We say that a [[Unary Operation]] $-$ defind over the [[Set|set]] $S$ has the [[Involution Property]] [[Biconditional|if and only if]] [[Universal Quantifier|for all]] $a\in S$, $-(-a) = a$ $\huge \begin{align} -: S &\to S \\ -(-a) &= a\\ (-)^{2} a &= a \\ \end{align}$ >[!tip] Thereom >The last notation refers to the use of [[Functional Powers|functional powers]]. >A more general thereom can be proved that states that for any $n \in \N$, the following [[Identity Function|identity]] holds >$ (-)^{2n} = \op{id}_{S} $