Markus-Lyapunov Fractals
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In other words, the Lyapunov exponent is a method for examining the rate of change of a system considered over infinite iterations, then taking that rate of change and making it easily identifiable as a value that induces either chaos or stability. | In other words, the Lyapunov exponent is a method for examining the rate of change of a system considered over infinite iterations, then taking that rate of change and making it easily identifiable as a value that induces either chaos or stability. | ||
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====In the Logistic Formula==== | ====In the Logistic Formula==== |
Revision as of 17:01, 7 June 2011
- A representation of the regions of chaos and stability over the space of two population growth rates.
Markus-Lyapunov Fractal |
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Contents |
Basic Description
The Markus-Lyapunov fractal is much more than a pretty picture; it is a map. The curving bodies and sweeping arms of the image are a color-coded plot that shows us how a population changes as its rate of growth moves between two values. All the rich variations of color in the fractal come from the different levels of stability and chaos possible in such change.The logistic map is one of the simplest mathematical representations of population growth. Depending on the rate of fecundity used in the map, it will generate either a neutral system, a stable oscillating system, or a chaotic system. To help determine which of these outcomes would occur, the mathematician Aleksandr Lyapunov developed a method for comparing changes in growth and time in order to calculate what has been dubbed the Lyapunov exponent . This is a useful indicator, and here's why:
- If it is zero, the population change is neutral; at some point in time, it reaches a fixed point and remains there.
- If it is less than zero, the population will become stable . The lower the number, the faster and more thoroughly the population will stabilize.
- If it is positive, the population will become chaotic.
What does all this have to do with the fantastical shapes of the Markus-Lyapunov fractal? The scientist Mario Markus wanted a way to visualize the potential represented by the Lyapunov exponent as a population moved between two different rates of growth. So he created a graphical space with one rate of growth measured along the x-axis and the other along the y. Thus for any point, (x,y), there is one specific Lyapunov exponent that predicts how a population with those rates of change will behave. Markus then created a color scheme to represent different Lyapunov exponents – one color represents positive numbers, and another represents negative numbers and zero. This second color he placed on a gradient from light to dark, so that lower negative numbers are lighter and those closer to zero are darker. The bands of black that appear in many fractals therefore show where the Lyapunov exponent is exactly zero, and bands of white indicate superstable points. By this code, Markus could color every point on his graph space based on its Lyapunov exponent.
Consider the main image on this page. The blue "background" shows all the points where the combination of the rates of change on the x and y axes will result in chaotic population growth. The "floating" yellow shapes show where the population will move toward stability. The lighter the yellow, the more stable the population.
A More Mathematical Explanation
The Lyapunov Exponent
The discrete form of the Lyapunov exponent is
In other words, the Lyapunov exponent represents the limit of the mean of the exponential rates of change that occur in each transition, x_{n} x_{(n+1)}, as the number of transitions approaches infinity.
What does this have to do with stability? The key is the log_{2} component, which renders numbers under 1 negative and those over 1 positive. This is what yields the properties of Lyapunov exponents laid out in the "Basic Explanation" – those mean overall rates of change that make the system finite must be less than 1, giving us a negative Lyapunov exponent, while those rates of change that expand the system to the point of chaos must be greater than one, giving us a positive exponent. When the mean overall rate of change is zero, the logarithmic component no longer exists, showing exactly what happens in a superstable system; the rate of change ceases to exist.
In other words, the Lyapunov exponent is a method for examining the rate of change of a system considered over infinite iterations, then taking that rate of change and making it easily identifiable as a value that induces either chaos or stability.
In the Logistic Formula
Basic differentiation shows us that, for the logistic formula (3):
Using this and a sufficiently large N number of iterations, we can approximate the Lyapunov exponent for the logistic formula to be:
Here we can see much more clearly a property that we have been assuming -- the variable that has the greatest impact on the stability of the logistic equation is r, not x_{n}. This is clear here because, as we differentiate the logistic formula in order to examine its overall rate of change, x_{n} is reduced to the constant value x_{0} (the starting volume of the population). It is therefore no longer a variable when we look at the formula on the level of its Lyapunov exponent, and so changing the value of x_{n} does not change whether the logistic function yields chaos or stability.
Forcing the Rates of Change
Mathematically, the important part of Markus's contribution to understanding this type of system was not his method for generating fractals, but his use of periodic rate-of-change forcing. We have been discussing the great impact of the r value in determining the output of the logistic formula, but this value can have still greater impact if we do not choose to keep it constant. Anyone who has studied biology, as Markus has, knows that the rates of change of a populations size do not simply fluctuate with changing supplies of food and space, but also often alternate between two or more specific potential rates of change depending on such things as weather and mating seasons.In terms of the logistic formula, this means we choose a set of rates of change, r_{1}, r_{2}, r_{3},..., r_{p}, where p is the period over which the rates of change loop. When we force the rates of change to follow such a loop, we have a new, modular logistic equation (3):
It is in these forced alterations in rates of change that the fascinating shapes of the Markus-Lyapunov fractal come out. Each of the fractals is formed from some pattern of two rates of change, a and b. So a pattern aba would mean each point on the fractal is colored based on the Lyapunov exponent of the logistic formula 7, where r_{1} = a, r_{2} = b, and r_{3} = a. That is, the r values would cycle a,b,a,a,b,a,a,b,a....
Because the axes used to map these fractals are measurements of changes in a and b, the pattern a would simply yield a set of vertical bars, just as the pattern b would yield horizontal bars. However, once the patterns start to become mixed, more interesting results come out. The image to the right shows an ab pattern. Note that it is much simpler than other images shown on this page; the main image, for instance, is a bbbbbbaaaaaa pattern.
Why It's Interesting
Fractal Properties
The movements from light to dark and the dramatic curves of the boundaries between stability and chaos here create an astonishing 3D effect. But the image is striking not only for its beauty but also for its self-similarity. Self-similarity is that trait that makes fractals what they are – zooming in on the image reveals smaller and smaller parts that resemble the whole. Consider the image to the right, enlarged from a section of the main image above. Here we see several shapes that repeat in smaller and smaller iterations. Perhaps ironically, this type of pattern is a common property of chaos.
For more images of the fractal properties of chaotic systems, see the Henon Attractor, the Harter-Heighway Dragon Curve, and Julia Sets.
Artistic Extensions
After Markus saw the incredible beauty and intriguing three-dimensionality of the images generated by his plotting system, he immediately sent the images to a gallery in the hopes that it would display his images in an exhibition.^{[1]} It's easy to see why he did so, and in fact, pictures based on these fractals have become a large part of what is called "fractalist" art. As with all domains of fractalist art, there is a great deal of debate in the art community over whether these images are truly "art" given their intrinsic reliance on a purely scientific, algorithmically-generated chart. One could say that such a process is devoid of creativity, but it is equally valid to say that the identification and presentation of the beauty in the science is an art in itself – a concept that is critical in modern art. Either way, there has been an undeniable artistic fascination with Markus-Lyapunov fractals; if the image seems familiar, you have likely seen it on posters, t-shirts, or any other canvas for graphic design.
Teaching Materials
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References
- ↑ Dewdney, A. K. (1991). Leaping into Lyapunov Space. Scientific American, (130-132).
- Other Sources Consulted
- Elert, G. (2007). The Chaos Hypertextbook. http://hypertextbook.com/chaos/
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