I encountered this question in Strogatz's "nonlinear dynamics and chaos", which requires to draw a phase portrait with exactly three closed orbits and one fixed point. Due to the index theorem, the fixed point must be enclosed by all three orbits, which seems to me can only be achieved by a solar-system like structure (fixed point as the sun). But after that I couldn't find any portrait that doesn't break the typological constraints (e.g. no crossed lines, etc). Does any one has a solution for this? Thanks!
[Math] Three closed orbits with only one fixed point in a phase portrait
dynamical systemsnonlinear systemordinary differential equations
Best Answer
You can fill the space between the closed orbits with spiraling trajectories connecting the orbits. No conservation of energy is assumed, i.e., the system is not Hamiltionian, so this is possible.
For instance, make the fixed point and the second closed orbit stable, the first and third closed orbit unstable.