Editing Lyapunov stability (section)

===System of deviations===
Instead of considering stability only near an equilibrium point (a constant solution <math>x(t)=x_e</math>), one can formulate similar definitions of stability near an arbitrary solution <math>x(t) = \phi(t)</math>. However, one can reduce the more general case to that of an equilibrium by a change of variables called a "system of deviations". Define <math>y = x - \phi(t)</math>, obeying the differential equation:

:<math>\dot{y} = f(t, y + \phi(t)) - \dot{\phi}(t) = g(t, y)</math>.

This is no longer an autonomous system, but it has a guaranteed equilibrium point at <math>y=0</math> whose stability is equivalent to the stability of the original solution <math>x(t) = \phi(t)</math>.