4.1 Inverse

A binary relation has an inverse relation. Let us consider a relation $L \subseteq X_1 \times X_2$ . The inverse $L^{-1}$ is a subset of $X_2 \times X_1$ . Furthermore, $\exists (x,y) \in L \Leftrightarrow \exists (y,x) \in L^{-1}$ .