# Function of compact support

A function defined in some domain of $E ^ {n}$, having compact support belonging to this domain. More precisely, suppose that the function $f ( x) = f ( x _ {1} \dots x _ {n} )$ is defined on a domain $\Omega \subset E ^ {n}$. The support of $f$ is the closure of the set of points $x \in \Omega$ for which $f ( x)$ is different from zero $( f ( x) \neq 0)$. Thus one can also say that a function of compact support in $\Omega$ is a function defined on $\Omega$ such that its support $\Lambda$ is a closed bounded set located at a distance from the boundary $\Gamma$ of $\Omega$ by a number greater than $\delta > 0$, where $\delta$ is sufficiently small.

One usually considers $k$- times continuously-differentiable functions of compact support, where $k$ is a given natural number. Even more often one considers infinitely-differentiable functions of compact support. The function

$$\psi ( x) = \ \left \{ \begin{array}{ll} e ^ {- 1/( | x | ^ {2} - 1) } , & | x | < 1, | x | ^ {2} = \sum _ {j = 1 } ^ { n } x _ {j} ^ {2} , \\ 0 , & | x | \geq 1 , \\ \end{array} \right .$$

can serve as an example of an infinitely-differentiable function of compact support in a domain $\Omega$ containing the sphere $| x | \leq 1$.

The set of all infinitely-differentiable functions of compact support in a domain $\Omega \subset E ^ {n}$ is denoted by $D$. On $D$ one can define linear functionals (generalized functions, cf. Generalized function). With the aid of functions $v \in D$ one can define generalized solutions (cf. Generalized solution) of boundary value problems.

In theorems concerned with problems on finding generalized solutions, it is often important to know whether $D$ is dense in some concrete space of functions. It is known, for example, that if the boundary $\Gamma$ of a bounded domain $\Omega \subset E ^ {n}$ is sufficiently smooth, then $D$ is dense in the space of functions

$${W ^ { o } } {} _ {p} ^ {r} ( \Omega ) = \ \left \{ {f } : { f \in W _ {p} ^ {r} ( \Omega ),\ \left . \frac{\partial ^ {s} f }{\partial n ^ {s} } \right | _ \Gamma = 0,\ s = 0 \dots r - 1 } \right \}$$

( $1 \leq p \leq \infty$), that is, in the Sobolev space of functions of class $W _ {p} ^ {r} ( \Omega )$ that vanish on $\Gamma$ along with their normal derivatives of order up to and including $r - 1$( $r = 1, 2 ,\dots$).

How to Cite This Entry:
Function of compact support. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Function_of_compact_support&oldid=47009
This article was adapted from an original article by S.M. Nikol'skii (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article