ABSTRACT
We have seen that the linear approximation of a system gives information about the stability of an equilibrium. Insight into other properties can also be expected. We will discuss this matter in detail for the case of two state variables only. An advantage of this special case is that the results can easily be presented graphically. A diagram where one state variable is plotted as a function of the other is called a phase plane. The curves in the phase plane are called trajectories or orbits. We will begin with a discussion of linear systems.
