Definition:Invertible Operation

Definition
Let $\left({S, \circ}\right)$ be an algebraic structure.

The operation $\circ$ is invertible :
 * $\forall a, b \in S: \exists r, s \in S: a \circ r = b = s \circ a$

Example
An example of a 4-element algebraic structure whose operation is invertible is given by the following Cayley table:


 * $\begin{array}{c|cccc}

\circ & a & b & c & d \\ \hline a & a & d & b & c \\ b & c & b & d & a \\ c & d & a & c & b \\ d & b & c & a & d \\ \end{array}$

The invertible nature of $\circ$ can readily be determined by inspection:


 * $a \circ c = b = d \circ a$
 * $a \circ d = c = b \circ a$
 * $a \circ b = d = c \circ a$

etc.