Definition:Self-Inverse Element/Definition 1

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Let $\struct {S, \circ}$ be a monoid whose identity element is $e_S$.

Let $x \in S$ be an element of $S$.

$x$ is a self-inverse element of $\struct {S, \circ}$ if and only if $x \circ x = e_S$.

Also known as

The definition of a self-inverse element is usually made in the context of a group.

Some sources refer to such an element as an involution.

Also see