Companion matrices. The equivalence of x’ = Ax and ψ(z)y = 0.

In some sense this section is a digression and could be postponed. However, the question we will ask is at this point a natural one, and its answer and the concept of companion matrices are general interest. What we do here is of special interest within the theory of the control and stability of continuous, as well as discrete, systems. We will see this for discrete systems in Section 15. The reader can, if he wishes,simply skim through this section and move on,and then come back later to pick up what is needed.