$${{\mathtt{xi}}}^{{\mathtt{2}}} = {\mathtt{\,-\,}}\left({{\mathtt{x}}}^{{\mathtt{2}}}\right) \Rightarrow \left\{ \begin{array}{l}{\mathtt{xi}} = {\mathtt{\,-\,}}\left({i}{\mathtt{\,\times\,}}{\mathtt{x}}\right)\\
{\mathtt{xi}} = {i}{\mathtt{\,\times\,}}{\mathtt{x}}\\
\end{array} \right\}$$
.