Find the Spectral Radius of $A=$ $\mbox{} \left[ \begin{array}{cc} 1 & 0 & 0 & 0 \\ 0 & 0 & c & 0 \\ 0 & -c & 0 & 0 \\ 0 & 0 & 0 & 0 \end{array} \right]$
After going through the motions, I got that the $-x^{3}-c^{2}x=0$ $\Rightarrow$ $-x(x^{2}+c^{2})$
Now C is supposed to be in $\mathbb{R}$ while $A \in \mathbb{C}$
When I use the quadratic formula, I get that 2iC=0, and by the definition the spectral radius is the largest absolute value of the eigenvalues.
So what am I to make of the results of the quadratic formula? On the right path?