Complex squaring map: Difference between revisions

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In [[mathematics]], the '''complex squaring map''', a [[polynomial]] mapping of [[Quadraticdegree functionof a polynomial|degree]] [[quadratic function|two]], is a simple and accessible demonstration of [[chaos theory|chaos]] in [[dynamical systemssystem]]s. It can be constructed by performing the following steps:

# Choose any [[complex number]] on the [[unit circle]] whose [[Arg (mathematics)Complex_number#Polar_complex_plane|argument]] (complex angle) is not a [[rational fractionnumber|rational]] multiple of π,

# Repeatedly square that number.

This repetition (iteration) produces a [[sequence]] of complex numbers that can be described alone by their complex anglearguments. Any choice of starting angle that satisfies (1) above will produce an extremely complicated sequence of angles, that belies the simplicity of the steps. It can be shown that the sequence will be [[Chaos theory|chaotic]], i.e. it is sensitive to the detailed choice of starting angle.

== Chaos and the complex squaring map ==

The informal reason why the iteration is chaotic is that the angle doubles on every iteration and doubling grows very quickly as the angle becomes ever larger, but angles which differ by multiples of 2π ([[radian]]s) are identical. Thus, when the angle exceeds 2π, it must ''wrap'' to the remainder on division by 2π. Therefore, the angle is transformed according to the [[dyadic transformation]] (also known as the 2x2''x'' mod 1 map). As the initial value ''z''₀ has been chosen so that its argument is not a rational multiple of π, the [[Orbit (dynamics)|forward orbit]] of ''z''_''n'' cannot repeat itself and become periodic.

More formally, the iteration can be written as:

:<math> \qquad z_{n+1} = z_n^2 </math>

where <math>z_n</math> is the resulting sequence of complex numbers obtained by iterating the steps above, and <math>z_0</math> represents the initial starting number. We can solve this iteration exactly:

:<math> \qquad z_n = z_0^{2^n} </math>

Starting with angle ''θ'', we can write the initial term as <math>z_0 = \exp(i\theta)</math> so that <math>z_n = \exp(i2^n\theta)</math>. This makes the successive doubling of the angle clear. (This is equivalent to the relation <math>z_n = \cos(2^n\theta)+i \sin(2^n\theta)</math> by [[Euler's formula]].)

== Generalisations ==

This map is a special case of the [[complex quadratic map]], which has exact solutions for many special cases.<ref>M. Little, D. Heesch (2004), [http://www.maxlittle.net/publications/GDEA41040.pdf Chaotic root-finding for a small class of polynomials], ''Journal of Difference Equations and Applications'', '''10'''(11):949&ndash;953.</ref> The complex map obtained by raising the previous number to any [[natural number]] power <math>z_{n+1} = z_n^p </math> is also exactly solvable as <math>z_n = z_0^{p^n}</math>. In the case ''p''&nbsp;=&nbsp;2, the dynamics can be mapped to the dyadic transformation, as described above, but for ''p''&nbsp;>&nbsp;2, we obtain a shift map in the [[number base]]&nbsp;''p''. For example, ''p''&nbsp;=&nbsp;10 is a decimal shift map.

== See also ==