P^{-1}=\left [\begin{array}{cccc} 0 & 1 & 0 & 0 \\ 0 & 0 & 0 & 1 \\ 1 & 0 & 0 & 0 \\ 0 & 0 & 1 & 0 \end{array} \right ], \quad 
L^{-1}=\left [\begin{array}{cccc} 1 & 0 & 0 & 0 \\ \frac{1}{2} & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ -\frac{1}{2} & -\frac{1}{2} & \frac{3}{4} & 1 \end{array} \right ]