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Let $S$ be a set.

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

Then $f$ is idempotent if and only if:

$\forall x \in S: \map f {\map f x} = \map f x$

That is, if and only if applying the same mapping a second time to an argument gives the same result as applying it once.

And of course, that means the same as applying it as many times as you want.

The condition for idempotence can also be written:

$f \circ f = f$

where $\circ$ denotes composition of mappings.

Also known as

The concept of idempotence can also be referred to as idempotency.

Also see

  • Results about idempotence can be found here.