Consider the following system of ODEs:
The eigenvalues of this simple canonical system are

. The extremum,

, is shown as a green dot.
If

, the extremum is an unstable focus.
If

, the extremum is a stable focus.
If

, the dynamic behavior is that of a limit cycle and the critical point is a center.
If

, the trajectories spiral clockwise around the origin.
If

, the trajectories spiral counterclockwise around the origin.
The red curve is the parametric plot of the solution of the system of ODEs with an initial condition

(shown as a cyan dot).