# Support of a generalized function

The set of those (and only those) points such that in any neighbourhood of them the generalized function does not vanish. A generalized function $f$ in $\mathcal D^\prime(O)$ vanishes in an open set $O^\prime \subset O$ if $(f,\phi) = 0$ for all $\phi \in \mathcal D(O^\prime)$. Using a partition of unity it can be proved that if a generalized function $f$ in $\mathcal D^\prime(O)$ vanishes in some neighbourhood $U_y \subset O$ for each point $y \in O$, then $f$ vanishes in $O$. The union of all neighbourhoods in which $f$ vanishes is called the zero set of $f$ and is denoted by $O_f$. The support of $f$, denoted by $\mathop{\mathrm{supp}} f$, is the complement of $O_f$ in $O$, that is, $\mathop{\mathrm{supp}} f = O \setminus O_f$ is a closed set in $O$. If $f$ is a continuous function in $O$, then an equivalent definition of the support of $f$ is the following: $\mathop{\mathrm{supp}} f$ is the closure in $O$ of the complement of the set of points at which $f$ vanishes (cf. Support of a function). For example, $\mathop{\mathrm{supp}} x = \mathbf{R}^1$, $\mathop{\mathrm{supp}} \delta = \{0\}$.

The singular support ($\mathop{\mathrm{sing}} \mathop{\mathrm{supp}}$) of a generalized function is the set of those (and only those) points such that in any neighbourhood of them the generalized function is not equal to a $C^\infty$-function. For example, $\mathop{\mathrm{sing}} \mathop{\mathrm{supp}} x = \emptyset$, $\mathop{\mathrm{sing}} \mathop{\mathrm{supp}} \delta = \{0\}$.

The notion of a zero set as used above is somewhat unusual and does not agree with the zero set of an ordinary function (not a generalized function) as the set of points where that function assumes the value zero. Of course, the statement "$f(x)=0$" has no meaning for generalized functions $f$.
A point $x_0$ in the support of a generalized function $f$ is called an essential point of $f$, cf. [a4].