-\frac{2}{3}
1 x + \frac{1}{6} x^{3} + {(-\frac{1}{40})} x^{5} + {(-\frac{55}{1008})} x^{7} + \mathcal{O}\left(x^{8}\right)
[ 1 -2 1/2]
[ 1 0 0]
[ -2 0 1]
\left(\begin{array}{rrr}
1 & -2 & \frac{1}{2} \\
1 & 0 & 0 \\
-2 & 0 & 1
\end{array}\right)