# Limit Cycles of a Simple Pendulum

In this pdf file the dynamical behavior of a simple pendulum is discussed. The equation of motion for a pendulum with no dissipation is:

$$\dot{\theta}=\omega, \qquad \dot{\omega}=-\frac{g}{l}\sin\theta$$

The phase portrait is given in the figure below.

The limit cycle is defined as an isolated closed trajectory in phase space. This means that there exists a neighborhood around the limit cycle, in which no other closed trajectories exist. Based on this definition the harmonic oscillator does not have a limit cycle, because the closed orbits are concentric ellipses and are arbitrarily close to each other. In other words, no neighborhood exists around a closed cycle which doesn't contain another closed cycle.

Based on this definition isn't this also the case for a simple pendulum for energies below the critical energy? It seems to me that for any neighborhood of a closed cycle, by changing the energy we can find another closed cycle in that neighborhood for the pendulum. Am I wrong? Can these closed orbits be characterized as limit cycles?

• There are many sources explaining the concept of the limit cycle. You can check this link if you want: youtu.be/EocHy9kufpA Aug 23, 2021 at 16:16
• The phase portrait is given in the image for a pendulum with no dissipation. Yet, the trajectories are called limit cycles in the portrait. So they received the name and it doesn't have dissipation My question is why and how. Aug 23, 2021 at 16:23
• It's not just the plot. Here is a part of the text: "The remaining orbits are periodic in time, and are called “limit cycles”. For small energies, near(0, 0)the limit cycles are the familiar simple harmonic motion, represented by circles or ellipses (stretched circles) in the phase space." Aug 23, 2021 at 16:58
• Ali, I turned my comments into an answer and deleted them, you might want to delete yours too. Aug 23, 2021 at 17:27

A limit cycle is a periodic orbit ("cycle") that is the accumulation point (for $$t \to \infty$$ or $$t \to -\infty$$) for all points in a given neighborhood — a condition which can't be satisfied by a Hamiltonian system (where phase space volume is preserved).