# Feigenbaum constants

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Feigenbaum constant δ expresses the limit of the ratio of distances between consecutive bifurcation diagram on Li / Li + 1

In mathematics, specifically bifurcation theory, the Feigenbaum constants are two mathematical constants which both express ratios in a bifurcation diagram for a non-linear map. They are named after the physicist Mitchell J. Feigenbaum.

## History

Feigenbaum originally related the first constant to the period-doubling bifurcations in the logistic map, but also showed it to hold for all one-dimensional maps with a single quadratic maximum. As a consequence of this generality, every chaotic system that corresponds to this description will bifurcate at the same rate. It was discovered in 1975.[1][2]

## The first constant

The first Feigenbaum constant is the limiting ratio of each bifurcation interval to the next between every period doubling, of a one-parameter map

${\displaystyle x_{i+1}=f(x_{i}),}$

where f(x) is a function parameterized by the bifurcation parameter a.

It is given by the limit[3]

${\displaystyle \delta =\lim _{n\to \infty }{\frac {a_{n-1}-a_{n-2}}{a_{n}-a_{n-1}}}=4.669\,201\,609\,\ldots ,}$

where an are discrete values of a at the n-th period doubling.

### Names

• Feigenbaum bifurcation velocity
• delta

### Value

• 30 decimal places : δ = 4.669201609102990671853203821578
• (sequence A006890 in the OEIS)
• A simple rational approximation is 4 * 307 / 263

### Illustration

#### Non-linear maps

To see how this number arises, consider the real one-parameter map

${\displaystyle f(x)=a-x^{2}.}$

Here a is the bifurcation parameter, x is the variable. The values of a for which the period doubles (e.g. the largest value for a with no period-2 orbit, or the largest a with no period-4 orbit), are a1, a2 etc. These are tabulated below:[4]

n Period Bifurcation parameter (an) Ratio an−1an−2/anan−1
1 2 0.75
2 4 1.25
3 8 1.3680989 4.2337
4 16 1.3940462 4.5515
5 32 1.3996312 4.6458
6 64 1.4008286 4.6639
7 128 1.4010853 4.6682
8 256 1.4011402 4.6689

The ratio in the last column converges to the first Feigenbaum constant. The same number arises for the logistic map

${\displaystyle f(x)=ax(1-x)}$

with real parameter a and variable x. Tabulating the bifurcation values again:[5]

n Period Bifurcation parameter (an) Ratio an−1an−2/anan−1
1 2 3
2 4 3.4494897
3 8 3.5440903 4.7514
4 16 3.5644073 4.6562
5 32 3.5687594 4.6683
6 64 3.5696916 4.6686
7 128 3.5698913 4.6692
8 256 3.5699340 4.6694

#### Fractals

Self-similarity in the Mandelbrot set shown by zooming in on a round feature while panning in the negative-x direction. The display center pans from (−1, 0) to (−1.31, 0) while the view magnifies from 0.5 × 0.5 to 0.12 × 0.12 to approximate the Feigenbaum ratio.

In the case of the Mandelbrot set for complex quadratic polynomial

${\displaystyle f(z)=z^{2}+c}$

the Feigenbaum constant is the ratio between the diameters of successive circles on the real axis in the complex plane (see animation on the right).

n Period = 2n Bifurcation parameter (cn) Ratio ${\displaystyle ={\dfrac {c_{n-1}-c_{n-2}}{c_{n}-c_{n-1}}}}$
1 2 −0.75
2 4 −1.25
3 8 −1.3680989 4.2337
4 16 −1.3940462 4.5515
5 32 −1.3996312 4.6458
6 64 −1.4008287 4.6639
7 128 −1.4010853 4.6682
8 256 −1.4011402 4.6689
9 512 −1.401151982029
10 1024 −1.401154502237
−1.4011551890

Bifurcation parameter is a root point of period-2n component. This series converges to the Feigenbaum point c = −1.401155...... The ratio in the last column converges to the first Feigenbaum constant.

Other maps also reproduce this ratio, in this sense the Feigenbaum constant in bifurcation theory is analogous to π in geometry and e in calculus.

## The second constant

The second Feigenbaum constant (sequence A006891 in the OEIS),

α = 2.502907875095892822283902873218,

is the ratio between the width of a tine and the width of one of its two subtines (except the tine closest to the fold). A negative sign is applied to α when the ratio between the lower subtine and the width of the tine is measured.[6]

These numbers apply to a large class of dynamical systems (for example, dripping faucets to population growth).[6]

A simple rational approximation is (13/11) * (17/11) * (37/27).

## Properties

Both numbers are believed to be transcendental, although they have not been proven to be so.[7] There also exists no known proof that either constant is irrational.

The first proof of the universality of the Feigenbaum constants carried out by Oscar Lanford in 1982[8] (with a small correction by Jean-Pierre Eckmann and Peter Wittwer of the University of Geneva in 1987[9]) was computer-assisted. Over the years, non-numerical methods were discovered for different parts of the proof, aiding Mikhail Lyubich in producing the first complete non-numerical proof.[10]

## Notes

1. ^ Feigenbaum, M. J. (1976) "Universality in complex discrete dynamics", Los Alamos Theoretical Division Annual Report 1975-1976
2. ^ Chaos: An Introduction to Dynamical Systems, K.T. Alligood, T.D. Sauer, J.A. Yorke, Springer, 1996, ISBN 978-0-38794-677-1
3. ^ Non-Linear Ordinary Differential Equations: Introduction for Scientists and Engineers (4th Edition), D. W. Jordan, P. Smith, Oxford University Press, 2007, ISBN 978-0-19-920825-8.
4. ^ Alligood, p. 503.
5. ^ Alligood, p. 504.
6. ^ a b Nonlinear Dynamics and Chaos, Steven H. Strogatz, Studies in Nonlinearity ,Perseus Books Publishing, 1994, ISBN 978-0-7382-0453-6
7. ^ Briggs, Keith (1997). Feigenbaum scaling in discrete dynamical systems (PDF) (PhD thesis). University of Melbourne.
8. ^ Lanford III, Oscar (1982). "A computer-assisted proof of the Feigenbaum conjectures". Bull. Amer. Math. Soc. 6 (3): 427–434. doi:10.1090/S0273-0979-1982-15008-X.
9. ^ Eckmann, J. P.; Wittwer, P. (1987). "A complete proof of the Feigenbaum conjectures". Journal of Statistical Physics. 46 (3–4): 455. Bibcode:1987JSP....46..455E. doi:10.1007/BF01013368.
10. ^ Lyubich, Mikhail (1999). "Feigenbaum-Coullet-Tresser universality and Milnor's Hairiness Conjecture". Annals of Mathematics. 149 (2): 319–420. arXiv:math/9903201. Bibcode:1999math......3201L. doi:10.2307/120968. JSTOR 120968.