Differential Equations Lec 31 (Class 34), Hyperbolicity, Stability, Hamiltonian & Lyapunov Functions

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Differential Equations Course, Lecture 31. (0:00) Encouragement to pay attention and take notes for subtle things in this lecture and mention one more reading assignment. (0:57) Review hyperbolic equilibrium points and Hartman-Grobman theorem (linearization). (4:32) Comments about hyperbolic equilibrium points being structurally stable. (5:46) Lyapunov stable and unstable equilibrium points. Draw pictures to help understand the definitions. (18:24) Physical importance of stable versus unstable equilibrium points. Relate it to the pendulum and competing species models. (21:40) Hamiltonian systems and the ideal (undamped) pendulum. (23:25) Basic consequences of the nonlinearity, small-angle linear approximation, and solution curves in the phase plane. (28:57) Find the Hamiltonian function for the ideal pendulum (correction from the end of Class #33 (Lecture #30)) and graph the level curves with ContourPlot along with solution curves on top of the level curves. (36:34) Construct a Hamiltonian system by starting with the Hamiltonian function and then study the resulting system. (42:39) Saddles and centers are the only kinds of nondegenerate equilibrium points that can occur in Hamiltonian systems. (43:19) Adding friction to the pendulum model results in a dissipative system. The old Hamiltonian function for the system without friction now becomes a Lyapunov function for the system with friction. (43:54) Comment about the possibility of doing an experiment as part of the project as a way of getting more points. (44:27) The corresponding system. (44:57) See solutions in the phase plane and how they move from higher to lower values of the Lyapunov function. (48:18) Confirm symbolically with the Chain Rule that the Lyapunov function decreases along solution curves.

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Nice Close up of the Lyapunov Stability explanation.

videofountain
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At 42:50, you say that in non degenerate Hamiltonian case we always get equilibrium points as centres or saddles. I think the close up of the slide at 37:27 proves that ---- because the Trace of the Jacobian at equilibrium point is zero, hence the point (T, D) is always on the vertical D axis in the TD plane.

swastiktiwari