I'm studying for an engineering exam, but I haven't understood a mathematical concept..
Let's consider $\Delta(s)=a_ns^n+a_{n-1}s^{n-1}+...+a_0$. We say that we have stability if all the roots of $\Delta(s)$ have negative real part.
Now let's consider a set of polynomials that correspond to another given set P.
and switch to the coefficient space, suppose we have a convex domain like this one:
[$\underline{a_0},\bar{a_0}$]+[$\underline{a_1},\bar{a_1}$]$s+$[$\underline{a_2},\bar{a_2}$]$s^2$
My problem now is:
'If you want to know if any polynomial belonging to the box is stable, you have to check the stability of only 4 polynomials, following these steps (starting from 0 degree)':
They are related to the verteces. Well, I can't understand. Any vertex is defined by 3 parameters, why in the picture there are 6 bars?
Maybe its related to some Vladimir Kharitonov's works.. I have googled something but I haven't found anything, and nothing is written in my book... many thanks for your help