involution

An involution is a function $f$ that is its own inverse function,

for all $x$ in the domain of $f$.

Any involution is a bijection.

Examples

• Trivially, the identity map $(x\mapsto x)$
• negation $(x\mapsto -x)$
• reciprocal $(x\mapsto \frac{1}{x})$
• complex conjugate $(z\mapsto \bar{z})$
• reflection
• Half-turn rotation
• circle inversion
• set complement