Definition:Closed Element

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Definition

Let $\left({S, \preceq}\right)$ be an ordered set.

Let $\operatorname{cl}$ be a closure operator on $S$.

Let $x \in S$.


Definition 1

The element $x$ is a closed element of $S$ (with respect to $\operatorname{cl}$) if and only if $x$ is a fixed point of $\operatorname{cl}$:

$\operatorname{cl} \left({x}\right) = x$


Definition 2

The element $x$ is a closed element of $S$ (with respect to $\operatorname{cl}$) if and only if $x$ is in the image of $\operatorname{cl}$:

$x \in \operatorname{im} \left({\operatorname{cl} }\right)$


Also see


Special case