}},{\overline {U_{2}}},\ldots \right\}}$ where ${\displaystyle U=\cup _{i=1}^{\infty }U_{i},}$ and for all i, ${\displaystyle {\overline {U_{i}}}\subseteq U_{i+1}}$ and ${\displaystyle U_{i}}$ is a relatively compact non-empty open subset of ${\displaystyle U}$ (here, "relatively compact" means that the closure of ${\displaystyle U_{i},}$ in either U or ${\displaystyle \mathbb {R} ^{n},}$ is compact).

We make ${\displaystyle \mathbb {K} }$ into a directed set by defining ${\displaystyle K_{1}\leq K_{2}}$ if and only if ${\displaystyle K_{1}\subseteq K_{2}.}$ Note that although the definitions of the subsequently defined topologies explicitly reference ${\displaystyle \mathbb {K} ,}$ in reality they do not depend on the choice of ${\displaystyle \mathbb {K} ;}$ that is, if ${\displaystyle \mathbb {K} _{1}}$ and ${\displaystyle \mathbb {K} _{2}}$ are any two such collections of compact subsets of ${\displaystyle U,}$ then the topologies defined on ${\displaystyle C^{k}(U)}$ and ${\displaystyle C_{c}^{k}(U)}$ by using ${\displaystyle \mathbb {K} _{1}}$ in place of ${\displaystyle \mathbb {K} }$ are the same as those defined by using ${\displaystyle \mathbb {K} _{2}}$ in place of ${\displaystyle \mathbb {K} .}$

### Topology on Ck(U)

We now introduce the seminorms that will define the topology on ${\displaystyle C^{k}(U).}$ Different authors sometimes use different families of seminorms so we list the most common families below. However, the resulting topology is the same no matter which family is used.

Suppose ${\displaystyle k\in \{0,1,2,\ldots ,\infty \}}$ and ${\displaystyle K}$ is an arbitrary compact subset of ${\displaystyle U.}$ Suppose ${\displaystyle i}$ an integer such that ${\displaystyle 0\leq i\leq k.}$ [note 4] and ${\displaystyle p}$ is a multi-index with length ${\displaystyle |p|\leq k.}$ For ${\displaystyle K\neq \varnothing ,}$ define:
{\displaystyle {\begin{aligned}s_{p,K}(f)&:=\sup _{x_{0}\in K}\left|\partial ^{p}f(x_{0})\right|\\[4pt]q_{i,K}(f)&:=\sup _{|p|\leq i}\left(\sup _{x_{0}\in K}\left|\partial ^{p}f(x_{0})\right|\right)=\sup _{|p|\leq i}\left(s_{p,K}(f)\right)\\[4pt]r_{i,K}(f)&:=\sup _{\stackrel {|p|\leq i}{x_{0}\in K}}\left|\partial ^{p}f(x_{0})\right|\\[4pt]t_{i,K}(f)&:=\sup _{x_{0}\in K}\left(\sum _{|p|\leq i}\left|\partial ^{p}f(x_{0})\right|\right)\end{aligned}}}
while for ${\displaystyle K=\varnothing }$ we define all the functions above to be the constant 0 map.

Each of the functions above are non-negative ${\displaystyle \mathbb {R} }$-valued [note 5] seminorms on ${\displaystyle C^{k}(U).}$

Each of the following families of seminorms generates the same locally convex vector topology on ${\displaystyle C^{k}(U)}$:

{\displaystyle {\begin{alignedat}{4}(1)\quad &\{q_{i,K}&&:\;K\in \mathbb {K} {\text{ and }}\;&&i\in \mathbb {N} {\text{ satisfies }}\;&&0\leq i\leq k\}\\(2)\quad &\{r_{i,K}&&:\;K\in \mathbb {K} {\text{ and }}\;&&i\in \mathbb {N} {\text{ satisfies }}\;&&0\leq i\leq k\}\\(3)\quad &\{t_{i,K}&&:\;K\in \mathbb {K} {\text{ and }}\;&&i\in \mathbb {N} {\text{ satisfies }}\;&&0\leq i\leq k\}\\(4)\quad &\{s_{p,K}&&:\;K\in \mathbb {K} {\text{ and }}\;&&p\in \mathbb {N} ^{n}{\text{ satisfies }}\;&&|p|\leq k\}\end{alignedat}}}
Assumption: We will henceforth assume that ${\displaystyle C^{k}(U)}$ is endowed with the locally convex topology defined by any (or equivalently, all) of the families of seminorms described above.

With this topology, ${\displaystyle C^{k}(U)}$ becomes a locally convex ( non-normable) Fréchet space and all of the seminorms defined above are continuous on this space. All of the seminorms defined above are continuous functions on ${\displaystyle C^{k}(U).}$ Under this topology, a net ${\displaystyle (f_{i})_{i\in I}}$ in ${\displaystyle C^{k}(U)}$ converges to ${\displaystyle f\in C^{k}(U)}$ if and only if for every multi-index ${\displaystyle p}$ with ${\displaystyle |p| and every ${\displaystyle K\in \mathbb {K} ,}$ the net of partial derivatives ${\displaystyle \left(\partial ^{p}f_{i}\right)_{i\in I}}$ converges uniformly to ${\displaystyle \partial ^{p}f}$ on ${\displaystyle K.}$ [3] For any ${\displaystyle k\in \{0,1,2,\ldots ,\infty \},}$ any (von Neumann) bounded subset of ${\displaystyle C^{k+1}(U)}$ is a relatively compact subset of ${\displaystyle C^{k}(U).}$ [4] In particular, a subset of ${\displaystyle C^{\infty }(U)}$ is bounded if and only if it is bounded in ${\displaystyle C^{i}(U)}$ for all ${\displaystyle i\in \mathbb {N} .}$ [4] The space ${\displaystyle C^{k}(U)}$ is a Montel space if and only if ${\displaystyle k=\infty .}$ [5]

The topology on ${\displaystyle C^{\infty }(U)}$ is the superior limit of the subspace topologies induced on ${\displaystyle C^{\infty }(U)}$ by the TVSs ${\displaystyle C^{i}(U)}$ as i ranges over the non-negative integers. [3] A subset ${\displaystyle W}$ of ${\displaystyle C^{\infty }(U)}$ is open in this topology if and only if there exists ${\displaystyle i\in \mathbb {N} }$ such that ${\displaystyle W}$ is open when ${\displaystyle C^{\infty }(U)}$ is endowed with the subspace topology induced on it by ${\displaystyle C^{i}(U).}$

Metric defining the topology

If the family of compact sets ${\displaystyle \mathbb {K} =\left\{{\overline {U_{1}}},{\overline {U_{2}}},\ldots \right\}}$ satisfies ${\displaystyle U=\bigcup _{j=1}^{\infty }U_{j}}$ and ${\displaystyle {\overline {U_{i}}}\subseteq U_{i+1}}$ for all ${\displaystyle i,}$ then a complete translation-invariant metric on ${\displaystyle C^{\infty }(U)}$ can be obtained by taking a suitable countable Fréchet combination of any one of the above families. For example, using the seminorms ${\displaystyle \left(r_{i,K_{i}}\right)_{i=1}^{\infty }}$ results in the metric

${\displaystyle d(f,g):=\sum _{i=1}^{\infty }{\frac {1}{2^{i}}}{\frac {r_{i,{\overline {U_{i}}}}(f-g)}{1+r_{i,{\overline {U_{i}}}}(f-g)}}=\sum _{i=1}^{\infty }{\frac {1}{2^{i}}}{\frac {\sup _{|p|\leq i,x\in {\overline {U_{i}}}}\left|\partial ^{p}(f-g)(x)\right|}{\left[1+\sup _{|p|\leq i,x\in {\overline {U_{i}}}}\left|\partial ^{p}(f-g)(x)\right|\right]}}.}$

Often, it is easier to just consider seminorms.

### Topology on Ck(K)

As before, fix ${\displaystyle k\in \{0,1,2,\ldots ,\infty \}.}$ Recall that if ${\displaystyle K}$ is any compact subset of ${\displaystyle U}$ then ${\displaystyle C^{k}(K)\subseteq C^{k}(U).}$

Assumption: For any compact subset ${\displaystyle K\subseteq U,}$ we will henceforth assume that ${\displaystyle C^{k}(K)}$ is endowed with the subspace topology it inherits from the Fréchet space ${\displaystyle C^{k}(U).}$

For any compact subset ${\displaystyle K\subseteq U,}$ ${\displaystyle C^{k}(K)}$ is a closed subspace of the Fréchet space ${\displaystyle C^{k}(U)}$ and is thus also a Fréchet space. For all compact ${\displaystyle K,L\subseteq U}$ satisfying ${\displaystyle K\subseteq L,}$ denote the inclusion map by ${\displaystyle \operatorname {In} _{K}^{L}:C^{k}(K)\to C^{k}(L).}$ Then this map is a linear embedding of TVSs (that is, it is a linear map that is also a topological embedding) whose image (or "range") is closed in its codomain; said differently, the topology on ${\displaystyle C^{k}(K)}$ is identical to the subspace topology it inherits from ${\displaystyle C^{k}(L),}$ and also ${\displaystyle C^{k}(K)}$ is a closed subset of ${\displaystyle C^{k}(L).}$ The interior of ${\displaystyle C^{\infty }(K)}$ relative to ${\displaystyle C^{\infty }(U)}$ is empty. [6]

If ${\displaystyle k}$ is finite then ${\displaystyle C^{k}(K)}$ is a Banach space [7] with a topology that can be defined by the norm

${\displaystyle r_{K}(f):=\sup _{|p|

And when ${\displaystyle k=2,}$ then ${\displaystyle \,C^{k}(K)}$ is even a Hilbert space. [7] The space ${\displaystyle C^{\infty }(K)}$ is a distinguished Schwartz Montel space so if ${\displaystyle C^{\infty }(K)\neq \{0\}}$ then it is not normable and thus not a Banach space (although like all other ${\displaystyle C^{k}(K),}$ it is a Fréchet space).

#### Trivial extensions and independence of Ck(K)'s topology from U

The definition of ${\displaystyle C^{k}(K)}$ depends on U so we will let ${\displaystyle C^{k}(K;U)}$ denote the topological space ${\displaystyle C^{k}(K),}$ which by definition is a topological subspace of ${\displaystyle C^{k}(U).}$ Suppose ${\displaystyle V}$ is an open subset of ${\displaystyle \mathbb {R} ^{n}}$ containing ${\displaystyle U.}$ Given ${\displaystyle f\in C_{c}^{k}(U),}$ its trivial extension to V is by definition, the function ${\displaystyle F:V\to \mathbb {C} }$ defined by:

${\displaystyle F(x)={\begin{cases}f(x)&x\in U,\\0&{\text{otherwise}},\end{cases}}}$

so that ${\displaystyle F\in C^{k}(V).}$ Let ${\displaystyle I:C_{c}^{k}(U)\to C^{k}(V)}$ denote the map that sends a function in ${\displaystyle C_{c}^{k}(U)}$ to its trivial extension on V. This map is a linear injection and for every compact subset ${\displaystyle K\subseteq U,}$ we have ${\displaystyle I\left(C^{k}(K;U)\right)=C^{k}(K;V),}$ where ${\displaystyle C^{k}(K;V)}$ is the vector subspace of ${\displaystyle C^{k}(V)}$ consisting of maps with support contained in ${\displaystyle K}$ (since ${\displaystyle K\subseteq U\subseteq V,}$ ${\displaystyle K}$ is also a compact subset of ${\displaystyle V}$). It follows that ${\displaystyle I\left(C_{c}^{k}(U)\right)\subseteq C_{c}^{k}(V).}$ If I is restricted to ${\displaystyle C^{k}(K;U)}$ then the following induced linear map is a homeomorphism (and thus a TVS-isomorphism):

${\displaystyle C^{k}(K;U)\to C^{k}(K;V)}$

and thus the next two maps (which like the previous map are defined by ${\displaystyle f\mapsto I(f)}$) are topological embeddings:

${\displaystyle C^{k}(K;U)\to C^{k}(V),}$
${\displaystyle C^{k}(K;U)\to C_{c}^{k}(V),}$

(the topology on ${\displaystyle C_{c}^{k}(V)}$ is the canonical LF topology, which is defined later). Using ${\displaystyle C_{c}^{k}(U)\ni f\mapsto I(f)\in C_{c}^{k}(V)}$ we identify ${\displaystyle C_{c}^{k}(U)}$ with its image in ${\displaystyle C_{c}^{k}(V)\subseteq C^{k}(V).}$ Because ${\displaystyle C^{k}(K;U)\subseteq C_{c}^{k}(U),}$ through this identification, ${\displaystyle C^{k}(K;U)}$ can also be considered as a subset of ${\displaystyle C^{k}(V).}$ Importantly, the subspace topology ${\displaystyle C^{k}(K;U)}$ inherits from ${\displaystyle C^{k}(U)}$ (when it is viewed as a subset of ${\displaystyle C^{k}(U)}$) is identical to the subspace topology that it inherits from ${\displaystyle C^{k}(V)}$ (when ${\displaystyle C^{k}(K;U)}$ is viewed instead as a subset of ${\displaystyle C^{k}(V)}$ via the identification). Thus the topology on ${\displaystyle C^{k}(K;U)}$ is independent of the open subset U of ${\displaystyle \mathbb {R} ^{n}}$ that contains K. [6] This justifies the practice of written ${\displaystyle C^{k}(K)}$ instead of ${\displaystyle C^{k}(K;U).}$

### Canonical LF topology

Recall that ${\displaystyle C_{c}^{k}(U)}$ denote all those functions in ${\displaystyle C^{k}(U)}$ that have compact support in ${\displaystyle U,}$ where note that ${\displaystyle C_{c}^{k}(U)}$ is the union of all ${\displaystyle C^{k}(K)}$ as K ranges over ${\displaystyle \mathbb {K} .}$ Moreover, for every k, ${\displaystyle C_{c}^{k}(U)}$ is a dense subset of ${\displaystyle C^{k}(U).}$ The special case when ${\displaystyle k=\infty }$ gives us the space of test functions.

${\displaystyle C_{c}^{\infty }(U)}$ is called the space of test functions on ${\displaystyle U}$ and it may also be denoted by ${\displaystyle {\mathcal {D}}(U).}$

This section defines the canonical LF topology as a direct limit. It is also possible to define this topology in terms of its neighborhoods of the origin, which is described afterwards.

Topology defined by direct limits

For any two sets K and L, we declare that ${\displaystyle K\leq L}$ if and only if ${\displaystyle K\subseteq L,}$ which in particular makes the collection ${\displaystyle \mathbb {K} }$ of compact subsets of U into a directed set (we say that such a collection is directed by subset inclusion). For all compact ${\displaystyle K,L\subseteq U}$ satisfying ${\displaystyle K\subseteq L,}$ there are inclusion maps

${\displaystyle \operatorname {In} _{K}^{L}:C^{k}(K)\to C^{k}(L)\quad {\text{and}}\quad \operatorname {In} _{K}^{U}:C^{k}(K)\to C_{c}^{k}(U).}$

Recall from above that the map ${\displaystyle \operatorname {In} _{K}^{L}:C^{k}(K)\to C^{k}(L)}$ is a topological embedding. The collection of maps

${\displaystyle \left\{\operatorname {In} _{K}^{L}\;:\;K,L\in \mathbb {K} \;{\text{ and }}\;K\subseteq L\right\}}$

forms a direct system in the category of locally convex topological vector spaces that is directed by ${\displaystyle \mathbb {K} }$ (under subset inclusion). This system's direct limit (in the category of locally convex TVSs) is the pair ${\displaystyle (C_{c}^{k}(U),\operatorname {In} _{\bullet }^{U})}$ where ${\displaystyle \operatorname {In} _{\bullet }^{U}:=\left(\operatorname {In} _{K}^{U}\right)_{K\in \mathbb {K} }}$ are the natural inclusions and where ${\displaystyle C_{c}^{k}(U)}$ is now endowed with the (unique) strongest locally convex topology making all of the inclusion maps ${\displaystyle \operatorname {In} _{\bullet }^{U}=\left(\operatorname {In} _{K}^{U}\right)_{K\in \mathbb {K} }}$ continuous.

The canonical LF topology on ${\displaystyle C_{c}^{k}(U)}$ is the finest locally convex topology on ${\displaystyle C_{c}^{k}(U)}$ making all of the inclusion maps ${\displaystyle \operatorname {In} _{K}^{U}:C^{k}(K)\to C_{c}^{k}(U)}$ continuous (where K ranges over ${\displaystyle \mathbb {K} }$).
Assumption: As is common in mathematics literature, this article will henceforth assume that ${\displaystyle C_{c}^{k}(U)}$ is endowed with its canonical LF topology (unless explicitly stated otherwise).
Topology defined by neighborhoods of the origin

If U is a convex subset of ${\displaystyle C_{c}^{k}(U),}$ then U is a neighborhood of the origin in the canonical LF topology if and only if it satisfies the following condition:

For all ${\displaystyle K\in \mathbb {K} ,}$ ${\displaystyle U\cap C^{k}(K)}$ is a neighborhood of the origin in ${\displaystyle C^{k}(K).}$

(CN)

Note that any convex set satisfying this condition is necessarily absorbing in ${\displaystyle C_{c}^{k}(U).}$ Since the topology of any topological vector space is translation-invariant, any TVS-topology is completely determined by the set of neighborhood of the origin. This means that one could actually define the canonical LF topology by declaring that a convex balanced subset U is a neighborhood of the origin if and only if it satisfies condition CN.

Topology defined via differential operators

A linear differential operator in U with smooth coefficients is a sum

${\displaystyle P:=\sum _{\alpha \in \mathbb {N} ^{n}}c_{\alpha }\partial ^{\alpha }}$

where ${\displaystyle c_{\alpha }\in C^{\infty }(U)}$ and all but finitely many of ${\displaystyle c_{\alpha }}$ are identically 0. The integer ${\displaystyle \sup\{|\alpha |:c_{\alpha }\neq 0\}}$ is called the order of the differential operator ${\displaystyle P.}$ If ${\displaystyle P}$ is a linear differential operator of order k then it induces a canonical linear map ${\displaystyle C^{k}(U)\to C^{0}(U)}$ defined by ${\displaystyle \phi \mapsto P\phi ,}$ where we shall reuse notation and also denote this map by ${\displaystyle P.}$ [8]

For any ${\displaystyle 1\leq k\leq \infty ,}$ the canonical LF topology on ${\displaystyle C_{c}^{k}(U)}$ is the weakest locally convex TVS topology making all linear differential operators in U of order ${\displaystyle \, into continuous maps from ${\displaystyle C_{c}^{k}(U)}$ into ${\displaystyle C_{c}^{0}(U).}$ [8]

#### Properties of the canonical LF topology

Canonical LF topology's independence from ${\displaystyle \mathbb {K} }$

One benefit of defining the canonical LF topology as the direct limit of a direct system is that we may immediately use the universal property of direct limits. Another benefit is that we can use well-known results from category theory to deduce that the canonical LF topology is actually independent of the particular choice of the directed collection ${\displaystyle \mathbb {K} }$ of compact sets. And by considering different collections ${\displaystyle \mathbb {K} }$ (in particular, those ${\displaystyle \mathbb {K} }$ mentioned at the beginning of this article), we may deduce different properties of this topology. In particular, we may deduce that the canonical LF topology makes ${\displaystyle C_{c}^{k}(U)}$ into a Hausdorff locally convex strict LF-space (and also a strict LB-space if ${\displaystyle k\neq \infty }$), which of course is the reason why this topology is called "the canonical LF topology" (see this footnote for more details). [note 6]

Universal property

From the universal property of direct limits, we know that if ${\displaystyle u:C_{c}^{k}(U)\to Y}$ is a linear map into a locally convex space Y (not necessarily Hausdorff), then u is continuous if and only if u is bounded if and only if for every ${\displaystyle K\in \mathbb {K} ,}$ the restriction of u to ${\displaystyle C^{k}(K)}$ is continuous (or bounded). [9] [10]

Dependence of the canonical LF topology on U

Suppose V is an open subset of ${\displaystyle \mathbb {R} ^{n}}$ containing ${\displaystyle U.}$ Let ${\displaystyle I:C_{c}^{k}(U)\to C_{c}^{k}(V)}$ denote the map that sends a function in ${\displaystyle C_{c}^{k}(U)}$ to its trivial extension on V (which was defined above). This map is a continuous linear map. [11] If (and only if) ${\displaystyle U\neq V}$ then ${\displaystyle I(C_{c}^{\infty }(U))}$ is not a dense subset of ${\displaystyle C_{c}^{\infty }(V)}$ and ${\displaystyle I:C_{c}^{\infty }(U)\to C_{c}^{\infty }(V)}$ is not a topological embedding. [11] Consequently, if ${\displaystyle U\neq V}$ then the transpose of ${\displaystyle I:C_{c}^{\infty }(U)\to C_{c}^{\infty }(V)}$ is neither one-to-one nor onto. [11]

Bounded subsets

A subset B of ${\displaystyle C_{c}^{k}(U)}$ is bounded in ${\displaystyle C_{c}^{k}(U)}$ if and only if there exists some ${\displaystyle K\in \mathbb {K} }$ such that ${\displaystyle B\subseteq C^{k}(K)}$ and B is a bounded subset of ${\displaystyle C^{k}(K).}$ [10] Moreover, if ${\displaystyle K\subseteq U}$ is compact and ${\displaystyle S\subseteq C^{k}(K)}$ then S is bounded in ${\displaystyle C^{k}(K)}$ if and only if it is bounded in ${\displaystyle C^{k}(U).}$ For any ${\displaystyle 0\leq k\leq \infty ,}$ any bounded subset of ${\displaystyle C_{c}^{k+1}(U)}$ (resp. ${\displaystyle C^{k+1}(U)}$) is a relatively compact subset of ${\displaystyle C_{c}^{k}(U)}$ (resp. ${\displaystyle C^{k}(U)}$), where ${\displaystyle \infty +1=\infty .}$ [10]

Non-metrizability

For all compact ${\displaystyle K\subseteq U,}$ the interior of ${\displaystyle C^{k}(K)}$ in ${\displaystyle C_{c}^{k}(U)}$ is empty so that ${\displaystyle C_{c}^{k}(U)}$ is of the first category in itself. It follows from Baire's theorem that ${\displaystyle C_{c}^{k}(U)}$ is not metrizable and thus also not normable (see this footnote [note 7] for an explanation of how the non-metrizable space ${\displaystyle C_{c}^{k}(U)}$ can be complete even though it does not admit a metric). The fact that ${\displaystyle C_{c}^{\infty }(U)}$ is a nuclear Montel space makes up for the non-metrizability of ${\displaystyle C_{c}^{\infty }(U)}$ (see this footnote for a more detailed explanation). [note 8]

Relationships between spaces

Using the universal property of direct limits and the fact that the natural inclusions ${\displaystyle \operatorname {In} _{K}^{L}:C^{k}(K)\to C^{k}(L)}$ are all topological embedding, one may show that all of the maps ${\displaystyle \operatorname {In} _{K}^{U}:C^{k}(K)\to C_{c}^{k}(U)}$ are also topological embeddings. Said differently, the topology on ${\displaystyle C^{k}(K)}$ is identical to the subspace topology that it inherits from ${\displaystyle C_{c}^{k}(U),}$ where recall that ${\displaystyle C^{k}(K)}$'s topology was defined to be the subspace topology induced on it by ${\displaystyle C^{k}(U).}$ In particular, both ${\displaystyle C_{c}^{k}(U)}$ and ${\displaystyle C^{k}(U)}$ induces the same subspace topology on ${\displaystyle C^{k}(K).}$ However, this does not imply that the canonical LF topology on ${\displaystyle C_{c}^{k}(U)}$ is equal to the subspace topology induced on ${\displaystyle C_{c}^{k}(U)}$ by ${\displaystyle C^{k}(U)}$; these two topologies on ${\displaystyle C_{c}^{k}(U)}$ are in fact never equal to each other since the canonical LF topology is never metrizable while the subspace topology induced on it by ${\displaystyle C^{k}(U)}$ is metrizable (since recall that ${\displaystyle C^{k}(U)}$ is metrizable). The canonical LF topology on ${\displaystyle C_{c}^{k}(U)}$ is actually strictly finer than the subspace topology that it inherits from ${\displaystyle C^{k}(U)}$ (thus the natural inclusion ${\displaystyle C_{c}^{k}(U)\to C^{k}(U)}$ is continuous but not a topological embedding). [7]

Indeed, the canonical LF topology is so fine that if ${\displaystyle C_{c}^{\infty }(U)\to X}$ denotes some linear map that is a "natural inclusion" (such as ${\displaystyle C_{c}^{\infty }(U)\to C^{k}(U),}$ or ${\displaystyle C_{c}^{\infty }(U)\to L^{p}(U),}$ or other maps discussed below) then this map will typically be continuous, which as is shown below, is ultimately the reason why locally integrable functions, Radon measures, etc. all induce distributions (via the transpose of such a "natural inclusion"). Said differently, the reason why there are so many different ways of defining distributions from other spaces ultimately stems from how very fine the canonical LF topology is. Moreover, since distributions are just continuous linear functionals on ${\displaystyle C_{c}^{\infty }(U),}$ the fine nature of the canonical LF topology means that more linear functionals on ${\displaystyle C_{c}^{\infty }(U)}$ end up being continuous ("more" means as compared to a coarser topology that we could have placed on ${\displaystyle C_{c}^{\infty }(U)}$ such as for instance, the subspace topology induced by some ${\displaystyle C^{k}(U),}$ which although it would have made ${\displaystyle C_{c}^{\infty }(U)}$ metrizable, it would have also resulted in fewer linear functionals on ${\displaystyle C_{c}^{\infty }(U)}$ being continuous and thus there would have been fewer distributions; moreover, this particular coarser topology also has the disadvantage of not making ${\displaystyle C_{c}^{\infty }(U)}$ into a complete TVS [12]).

Other properties
• The differentiation map ${\displaystyle C_{c}^{\infty }(U)\to C_{c}^{\infty }(U)}$ is a surjective continuous linear operator. [13]
• The bilinear multiplication map ${\displaystyle C^{\infty }(\mathbb {R} ^{m})\times C_{c}^{\infty }(\mathbb {R} ^{n})\to C_{c}^{\infty }(\mathbb {R} ^{m+n})}$ given by ${\displaystyle (f,g)\mapsto fg}$ is not continuous; it is however, hypocontinuous. [14]

### Distributions

As discussed earlier, continuous linear functionals on a ${\displaystyle C_{c}^{\infty }(U)}$ are known as distributions on U. Thus the set of all distributions on U is the continuous dual space of ${\displaystyle C_{c}^{\infty }(U),}$ which when endowed with the strong dual topology is denoted by ${\displaystyle {\mathcal {D}}'(U).}$

By definition, a distribution on U is defined to be a continuous linear functional on ${\displaystyle C_{c}^{\infty }(U).}$ Said differently, a distribution on U is an element of the continuous dual space of ${\displaystyle C_{c}^{\infty }(U)}$ when ${\displaystyle C_{c}^{\infty }(U)}$ is endowed with its canonical LF topology.

We have the canonical duality pairing between a distribution T on U and a test function ${\displaystyle f\in C_{c}^{\infty }(U),}$ which is denoted using angle brackets by

${\displaystyle {\begin{cases}{\mathcal {D}}'(U)\times C_{c}^{\infty }(U)\to \mathbb {R} \\(T,f)\mapsto \langle T,f\rangle :=T(f)\end{cases}}}$

One interprets this notation as the distribution T acting on the test function ${\displaystyle f}$ to give a scalar, or symmetrically as the test function ${\displaystyle f}$ acting on the distribution T.

Characterizations of distributions

Proposition. If T is a linear functional on ${\displaystyle C_{c}^{\infty }(U)}$ then the following are equivalent:

1. T is a distribution;
2. Definition: T is continuous;
3. T is continuous at the origin;
4. T is uniformly continuous;
5. T is a bounded operator;
6. T is sequentially continuous;
• explicitly, for every sequence ${\displaystyle \left(f_{i}\right)_{i=1}^{\infty }}$ in ${\displaystyle C_{c}^{\infty }(U)}$ that converges in ${\displaystyle C_{c}^{\infty }(U)}$ to some ${\displaystyle f\in C_{c}^{\infty }(U),}$ ${\displaystyle \lim _{i\to \infty }T\left(f_{i}\right)=T(f);}$ [note 9]
7. T is sequentially continuous at the origin; in other words, T maps null sequences [note 10] to null sequences;
• explicitly, for every sequence ${\displaystyle \left(f_{i}\right)_{i=1}^{\infty }}$ in ${\displaystyle C_{c}^{\infty }(U)}$ that converges in ${\displaystyle C_{c}^{\infty }(U)}$ to the origin (such a sequence is called a null sequence), ${\displaystyle \lim _{i\to \infty }T\left(f_{i}\right)=0;}$
• a null sequence is by definition a sequence that converges to the origin;
8. T maps null sequences to bounded subsets;
• explicitly, for every sequence ${\displaystyle \left(f_{i}\right)_{i=1}^{\infty }}$ in ${\displaystyle C_{c}^{\infty }(U)}$ that converges in ${\displaystyle C_{c}^{\infty }(U)}$ to the origin, the sequence ${\displaystyle \left(T\left(f_{i}\right)\right)_{i=1}^{\infty }}$ is bounded;
9. T maps Mackey convergence null sequences [note 11] to bounded subsets;
• explicitly, for every Mackey convergent null sequence ${\displaystyle \left(f_{i}\right)_{i=1}^{\infty }}$ in ${\displaystyle C_{c}^{\infty }(U),}$ the sequence ${\displaystyle \left(T\left(f_{i}\right)\right)_{i=1}^{\infty }}$ is bounded;
• a sequence ${\displaystyle f_{\bullet }=\left(f_{i}\right)_{i=1}^{\infty }}$ is said to be Mackey convergent to 0 if there exists a divergent sequence ${\displaystyle r_{\bullet }=\left(r_{i}\right)_{i=1}^{\infty }\to \infty }$ of positive real number such that the sequence ${\displaystyle \left(r_{i}f_{i}\right)_{i=1}^{\infty }}$ is bounded; every sequence that is Mackey convergent to 0 necessarily converges to the origin (in the usual sense);
10. The kernel of T is a closed subspace of ${\displaystyle C_{c}^{\infty }(U);}$
11. The graph of T is closed;
12. There exists a continuous seminorm g on ${\displaystyle C_{c}^{\infty }(U)}$ such that ${\displaystyle |T|\leq g;}$
13. There exists a constant ${\displaystyle C>0,}$ a collection of continuous seminorms, ${\displaystyle {\mathcal {P}},}$ that defines the canonical LF topology of ${\displaystyle C_{c}^{\infty }(U),}$ and a finite subset ${\displaystyle \{g_{1},\ldots ,g_{m}\}\subseteq {\mathcal {P}}}$ such that ${\displaystyle |T|\leq C(g_{1}+\cdots g_{m});}$ [note 12]
14. For every compact subset ${\displaystyle K\subseteq U}$ there exist constants ${\displaystyle C>0}$ and ${\displaystyle N\in \mathbb {N} }$ such that for all ${\displaystyle f\in C^{\infty }(K),}$ [1]
${\displaystyle |T(f)|\leq C\sup\{|\partial ^{p}f(x)|:x\in U,|\alpha |\leq N\};}$
15. For every compact subset ${\displaystyle K\subseteq U}$ there exist constants ${\displaystyle C_{K}>0}$ and ${\displaystyle N_{K}\in \mathbb {N} }$ such that for all ${\displaystyle f\in C_{c}^{\infty }(U)}$ with support contained in ${\displaystyle K,}$ [15]
${\displaystyle |T(f)|\leq C_{K}\sup\{|\partial ^{\alpha }f(x)|:x\in K,|\alpha |\leq N_{K}\};}$
16. For any compact subset ${\displaystyle K\subseteq U}$ and any sequence ${\displaystyle \{f_{i}\}_{i=1}^{\infty }}$ in ${\displaystyle C^{\infty }(K),}$ if ${\displaystyle \{\partial ^{p}f_{i}\}_{i=1}^{\infty }}$ converges uniformly to zero for all multi-indices p, then ${\displaystyle T(f_{i})\to 0;}$
17. Any of the three statements immediately above (i.e. statements 14, 15, and 16) but with the additional requirement that compact set K belongs to ${\displaystyle \mathbb {K} .}$

#### Topology on the space of distributions

Definition and notation: The space of distributions on U, denoted by ${\displaystyle {\mathcal {D}}'(U),}$ is the continuous dual space of ${\displaystyle C_{c}^{\infty }(U)}$ endowed with the topology of uniform convergence on bounded subsets of ${\displaystyle C_{c}^{\infty }(U).}$ [7] More succinctly, the space of distributions on U is ${\displaystyle {\mathcal {D}}'(U):=(C_{c}^{\infty }(U))'_{b}.}$

The topology of uniform convergence on bounded subsets is also called the strong dual topology. [note 13] This topology is chosen because it is with this topology that ${\displaystyle {\mathcal {D}}'(U)}$ becomes a nuclear Montel space and it is with this topology that the kernels theorem of Schwartz holds. [16] No matter what dual topology is placed on ${\displaystyle {\mathcal {D}}'(U),}$ [note 14] a sequence of distributions converges in this topology if and only if it converges pointwise (although this need not be true of a net). No matter which topology is chosen, ${\displaystyle {\mathcal {D}}'(U)}$ will be a non- metrizable, locally convex topological vector space. The space ${\displaystyle {\mathcal {D}}'(U)}$ is separable [17] and has the strong Pytkeev property [18] but it is neither a k-space [18] nor a sequential space, [17] which in particular implies that it is not metrizable and also that its topology can not be defined using only sequences.

### Topological properties

Topological vector space categories

The canonical LF topology makes ${\displaystyle C_{c}^{k}(U)}$ into a complete distinguished strict LF-space (and a strict LB-space if and only if ${\displaystyle k\neq \infty }$ [19]), which implies that ${\displaystyle C_{c}^{k}(U)}$ is a meager subset of itself. [20] Furthermore, ${\displaystyle C_{c}^{k}(U),}$ as well as its strong dual space, is a complete Hausdorff locally convex barrelled bornological Mackey space. The strong dual of ${\displaystyle C_{c}^{k}(U)}$ is a Fréchet space if and only if ${\displaystyle k\neq \infty }$ so in particular, the strong dual of ${\displaystyle C_{c}^{\infty }(U),}$ which is the space ${\displaystyle {\mathcal {D}}'(U)}$ of distributions on U, is not metrizable (note that the weak-* topology on ${\displaystyle {\mathcal {D}}'(U)}$ also is not metrizable and moreover, it further lacks almost all of the nice properties that the strong dual topology gives ${\displaystyle {\mathcal {D}}'(U)}$).

The three spaces ${\displaystyle C_{c}^{\infty }(U),}$ ${\displaystyle C^{\infty }(U),}$ and the Schwartz space ${\displaystyle {\mathcal {S}}(\mathbb {R} ^{n}),}$ as well as the strong duals of each of these three spaces, are complete nuclear [21] Montel [22] bornological spaces, which implies that all six of these locally convex spaces are also paracompact [23] reflexive barrelled Mackey spaces. The spaces ${\displaystyle C^{\infty }(U)}$ and ${\displaystyle {\mathcal {S}}(\mathbb {R} ^{n})}$ are both distinguished Fréchet spaces. Moreover, both ${\displaystyle C_{c}^{\infty }(U)}$ and ${\displaystyle {\mathcal {S}}(\mathbb {R} ^{n})}$ are Schwartz TVSs.

#### Convergent sequences

Convergent sequences and their insufficiency to describe topologies

The strong dual spaces of ${\displaystyle C^{\infty }(U)}$ and ${\displaystyle {\mathcal {S}}(\mathbb {R} ^{n})}$ are sequential spaces but not Fréchet-Urysohn spaces. [17] Moreover, neither the space of test functions ${\displaystyle C_{c}^{\infty }(U)}$ nor its strong dual ${\displaystyle {\mathcal {D}}'(U)}$ is a sequential space (not even an Ascoli space), [17] [24] which in particular implies that their topologies can not be defined entirely in terms of convergent sequences.

A sequence ${\displaystyle (f_{i})_{i=1}^{\infty }}$ in ${\displaystyle C_{c}^{k}(U)}$ converges in ${\displaystyle C_{c}^{k}(U)}$ if and only if there exists some ${\displaystyle K\in \mathbb {K} }$ such that ${\displaystyle C^{k}(K)}$ contains this sequence and this sequence converges in ${\displaystyle C^{k}(K)}$; equivalently, it converges if and only if the following two conditions hold: [25]

1. There is a compact set ${\displaystyle K\subseteq U}$ containing the supports of all ${\displaystyle f_{i}.}$
2. For each multi-index ${\displaystyle \alpha ,}$ the sequence of partial derivatives ${\displaystyle \partial ^{\alpha }f_{i}}$ tends uniformly to ${\displaystyle \partial ^{\alpha }f.}$

Neither the space ${\displaystyle C_{c}^{\infty }(U)}$ nor its strong dual ${\displaystyle {\mathcal {D}}'(U)}$ is a sequential space, [17] [24] and consequently, their topologies can not be defined entirely in terms of convergent sequences. For this reason, the above characterization of when a sequence converges is not enough to define the canonical LF topology on ${\displaystyle C_{c}^{\infty }(U).}$ The same can be said of the strong dual topology on ${\displaystyle {\mathcal {D}}'(U).}$

What sequences do characterize

Nevertheless, sequences do characterize many important properties, as we now discuss. It is known that in the dual space of any Montel space, a sequence converges in the strong dual topology if and only if it converges in the weak* topology, [26] which in particular, is the reason why a sequence of distributions converges (in the strong dual topology) if and only if it converges pointwise (this leads many authors to use pointwise convergence to actually define the convergence of a sequence of distributions; this is fine for sequences but it does not extend to the convergence of nets of distributions since a net may converge pointwise but fail to convergence in the strong dual topology).

Sequences characterize continuity of linear maps valued in locally convex space. Suppose X is a locally convex bornological space (such as any of the six TVSs mentioned earlier). Then a linear map ${\displaystyle F:X\to Y}$ into a locally convex space Y is continuous if and only if it maps null sequences [note 10] in X to bounded subsets of Y. [note 15] More generally, such a linear map ${\displaystyle F:X\to Y}$ is continuous if and only if it maps Mackey convergent null sequences [note 11] to bounded subsets of ${\displaystyle Y.}$ So in particular, if a linear map ${\displaystyle F:X\to Y}$ into a locally convex space is sequentially continuous at the origin then it is continuous. [27] However, this does not necessarily extend to non-linear maps and/or to maps valued in topological spaces that are not locally convex TVSs.

For every ${\displaystyle k\in \{0,1,\ldots ,\infty \},C_{c}^{\infty }(U)}$ is sequentially dense in ${\displaystyle C_{c}^{k}(U).}$ [28] Furthermore, ${\displaystyle \{D_{\phi }:\phi \in C_{c}^{\infty }(U)\}}$ is a sequentially dense subset of ${\displaystyle {\mathcal {D}}'(U)}$ (with its strong dual topology) [29] and also a sequentially dense subset of the strong dual space of ${\displaystyle C^{\infty }(U).}$ [29]

Sequences of distributions

A sequence of distributions ${\displaystyle (T_{i})_{i=1}^{\infty }}$ converges with respect to the weak-* topology on ${\displaystyle {\mathcal {D}}'(U)}$ to a distribution T if and only if

${\displaystyle \langle T_{i},f\rangle \to \langle T,f\rangle }$

for every test function ${\displaystyle f\in {\mathcal {D}}(U).}$ For example, if ${\displaystyle f_{m}:\mathbb {R} \to \mathbb {R} }$ is the function

${\displaystyle f_{m}(x)={\begin{cases}m&{\text{if }}x\in [0,{\frac {1}{m}}]\\0&{\text{otherwise}}\end{cases}}}$

and ${\displaystyle T_{m}}$ is the distribution corresponding to ${\displaystyle f_{m},}$ then

${\displaystyle \langle T_{m},f\rangle =m\int _{0}^{\frac {1}{m}}f(x)\,dx\to f(0)=\langle \delta ,f\rangle }$

as ${\displaystyle m\to \infty ,}$ so ${\displaystyle T_{m}\to }$δ in ${\displaystyle {\mathcal {D}}'(\mathbb {R} ).}$ Thus, for large ${\displaystyle m,}$ the function ${\displaystyle f_{m}}$ can be regarded as an approximation of the Dirac delta distribution.

Other properties
• The strong dual space of ${\displaystyle {\mathcal {D}}'(U)}$ is TVS isomorphic to ${\displaystyle C_{c}^{\infty }(U)}$ via the canonical TVS-isomorphism ${\displaystyle C_{c}^{\infty }(U)\to ({\mathcal {D}}'(U))'_{b}}$ defined by sending ${\displaystyle f\in C_{c}^{\infty }(U)}$ to value at ${\displaystyle f}$ (that is, to the linear functional on ${\displaystyle {\mathcal {D}}'(U)}$ defined by sending ${\displaystyle d\in {\mathcal {D}}'(U)}$ to ${\displaystyle d(f)}$);
• On any bounded subset of ${\displaystyle {\mathcal {D}}'(U),}$ the weak and strong subspace topologies coincide; the same is true for ${\displaystyle C_{c}^{\infty }(U)}$;
• Every weakly convergent sequence in ${\displaystyle {\mathcal {D}}'(U)}$ is strongly convergent (although this does not extend to nets).

## Localization of distributions

There is no way to define the value of a distribution in ${\displaystyle {\mathcal {D}}'(U)}$ at a particular point of U. However, as is the case with functions, distributions on U restrict to give distributions on open subsets of U. Furthermore, distributions are locally determined in the sense that a distribution on all of U can be assembled from a distribution on an open cover of U satisfying some compatibility conditions on the overlaps. Such a structure is known as a sheaf.

### Restrictions to an open subset

Let U and V be open subsets of ${\displaystyle \mathbb {R} ^{n}}$ with ${\displaystyle V\subseteq U}$. Let ${\displaystyle E_{VU}:{\mathcal {D}}(V)\to {\mathcal {D}}(U)}$ be the operator which extends by zero a given smooth function compactly supported in V to a smooth function compactly supported in the larger set U. The transpose of ${\displaystyle E_{VU}}$ is called the restriction mapping and is denoted by ${\displaystyle \rho _{VU}:={}^{t}E_{VU}:{\mathcal {D}}'(U)\to {\mathcal {D}}'(V).}$

The map ${\displaystyle E_{VU}:{\mathcal {D}}(V)\to {\mathcal {D}}(U)}$ is a continuous injection where if ${\displaystyle V\subseteq U}$ then it is not a topological embedding and its range is not dense in ${\displaystyle {\mathcal {D}}(U),}$ which implies that this map's transpose is neither injective nor surjective and that the topology that ${\displaystyle E_{VU}}$ transfers from ${\displaystyle {\mathcal {D}}(V)}$ onto its image is strictly finer than the subspace topology that ${\displaystyle {\mathcal {D}}(U)}$ induces on this same set. [11] A distribution ${\displaystyle S\in {\mathcal {D}}'(V)}$ is said to be extendible to U if it belongs to the range of the transpose of ${\displaystyle E_{VU}}$ and it is called extendible if it is extendable to ${\displaystyle \mathbb {R} ^{n}.}$ [11]

For any distribution ${\displaystyle T\in {\mathcal {D}}'(U),}$ the restriction ρVU(T) is a distribution in ${\displaystyle {\mathcal {D}}'(V)}$ defined by:

${\displaystyle \langle \rho _{VU}T,\phi \rangle =\langle T,E_{VU}\phi \rangle \quad {\text{ for all }}\phi \in {\mathcal {D}}(V).}$

Unless U = V, the restriction to V is neither injective nor surjective. Lack of surjectivity follows since distributions can blow up towards the boundary of V. For instance, if ${\displaystyle U=\mathbb {R} }$ and ${\displaystyle V=(0,2),}$ then the distribution

${\displaystyle T(x)=\sum _{n=1}^{\infty }n\,\delta \left(x-{\frac {1}{n}}\right)}$

is in ${\displaystyle {\mathcal {D}}'(V)}$ but admits no extension to ${\displaystyle {\mathcal {D}}'(U).}$

### Gluing and distributions that vanish in a set

Theorem [30] — Let ${\displaystyle (U_{i})_{i\in I}}$ be a collection of open subsets of ${\displaystyle \mathbb {R} ^{n}.}$ For each ${\displaystyle i\in I,}$ let ${\displaystyle T_{i}\in {\mathcal {D}}'(U_{i})}$ and suppose that for all ${\displaystyle i,j\in I,}$ the restriction of ${\displaystyle T_{i}}$ to ${\displaystyle U_{i}\cap U_{j}}$ is equal to the restriction of ${\displaystyle T_{j}}$ to ${\displaystyle U_{i}\cap U_{j}}$ (note that both restrictions are elements of ${\displaystyle {\mathcal {D}}'(U_{i}\cap U_{j})}$). Then there exists a unique ${\displaystyle T\in {\mathcal {D}}'(\cup _{i\in I}U_{i})}$ such that for all ${\displaystyle i\in I,}$ the restriction of T to ${\displaystyle U_{i}}$ is equal to ${\displaystyle T_{i}.}$

Let V be an open subset of U. ${\displaystyle T\in {\mathcal {D}}'(U)}$ is said to vanish in V if for all ${\displaystyle f\in {\mathcal {D}}(U)}$ such that ${\displaystyle \operatorname {supp} (f)\subseteq V}$ we have ${\displaystyle Tf=0.}$ T vanishes in V if and only if the restriction of T to V is equal to 0, or equivalently, if and only if T lies in the kernel of the restriction map ρVU.

Corollary. [30] Let ${\displaystyle (U_{i})_{i\in I}}$ be a collection of open subsets of ${\displaystyle \mathbb {R} ^{n}}$ and let ${\displaystyle T\in {\mathcal {D}}'(\cup _{i\in I}U_{i}).}$ T = 0 if and only if for each ${\displaystyle i\in I,}$ the restriction of T to ${\displaystyle U_{i}}$ is equal to 0.
Corollary. [30] The union of all open subsets of U in which a distribution T vanishes is an open subset of U in which T vanishes.

### Support of a distribution

This last corollary implies that for every distribution T on U, there exists a unique largest subset V of U such that T vanishes in V (and does not vanish in any open subset of U that is not contained in V); the complement in U of this unique largest open subset is called the support of T. [30] Thus

${\displaystyle \operatorname {supp} (T)=U\setminus \bigcup \{V\mid \rho _{VU}T=0\}.}$

If ${\displaystyle f}$ is a locally integrable function on U and if ${\displaystyle D_{f}}$ is its associated distribution, then the support of ${\displaystyle D_{f}}$ is the smallest closed subset of U in the complement of which ${\displaystyle f}$ is almost everywhere equal to 0. [30] If ${\displaystyle f}$ is continuous, then the support of ${\displaystyle D_{f}}$ is equal to the closure of the set of points in U at which ${\displaystyle f}$ does not vanish. [30] The support of the distribution associated with the Dirac measure at a point ${\displaystyle x_{0}}$ is the set ${\displaystyle \{x_{0}\}.}$ [30] If the support of a test function ${\displaystyle f}$ does not intersect the support of a distribution T then Tf = 0. A distribution T is 0 if and only if its support is empty. If ${\displaystyle f\in C^{\infty }(U)}$ is identically 1 on some open set containing the support of a distribution T then fT = T. If the support of a distribution T is compact then it has finite order and furthermore, there is a constant C and a non-negative integer N such that: [6]

${\displaystyle \qquad |T\phi |\leq C\|\phi \|_{N}:=C\sup \left\{\left|\partial ^{\alpha }\phi (x)\right|:x\in U,|\alpha |\leq N\right\}\quad {\text{ for all }}\phi \in {\mathcal {D}}(U).}$

If T has compact support then it has a unique extension to a continuous linear functional ${\displaystyle {\widehat {T}}}$ on ${\displaystyle C^{\infty }(U)}$; this functional can be defined by ${\displaystyle {\widehat {T}}(f):=T(\psi f),}$ where ${\displaystyle \psi \in {\mathcal {D}}(U)}$ is any function that is identically 1 on an open set containing the support of T. [6]

If ${\displaystyle S,T\in {\mathcal {D}}'(U)}$ and ${\displaystyle \lambda \neq 0}$ then ${\displaystyle \operatorname {supp} (S+T)\subseteq \operatorname {supp} (S)\cup \operatorname {supp} (T)}$ and ${\displaystyle \operatorname {supp} (\lambda T)=\operatorname {supp} (T).}$ Thus, distributions with support in a given subset ${\displaystyle A\subseteq U}$ form a vector subspace of ${\displaystyle {\mathcal {D}}'(U)}$; such a subspace is weakly closed in ${\displaystyle {\mathcal {D}}'(U)}$ if and only if A is closed in U. [31] Furthermore, if ${\displaystyle P}$ is a differential operator in U, then for all distributions T on U and all ${\displaystyle f\in C^{\infty }(U)}$ we have ${\displaystyle \operatorname {supp} (P(x,\partial )T)\subseteq \operatorname {supp} (T)}$ and ${\displaystyle \operatorname {supp} (fT)\subseteq \operatorname {supp} (f)\cap \operatorname {supp} (T).}$ [31]

### Distributions with compact support

Support in a point set and Dirac measures

For any ${\displaystyle x\in U,}$ let ${\displaystyle \delta _{x}\in {\mathcal {D}}'(U)}$ denote the distribution induced by the Dirac measure at x. For any ${\displaystyle x_{0}\in U}$ and distribution ${\displaystyle T\in {\mathcal {D}}'(U),}$ the support of T is contained in ${\displaystyle \{x_{0}\}}$ if and only if T is a finite linear combination of derivatives of the Dirac measure at ${\displaystyle x_{0}.}$ [32] If in addition the order of T is ${\displaystyle \leq k}$ then there exist constants ${\displaystyle \alpha _{p}}$ such that: [33]

${\displaystyle T=\sum _{|p|\leq k}\alpha _{p}\partial ^{p}\delta _{x_{0}}.}$

Said differently, if T has support at a single point ${\displaystyle \{P\},}$ then T is in fact a finite linear combination of distributional derivatives of the δ function at P. That is, there exists an integer m and complex constants aα such that

${\displaystyle T=\sum _{|\alpha |\leq m}a_{\alpha }\partial ^{\alpha }(\tau _{P}\delta )}$

where ${\displaystyle \tau _{P}}$ is the translation operator.

Distribution with compact support

Theorem [6] — Suppose T is a distribution on U with compact support K. There exists a continuous function ${\displaystyle f}$ defined on U and a multi-index p such that

${\displaystyle T=\partial ^{p}f,}$

where the derivatives are understood in the sense of distributions. That is, for all test functions ${\displaystyle \phi }$ on U,

${\displaystyle T\phi =(-1)^{|p|}\int _{U}f(x)(\partial ^{p}\phi )(x)\,dx.}$
Distributions of finite order with support in an open subset

Theorem [6] — Suppose T is a distribution on U with compact support K and let V be an open subset of U containing K. Since every distribution with compact support has finite order, take N to be the order of T and define ${\displaystyle P:=\{0,1,\ldots ,N+2\}^{n}.}$ There exists a family of continuous functions ${\displaystyle (f_{p})_{p\in P}}$ defined on U with support in V such that

${\displaystyle T=\sum _{p\in P}\partial ^{p}f_{p},}$

where the derivatives are understood in the sense of distributions. That is, for all test functions ${\displaystyle \phi }$ on U,

${\displaystyle T\phi =\sum _{p\in P}(-1)^{|p|}\int _{U}f_{p}(x)(\partial ^{p}\phi )(x)\,dx.}$

### Global structure of distributions

The formal definition of distributions exhibits them as a subspace of a very large space, namely the topological dual of ${\displaystyle {\mathcal {D}}(U)}$ (or the Schwartz space ${\displaystyle {\mathcal {S}}(\mathbb {R} ^{n})}$ for tempered distributions). It is not immediately clear from the definition how exotic a distribution might be. To answer this question, it is instructive to see distributions built up from a smaller space, namely the space of continuous functions. Roughly, any distribution is locally a (multiple) derivative of a continuous function. A precise version of this result, given below, holds for distributions of compact support, tempered distributions, and general distributions. Generally speaking, no proper subset of the space of distributions contains all continuous functions and is closed under differentiation. This says that distributions are not particularly exotic objects; they are only as complicated as necessary.

Distributions as sheafs

Theorem [34] — Let T be a distribution on U. There exists a sequence ${\displaystyle (T_{i})_{i=1}^{\infty }}$ in ${\displaystyle {\mathcal {D}}'(U)}$ such that each Ti has compact support and every compact subset ${\displaystyle K\subseteq U}$ intersects the support of only finitely many Ti, and the sequence of partial sums ${\displaystyle (S_{j})_{j=1}^{\infty },}$ defined by ${\displaystyle S_{j}:=T_{1}+\cdots +T_{j},}$ converges in ${\displaystyle {\mathcal {D}}'(U)}$ to T; in other words we have:

${\displaystyle T=\sum _{i=1}^{\infty }T_{i}.}$

Recall that a sequence converges in ${\displaystyle {\mathcal {D}}'(U)}$ (with its strong dual topology) if and only if it converges pointwise.

#### Decomposition of distributions as sums of derivatives of continuous functions

By combining the above results, one may express any distribution on U as the sum of a series of distributions with compact support, where each of these distributions can in turn be written as a finite sum of distributional derivatives of continuous functions on U. In other words for arbitrary ${\displaystyle T\in {\mathcal {D}}'(U)}$ we can write:

${\displaystyle T=\sum _{i=1}^{\infty }\sum _{p\in P_{i}}\partial ^{p}f_{ip},}$

where ${\displaystyle P_{1},P_{2},\ldots }$ are finite sets of multi-indices and the functions ${\displaystyle f_{ip}}$ are continuous.

Theorem [35] — Let T be a distribution on U. For every multi-index p there exists a continuous function gp on U such that

1. any compact subset K of U intersects the support of only finitely many gp, and
2. ${\displaystyle T=\sum \nolimits _{p}\partial ^{p}g_{p}.}$

Moreover, if T has finite order, then one can choose gp in such a way that only finitely many of them are non-zero.

Note that the infinite sum above is well-defined as a distribution. The value of T for a given ${\displaystyle f\in {\mathcal {D}}(U)}$ can be computed using the finitely many gα that intersect the support of ${\displaystyle f.}$

## Operations on distributions

Many operations which are defined on smooth functions with compact support can also be defined for distributions. In general, if ${\displaystyle A:{\mathcal {D}}(U)\to {\mathcal {D}}(U)}$ is a linear map which is continuous with respect to the weak topology, then it is possible to extend A to a map ${\displaystyle A:{\mathcal {D}}'(U)\to {\mathcal {D}}'(U)}$ by passing to the limit. [note 16][ citation needed][ clarification needed]

### Preliminaries: Transpose of a linear operator

Operations on distributions and spaces of distributions are often defined by means of the transpose of a linear operator because it provides a unified approach that the many definitions in the theory of distributions and because of its many well-known topological properties. [36] In general the transpose of a continuous linear map ${\displaystyle A:X\to Y}$ is the linear map ${\displaystyle {}^{t}A:Y'\to X'}$ defined by ${\displaystyle {}^{t}A(y'):=y'\circ A,}$ or equivalently, it is the unique map satisfying ${\displaystyle \langle y',A(x)\rangle =\left\langle {}^{t}A(y'),x\right\rangle }$ for all ${\displaystyle x\in X}$ and all ${\displaystyle y'\in Y'.}$ Since A is continuous, the transpose ${\displaystyle {}^{t}A:Y'\to X'}$ is also continuous when both duals are endowed with their respective strong dual topologies; it is also continuous when both duals are endowed with their respective weak* topologies (see the articles polar topology and dual system for more details).

In the context of distributions, the characterization of the transpose can be refined slightly. Let ${\displaystyle A:{\mathcal {D}}(U)\to {\mathcal {D}}(U)}$ be a continuous linear map. Then by definition, the transpose of A is the unique linear operator ${\displaystyle A^{t}:{\mathcal {D}}'(U)\to {\mathcal {D}}'(U)}$ that satisfies:

${\displaystyle \langle {}^{t}A(T),\phi \rangle =\langle T,A(\phi )\rangle }$       for all ${\displaystyle \phi \in {\mathcal {D}}(U)}$ and all ${\displaystyle T\in {\mathcal {D}}'(U).}$

However, since the image of ${\displaystyle {\mathcal {D}}(U)}$ is dense in ${\displaystyle {\mathcal {D}}'(U),}$ it is sufficient that the above equality hold for all distributions of the form ${\displaystyle T=D_{\psi }}$ where ${\displaystyle \psi \in {\mathcal {D}}(U).}$ Explicitly, this means that the above condition holds if and only if the condition below holds:

${\displaystyle \langle {}^{t}A(D_{\psi }),\phi \rangle =\langle D_{\psi },A(\phi )\rangle =\langle \psi ,A(\phi )\rangle =\int _{U}\psi (A\phi )\,dx}$       for all ${\displaystyle \phi ,\psi \in {\mathcal {D}}(U).}$

### Differential operators

#### Differentiation of distributions

Let ${\displaystyle A:{\mathcal {D}}(U)\to {\mathcal {D}}(U)}$ is the partial derivative operator ${\displaystyle {\tfrac {\partial }{\partial x_{k}}}.}$ In order to extend ${\displaystyle A}$ we compute its transpose:

{\displaystyle {\begin{aligned}\langle {}^{t}A(D_{\psi }),\phi \rangle &=\int _{U}\psi (A\phi )\,dx&&{\text{(See above.)}}\\&=\int _{U}\psi {\frac {\partial \phi }{\partial x_{k}}}\,dx\\[4pt]&=-\int _{U}\phi {\frac {\partial \psi }{\partial x_{k}}}\,dx&&{\text{(integration by parts)}}\\[4pt]&=-\left\langle {\frac {\partial \psi }{\partial x_{k}}},\phi \right\rangle \\[4pt]&=-\langle A\psi ,\phi \rangle \end{aligned}}}

Therefore ${\displaystyle {}^{t}A=-A.}$ Therefore the partial derivative of ${\displaystyle T}$ with respect to the coordinate ${\displaystyle x_{k}}$ is defined by the formula

${\displaystyle \left\langle {\frac {\partial T}{\partial x_{k}}},\phi \right\rangle =-\left\langle T,{\frac {\partial \phi }{\partial x_{k}}}\right\rangle \qquad {\text{for all }}\phi \in {\mathcal {D}}(U).}$

With this definition, every distribution is infinitely differentiable, and the derivative in the direction ${\displaystyle x_{k}}$ is a linear operator on ${\displaystyle {\mathcal {D}}'(U).}$

More generally, if ${\displaystyle \alpha }$ is an arbitrary multi-index, then the partial derivative ${\displaystyle \partial ^{\alpha }T}$ of the distribution ${\displaystyle T\in {\mathcal {D}}'(U)}$ is defined by

${\displaystyle \langle \partial ^{\alpha }T,\phi \rangle =(-1)^{|\alpha |}\langle T,\partial ^{\alpha }\phi \rangle \qquad {\text{for all }}\phi \in {\mathcal {D}}(U).}$

Differentiation of distributions is a continuous operator on ${\displaystyle {\mathcal {D}}'(U);}$ this is an important and desirable property that is not shared by most other notions of differentiation.

If T is a distribution in ${\displaystyle \mathbb {R} }$ then

${\displaystyle \lim _{x\to 0}{\frac {T-\tau _{x}T}{x}}=T'\in {\mathcal {D}}'(\mathbb {R} ),}$

where ${\displaystyle T'}$ is the derivative of T and τx is translation by x; thus the derivative of T may be viewed as a limit of quotients. [37]

#### Differential operators acting on smooth functions

A linear differential operator in U with smooth coefficients acts on the space of smooth functions on ${\displaystyle U.}$ Given ${\displaystyle \textstyle P:=\sum \nolimits _{\alpha }c_{\alpha }\partial ^{\alpha }}$ we would like to define a continuous linear map, ${\displaystyle D_{P}}$ that extends the action of ${\displaystyle P}$ on ${\displaystyle C^{\infty }(U)}$ to distributions on ${\displaystyle U.}$ In other words we would like to define ${\displaystyle D_{P}}$ such that the following diagram commutes:

${\displaystyle {\begin{matrix}{\mathcal {D}}'(U)&{\stackrel {D_{P}}{\longrightarrow }}&{\mathcal {D}}'(U)\\\uparrow &&\uparrow \\C^{\infty }(U)&{\stackrel {P}{\longrightarrow }}&C^{\infty }(U)\end{matrix}}}$

Where the vertical maps are given by assigning ${\displaystyle f\in C^{\infty }(U)}$ its canonical distribution ${\displaystyle D_{f}\in {\mathcal {D}}'(U),}$ which is defined by: ${\displaystyle D_{f}(\phi )=\langle f,\phi \rangle }$ for all ${\displaystyle \phi \in {\mathcal {D}}(U).}$ With this notation the diagram commuting is equivalent to:

${\displaystyle D_{P(f)}=D_{P}D_{f}\qquad {\text{ for all }}f\in C^{\infty }(U).}$

In order to find ${\displaystyle D_{P}}$ we consider the transpose ${\displaystyle {}^{t}P:{\mathcal {D}}'(U)\to {\mathcal {D}}'(U)}$ of the continuous induced map ${\displaystyle P:{\mathcal {D}}(U)\to {\mathcal {D}}(U)}$ defined by ${\displaystyle \phi \mapsto P(\phi ).}$ As discussed above, for any ${\displaystyle \phi \in {\mathcal {D}}(U),}$ the transpose may be calculated by:

{\displaystyle {\begin{aligned}\left\langle {}^{t}P(D_{f}),\phi \right\rangle &=\int _{U}f(x)P(\phi )(x)\,dx\\&=\int _{U}f(x)\left[\sum \nolimits _{\alpha }c_{\alpha }(x)(\partial ^{\alpha }\phi )(x)\right]\,dx\\&=\sum \nolimits _{\alpha }\int _{U}f(x)c_{\alpha }(x)(\partial ^{\alpha }\phi )(x)\,dx\\&=\sum \nolimits _{\alpha }(-1)^{|\alpha |}\int _{U}\phi (x)(\partial ^{\alpha }(c_{\alpha }f))(x)\,dx\end{aligned}}}

For the last line we used integration by parts combined with the fact that ${\displaystyle \phi }$ and therefore all the functions ${\displaystyle f(x)c_{\alpha }(x)\partial ^{\alpha }\phi (x)}$ have compact support. [note 17] Continuing the calculation above we have for all ${\displaystyle \phi \in {\mathcal {D}}(U):}$

{\displaystyle {\begin{aligned}\left\langle {}^{t}P(D_{f}),\phi \right\rangle &=\sum \nolimits _{\alpha }(-1)^{|\alpha |}\int _{U}\phi (x)(\partial ^{\alpha }(c_{\alpha }f))(x)\,dx&&{\text{As shown above}}\\[4pt]&=\int _{U}\phi (x)\sum \nolimits _{\alpha }(-1)^{|\alpha |}(\partial ^{\alpha }(c_{\alpha }f))(x)\,dx\\[4pt]&=\int _{U}\phi (x)\sum _{\alpha }\left[\sum _{\gamma \leq \alpha }{\binom {\alpha }{\gamma }}(\partial ^{\gamma }c_{\alpha })(x)(\partial ^{\alpha -\gamma }f)(x)\right]\,dx&&{\text{Leibniz rule}}\\&=\int _{U}\phi (x)\left[\sum _{\alpha }\sum _{\gamma \leq \alpha }(-1)^{|\alpha |}{\binom {\alpha }{\gamma }}(\partial ^{\gamma }c_{\alpha })(x)(\partial ^{\alpha -\gamma }f)(x)\right]\,dx\\&=\int _{U}\phi (x)\left[\sum _{\alpha }\left[\sum _{\beta \geq \alpha }(-1)^{|\beta |}{\binom {\beta }{\alpha }}\left(\partial ^{\beta -\alpha }c_{\beta }\right)(x)\right](\partial ^{\alpha }f)(x)\right]\,dx&&{\text{Grouping terms by derivatives of }}f\\&=\int _{U}\phi (x)\left[\sum \nolimits _{\alpha }b_{\alpha }(x)(\partial ^{\alpha }f)(x)\right]\,dx&&b_{\alpha }:=\sum _{\beta \geq \alpha }(-1)^{|\beta |}{\binom {\beta }{\alpha }}\partial ^{\beta -\alpha }c_{\beta }\\&=\left\langle \left(\sum \nolimits _{\alpha }b_{\alpha }\partial ^{\alpha }\right)(f),\phi \right\rangle \end{aligned}}}

Define the formal transpose of ${\displaystyle P,}$ which will be denoted by ${\displaystyle P_{*}}$ to avoid confusion with the transpose map, to be the following differential operator on U:

${\displaystyle P_{*}:=\sum \nolimits _{\alpha }b_{\alpha }\partial ^{\alpha }}$

The computations above have shown that:

Lemma. Let ${\displaystyle P}$ be a linear differential operator with smooth coefficients in ${\displaystyle U.}$ Then for all ${\displaystyle \phi \in {\mathcal {D}}(U)}$ we have
${\displaystyle \left\langle {}^{t}P(D_{f}),\phi \right\rangle =\left\langle D_{P_{*}(f)},\phi \right\rangle ,}$
which is equivalent to:
${\displaystyle {}^{t}P(D_{f})=D_{P_{*}(f)}.}$

The Lemma combined with the fact that the formal transpose of the formal transpose is the original differential operator, i.e. ${\displaystyle P_{**}=P,}$ [8] enables us to arrive at the correct definition: the formal transpose induces the (continuous) canonical linear operator ${\displaystyle P_{*}:C_{c}^{\infty }(U)\to C_{c}^{\infty }(U)}$ defined by ${\displaystyle \phi \mapsto P_{*}(\phi ).}$ We claim that the transpose of this map, ${\displaystyle {}^{t}P_{*}:{\mathcal {D}}'(U)\to {\mathcal {D}}'(U),}$ can be taken as ${\displaystyle D_{P}.}$ To see this, for every ${\displaystyle \phi \in {\mathcal {D}}(U)}$, compute its action on a distribution of the form ${\displaystyle D_{f}}$ with ${\displaystyle f\in C^{\infty }(U)}$:

{\displaystyle {\begin{aligned}\left\langle {}^{t}P_{*}(D_{f}),\phi \right\rangle &=\left\langle D_{P_{**}(f)},\phi \right\rangle &&{\text{Using Lemma above with }}P_{*}{\text{ in place of }}P\\&=\left\langle D_{P(f)},\phi \right\rangle &&P_{**}=P\end{aligned}}}

We call the continuous linear operator ${\displaystyle D_{P}:={}^{t}P_{*}:{\mathcal {D}}'(U)\to {\mathcal {D}}'(U)}$ the differential operator on distributions extending P. [8] Its action on an arbitrary distribution ${\displaystyle S}$ is defined via:

${\displaystyle D_{P}(S)(\phi )=S(P_{*}(\phi ))\quad {\text{ for all }}\phi \in {\mathcal {D}}(U).}$

If ${\displaystyle (T_{i})_{i=1}^{\infty }}$ converges to ${\displaystyle T\in {\mathcal {D}}'(U)}$ then for every multi-index ${\displaystyle \alpha ,(\partial ^{\alpha }T_{i})_{i=1}^{\infty }}$ converges to ${\displaystyle \partial ^{\alpha }T\in {\mathcal {D}}'(U).}$

#### Multiplication of distributions by smooth functions

A differential operator of order 0 is just multiplication by a smooth function. And conversely, if ${\displaystyle f}$ is a smooth function then ${\displaystyle P:=f(x)}$ is a differential operator of order 0, whose formal transpose is itself (i.e. ${\displaystyle P_{*}=P}$). The induced differential operator ${\displaystyle D_{P}:{\mathcal {D}}'(U)\to {\mathcal {D}}'(U)}$ maps a distribution T to a distribution denoted by ${\displaystyle fT:=D_{P}(T).}$ We have thus defined the multiplication of a distribution by a smooth function.

We now give an alternative presentation of multiplication by a smooth function. If ${\displaystyle m:U\to \mathbb {R} }$ is a smooth function and T is a distribution on U, then the product mT is defined by

${\displaystyle \langle mT,\phi \rangle =\langle T,m\phi \rangle \qquad {\text{for all }}\phi \in {\mathcal {D}}(U).}$

This definition coincides with the transpose definition since if ${\displaystyle M:{\mathcal {D}}(U)\to {\mathcal {D}}(U)}$ is the operator of multiplication by the function m (i.e., ${\displaystyle (M\phi )(x)=m(x)\phi (x)}$), then

${\displaystyle \int _{U}(M\phi )(x)\psi (x)\,dx=\int _{U}m(x)\phi (x)\psi (x)\,dx=\int _{U}\phi (x)m(x)\psi (x)\,dx=\int _{U}\phi (x)(M\psi )(x)\,dx,}$

so that ${\displaystyle {}^{t}M=M.}$

Under multiplication by smooth functions, ${\displaystyle {\mathcal {D}}'(U)}$ is a module over the ring ${\displaystyle C^{\infty }(U).}$ With this definition of multiplication by a smooth function, the ordinary product rule of calculus remains valid. However, a number of unusual identities also arise. For example, if δ is the Dirac delta distribution on ${\displaystyle \mathbb {R} }$, then = m(0)δ, and if δ is the derivative of the delta distribution, then

${\displaystyle m\delta '=m(0)\delta '-m'\delta =m(0)\delta '-m'(0)\delta .}$

The bilinear multiplication map ${\displaystyle C^{\infty }(\mathbb {R} ^{n})\times {\mathcal {D}}'(\mathbb {R} ^{n})\to {\mathcal {D}}'(\mathbb {R} ^{n})}$ given by ${\displaystyle (f,T)\mapsto fT}$ is not continuous; it is however, hypocontinuous. [14]

Example. For any distribution T, the product of T with the function that is identically 1 on U is equal to T.

Example. Suppose ${\displaystyle (f_{i})_{i=1}^{\infty }}$ is a sequence of test functions on U that converges to the constant function ${\displaystyle 1\in C^{\infty }(U).}$ For any distribution T on U, the sequence ${\displaystyle (f_{i}T)_{i=1}^{\infty }}$ converges to ${\displaystyle T\in {\mathcal {D}}'(U).}$ [38]

If ${\displaystyle (T_{i})_{i=1}^{\infty }}$ converges to ${\displaystyle T\in {\mathcal {D}}'(U)}$ and ${\displaystyle (f_{i})_{i=1}^{\infty }}$ converges to ${\displaystyle f\in C^{\infty }(U)}$ then ${\displaystyle (f_{i}T_{i})_{i=1}^{\infty }}$ converges to ${\displaystyle fT\in {\mathcal {D}}'(U).}$

##### Problem of multiplying distributions

It is easy to define the product of a distribution with a smooth function, or more generally the product of two distributions whose singular supports are disjoint. With more effort it is possible to define a well-behaved product of several distributions provided their wave front sets at each point are compatible. A limitation of the theory of distributions (and hyperfunctions) is that there is no associative product of two distributions extending the product of a distribution by a smooth function, as has been proved by Laurent Schwartz in the 1950s. For example, if p.v.