Definition:Closed under Mapping

Definition
Let $f: S \to T$ be a mapping.

Let $S' \subseteq S$.

Then $S'$ is closed under $f$ iff:


 * $f \left[{S'}\right] \subseteq S'$

where $f \left[{S'}\right]$ is the image of $S'$ under $f$.

Arbitrary Product
Let $\phi: X^I \to T$ be a mapping or a partial mapping, taking $I$-indexed families as arguments.

Denote with $\operatorname{dom} \phi$ the domain of $\phi$ (if $\phi$ is a mapping, this is simply $X^I$).

A set $S$ is closed under $\phi$ iff:


 * $\forall \left\langle{s_i}\right\rangle_{i \in I} \in S^I \cap \operatorname{dom} \phi: \phi \left({\left\langle{s_i}\right\rangle_{i \in I}}\right) \in S$

Phrased in terms of image of a mapping, this translates to:


 * $\phi \left({S^I \cap \operatorname{dom} \phi}\right) \subseteq S$

Thus, in words, $S$ is closed under $\phi$, iff:


 * Whenever $\phi$ is defined for an $I$-indexed family from $S$, it maps that indexed family into $S$ again.

Also see

 * Closed Algebraic Structure, an analogous concept in abstract algebra
 * Closed for Scalar Product