Systems of differential equations: Linear systems of differential equations
Qualitative exploration of stability
The various forms of stability of the equilibrium can often be understood by means of a qualitative analysis of directions in the phase plane. We present some examples.
Repelling equilibrium The -nullcline is the red line and the -nullcline is the green line . The phase portrait and diagram of directions are as follows:
All directions are away from the equilibrium: so this is a repelling equilibrium.
Attracting equilibrium The -nullcline is the red line and the -nullcline is the green line . The phase portrait and the diagram of directions are as follows:
All directions are towards the equilibrium: so this is an attracting equilibrium.
Saddle point: an attracting and repelling direction The -nullcline is the red line and the -nullcline is the green line . The phase portrait and the diagram of directions are as follows:
The directions are on the lower-left and upper- right side of the phase plane towards the equilibrium, but on the upper-left and the lower-right side of the phase plane away from the equilibrium. The equilibrium therefore has both a stable and unstable direction: it is a saddle point.
Inward spiral The -nullcline is the red line and the -nullcline is the green line . The phase portrait and the diagram of directions are as follows::
The directions indicate a rotational movement about an equilibrium but whether this is spiralling inwardly, spirals outwardly, or forms a periodic orbit about the equilibrium cannot be inferred from the directions.