Exercise 3.2.55

$A=\left[\begin{array}{cc}I\hfill & I\hfill \end{array}\right]$has $N=\left[\begin{array}{c}\hfill I\\ \hfill -I\end{array}\right];B=\left[\begin{array}{cc}I\hfill & I\hfill \\ 0\hfill & 0\hfill \end{array}\right]$has the same $N;C=\left[\begin{array}{ccc}I\hfill & I\hfill & I\end{array}\right]$ has $N=\left[\begin{array}{cc}\hfill -I& \hfill -I\\ \hfill I& \hfill 0\\ \hfill 0& \hfill I\end{array}\right]$