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Hyperbolic Dynamics and Anosov Systems
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Structural Stability
Refers to a property of a dynamical system that stays qualitatively the same under small perturbations of the system. Example: Anosov diffeomorphisms are structurally stable as their qualitative behavior does not change under small perturbations.
Hyperbolic Fixed Point
A fixed point in a dynamical system where the Jacobian matrix has no eigenvalues on the unit circle in the complex plane. Nearby trajectories diverge rapidly from the fixed point. Example: In the system , the point is a hyperbolic fixed point.
Nonuniform Hyperbolicity
A generalization of uniform hyperbolicity where stable and unstable manifolds still exist but their rates of expansion and contraction may vary from point to point. Example: The Hénon map is a system exhibiting nonuniform hyperbolicity for certain parameter values.
Anosov Diffeomorphism
A diffeomorphism of a compact manifold that is uniformly hyperbolic on the entire manifold. Points are stretched along one direction and compressed along another under its tangent map. Example: The Arnold's cat map on the torus is an Anosov diffeomorphism.
Poincaré Map
A tool for studying the behavior of a dynamical system by considering its intersection with a lower-dimensional slice (section) and the associated return map. Example: The Poincaré map of a periodic orbit in a continuous-time dynamical system provides a discrete map that can reveal the system's stability.
Hyperbolicity in Dynamical Systems
Refers to systems where the tangent space at each point can be split into stable and unstable directions, with distinct behavior (exponential shrinking and expanding respectively). Example: The Lorenz system exhibits hyperbolicity in certain parameter regimes.
Stable Manifold Theorem
States that for each hyperbolic fixed point or periodic point, there exist stable and unstable manifolds, which are as smooth as the map, tangent to the eigenspaces of the Jacobian at the point. Example: The stable manifold of a saddle point in a linear system is the set of points that approach the saddle point as time goes to infinity.
Periodic Orbits
Orbits that repeat themselves after a fixed period. In hyperbolic systems, they often serve as the 'skeleton' around which the dynamics are organized. Example: In the logistic map, periodic orbits can be found for certain parameter values, and they play a crucial role in understanding the map's bifurcations.
Ergodic Theory
A branch of mathematics that studies the statistical properties of dynamical systems that are invariant under time evolution. It relates the long-term average behavior of orbits to the structure of the space they occupy. Example: Ergodic theory deals with questions like whether the time average of a function along the orbits equals its space average.
Shadowing Lemma
Asserts that for Anosov systems, given any 'pseudo-orbit' there exists a true orbit staying close to the pseudo-orbit. This is important for numerical simulations, where approximate trajectories are computed. Example: A numerical solution to a chaotic dynamical system can be shown to correspond closely to a true trajectory due to the Shadowing Lemma.
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