Arnold's cat map

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From order to chaos and back. Sample mapping on a picture of 150x150 pixels. The numbers shows the iteration step. After 300 iterations we obtain the original image.

In mathematics, Arnold's cat map is a chaotic map from the torus into itself, named after Vladimir Arnold, who demonstrated its effects in the 1960s using an image of a cat. One of this map's features is that image being apparently randomized by the transformation but returning to its original state after a number of steps. As can be seen in the picture to the right, the original image of the cat is sheared and then wrapped around in the first iteration of the transformation. After some iterations, the resulting image appears rather random or disordered, yet after further iterations the image appears to have further order—ghost-like images of the cat—and ultimately returns to the original image.

Arnold's cat map is the transformation $\Gamma : \mathbb{T}^2 \to \mathbb{T}^2$ given by the formula

$\Gamma \, : \, (x,y) \to (x+y,x+2y) \bmod 1$

Equivalently, in matrix notation, this is

$\Gamma \left( \begin{bmatrix} x \\ y \end{bmatrix} \right) = \begin{bmatrix} 1 & 1 \\ 1 & 2 \end{bmatrix} \begin{bmatrix} x \\ y \end{bmatrix} \bmod 1 = \begin{bmatrix} 1 & 0 \\ 1 & 1 \end{bmatrix} \begin{bmatrix} 1 & 1 \\ 0 & 1 \end{bmatrix} \begin{bmatrix} x \\ y \end{bmatrix} \bmod 1$

That is, with a unit size equal to the width of the square image, the image is sheared one unit to the right, then one unit up, and all that lies without that unit square is wrapped around on the other respective side to be within it.

Arnold's cat map is an important example of Anosov diffeomorphism.

The map describes the phase space flow corresponding to the discrete dynamics of a bead hopping from site q_t (0 =< q_t < N) to site q_t+1 on a circular ring with circumference N, according to the second order equation:

q_t+1 - 3q_t + q_t-1 = 0 mod N

Defining the momentum variable p_t = q_t - q_t-1, the above second order dynamics can be re-written as a mapping of the square 0 =< q, p < N (the phase space of the discrete dynamical system) onto itself:

q_t+1 = 2q_t + p_t mod N

p_t+1 = q_t + p_t mod N

This Arnold cat mapping shows mixing behavior typical for chaotic systems. However, since the transformation has a determinant equal to unity, it is area-preserving and therefore invertible the inverse transformation being:

q_t-1 = 2q_t - p_t mod N

p_t-1 = -q_t + p_t mod N

For real variables q and p, it is common to set N = 1. In that case a mapping of the unit square with periodic boundary conditions onto itself results.

When N is set to an integer value, the position and momentum variables can be restricted to integers and the mapping becomes a mapping of a toroidial square grid of points onto itself. Such an integer cat map is commonly used to demonstrate mixing behavior with Poincaré recurrence utilising digital images. The number of iterations needed to restore the image can be shown never to exceed 3N.

[edit] See also

List of chaotic maps

[edit] References

(French) V. I. Arnold; A. Avez (1967). Problèmes Ergodiques de la Mécanique Classique. Paris: Gauthier-Villars.
- English translation: V. I. Arnold; A. Avez (1968). Ergodic Problems in Classical Mechanics. New York: Benjamin.