Fixed Point of Idempotent Mapping

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Let $S$ be a set.

Let $f: S \to S$ be an idempotent mapping.

Let $f \left[{S}\right]$ be the image of $S$ under $f$.

Let $x \in S$.

Then $x$ is a fixed point of $f$ if and only if $x \in f \left[{S}\right]$.


Necessary Condition

Let $x$ be a fixed point of $f$.


$f \left({x}\right) = x$

and so by the definition of image of mapping:

$x \in f \left[{S}\right]$


Sufficient Condition

Let $x \in f \left[{S}\right]$.

Then by the definition of image:

$\exists y \in S: f \left({y}\right) = x$


\(\displaystyle x\) \(=\) \(\displaystyle f \left({y}\right)\)
\(\displaystyle \implies \ \ \) \(\displaystyle f \left({x}\right)\) \(=\) \(\displaystyle f \left({f \left({y}\right)}\right)\) Definition of Mapping
\(\displaystyle \) \(=\) \(\displaystyle f \left({y}\right)\) Definition of Idempotent Mapping
\(\displaystyle \) \(=\) \(\displaystyle x\)

Thus by definition $x$ is a fixed point of $f$.