 Circle map

In mathematics, a circle map is a member of a family of dynamical systems on the circle first defined by Andrey Kolmogorov. Kolmogorov proposed this family as a simplified model for driven mechanical rotors (specifically, a freespinning wheel weakly coupled by a spring to a motor). The circle map equations also describe a simplified model of the phaselocked loop in electronics. The circle map exhibits certain regions of its parameters where it is locked to the driving frequency (phaselocking or modelocking in the language of electronic circuits); these are referred to as Arnold tongues, after Vladimir Arnold. Among other applications, the circle map has been used to study the dynamical behaviour of a beating heart.
Contents
Definition
The circle map is given by iterating the map
where θ is to be interpreted as polar angle such that its value lies between 0 and 1.
It has two parameters, the coupling strength K and the driving phase Ω. As a model for phaselocked loops, Ω may be interpreted as a driving frequency. For K = 0 and Ω irrational, the map reduces to an irrational rotation.
Mode locking
For small to intermediate values of K (that is, in the range of K = 0 to about K = 1), and certain values of Ω, the map exhibits a phenomenon called mode locking or phase locking. In a phaselocked region, the values θ_{n} advance essentially as a rational multiple of n, although they may do so chaotically on the small scale.
The limiting behavior in the modelocked regions is given by the rotation number
which is also sometimes referred to as the map winding number.
The phaselocked regions, or Arnold tongues, are illustrated in black in the figure above. Each such Vshaped region touches down to a rational value Ω = p / q in the limit of . The values of (K,Ω) in one of these regions will all result in a motion such that the rotation number ω = p / q. For example, all values of (K,Ω) in the large Vshaped region in the bottomcenter of the figure correspond to a rotation number of ω = 1 / 2. One reason the term "locking" is used is that the individual values θ_{n} can be perturbed by rather large random disturbances (up to the width of the tongue, for a given value of K), without disturbing the limiting rotation number. That is, the sequence stays "locked on" to the signal, despite the addition of significant noise to the series θ_{n}. This ability to "lock on" in the presence of noise is central to the utility of phaselocked loop electronic circuit.
There is a modelocked region for every rational number p / q. It is sometimes said that the circle map maps the rationals, a set of measure zero at K = 0, to a set of nonzero measure for . The largest tongues, ordered by size, occur at the Farey fractions. Fixing K and taking a crosssection through this image, so that ω is plotted as a function of Ω gives the Devil's staircase, a shape that is generically similar to the Cantor function.
The circle map also exhibits subharmonic routes to chaos, that is, period doubling of the form 3,6,12,24,....
Chirikov standard map
The Chirikov standard map is related to the circle map, having similar recurrence relations, which may be written as
 p_{n + 1} = θ_{n + 1} − θ_{n}
with both iterates taken modulo 1. In essence, the standard map introduces a momentum p_{n} which is allowed to dynamically vary, rather than being forced fixed, as it is in the circle map. The standard map is studied in physics by means of the kicked rotor Hamiltonian.
References
 Weisstein, Eric W., "Circle Map" from MathWorld.
 Philip L. Boyland : Bifurcations of circle maps: Arnol'd tongues, bistability and rotation intervals Comm. Math. Phys. Volume 106, Number 3 (1986), 353381.
 Robert Gilmore and Marc Lefranc, The Topology of Chaos, Alice in Stretch and Squeezeland, (2002) Wiley Interscience ISBN 0471408166 (Provides a brief review of basic facts in section 2.12).
 Leon Glass, Micheal R. Guevara, Alvin Shrier, Rafael Perez, "Bifurcation and Chaos in a Periodically Stimulated Cardiac Oscillator", Physica 7D (1983) pp 89–101. Bound as Order in Chaos, Proceedings of the International Conference on Order and Chaos held at the Center for Nonlinear Studies, Los Alamos, New Mexico 87545,USA 24–28 May 1982, Eds. David Campbell, Harvey Rose; NorthHolland Amsterdam ISBN 0444867279. (Performs a detailed analysis of heart cardiac rhythms in the context of the circle map.)
 Mark McGuinness and Young Hong, Arnold tongues in human cardiorespiratory system, Chaos, March 2004, 14, 1.
External links
 Circle map with interactive Java applet
Categories: Chaotic maps
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