Fundamental vector field

In the study of dynamical systems, a hyperbolic equilibrium point or hyperbolic fixed point is a fixed point that does not have any center manifolds. Near a hyperbolic point the orbits of a two-dimensional, non-dissipative system resemble hyperbolas. This fails to hold in general. Strogatz notes that "hyperbolic is an unfortunate name – it sounds like it should mean 'saddle point' – but it has become standard." Several properties hold about a neighborhood of a hyperbolic point, notably

• A stable manifold and an unstable manifold exist,
• Shadowing occurs,
• The dynamics on the invariant set can be represented via symbolic dynamics,
• A natural measure can be defined,
• The system is structurally stable.

File:Phase Portrait Sadle.svg

Orbits near a two-dimensional saddle point, an example of a hyperbolic equilibrium.

Maps

If T : Rⁿ → Rⁿ is a C¹ map and p is a fixed point then p is said to be a hyperbolic fixed point when the differentialTemplate:Disambiguation needed DT(p) has no eigenvalues on the unit circle.

One example of a map that its only fixed point is hyperbolic is the Arnold Map or cat map:

$\left[\begin{array}{c}{x}_{n+1}\\ y_{n+1}\end{array}\right]=\left[\begin{array}{cc}1& 1\\ 1& 2\end{array}\right]\left[\begin{array}{c}{x}_n\\ y_n\end{array}\right]\text{modulo }1$

Since the eigenvalues are given by

${\lambda }_1=\frac{3+\sqrt{5}}{2}>1$

${\lambda }_2=\frac{3-\sqrt{5}}{2}<1$

Flows

Let F : Rⁿ → Rⁿ be a C¹ (that is, continuously differentiable) vector field with a critical point p and let J denote the Jacobian matrix of F at p. If the matrix J has no eigenvalues with zero real parts then p is called hyperbolic. Hyperbolic fixed points may also be called hyperbolic critical points or elementary critical points.

The Hartman-Grobman theorem states that the orbit structure of a dynamical system in a neighbourhood of a hyperbolic equilibrium point is topologically equivalent to the orbit structure of the linearized dynamical system.

Example

Consider the nonlinear system

$\frac{dx}{dt}=y,$

$\frac{dy}{dt}=-x-x^3-\alpha y,\alpha \ne 0$

(0, 0) is the only equilibrium point. The linearization at the equilibrium is

$J(0,0)=\left(\begin{array}{cc}0& 1\\ -1& -\alpha \end{array}\right)$.

The eigenvalues of this matrix are $\frac{-\alpha \pm \sqrt{{\alpha }^2-4}}{2}$. For all values of α ≠ 0, the eigenvalues have non-zero real part. Thus, this equilibrium point is a hyperbolic equilbrium point. The linearized system will behave similar to the non-linear system near (0, 0). When α = 0, the system has a nonhyperbolic equilibrium at (0, 0).

Comments

In the case of an infinite dimensional system - for example systems involving a time delay - the notion of the "hyperbolic part of the spectrum" refers to the above property.

See also

• Anosov flow
• Hyperbolic set
• Normally hyperbolic invariant manifold

References

• Template:Scholarpedia