Let $a , b , c$ and $d$ be four real numbers. We consider the matrix of $\mathcal { M } _ { 2 } ( \mathbb { R } )$
$$L = \left( \begin{array} { l l } a & b \\ c & d \end{array} \right)$$
Write the equations on $a , b , c , d$ expressing the membership of $L$ in $O ( 1,1 )$.