the logistic map at 3.2, iterated from 0.2
cobweb is one function. Everything below came out of it during this
build, at parameters taken from the essays rather than invented for this page — so a figure
here is the same figure a reader meets in an essay, and if the generator changes, this page
changes with it.
With nothing chosen
x ↦ cos x: two starts, one destination
A fixed point that attracts, and one that does not
The logistic map at 2.8, and the same map applied twice
How fast an iteration arrives
A map drawn through the cycle 0 → 1/3 → 1, and the graph its pieces make
What it checks while it draws
Collected by running the family and recording what it asserted, not written here. The count is how many separate times the claim was put to the test while these drawings were made.
- T2: bin 1 matches the arcsine share ×90
- bin 1 holds the share the density says it should ×60
- at r = 4 the 0-th iterate has 2^0 folds ×21
- T2 composed 1 times has 2 fixed points ×16
- T2 composed 1 times is T2 ×16
- step 0 is cos(3^0 θ) ×13
- returns after 1 steps against the trace ×10
- the orbits of exact period 1, walked and by inversion ×10
- the points of exact period 1 make whole orbits ×10
- walks of length 1 from the start state are the admissible strings ×10
- sampling finds every admissible word of length 1 (the golden ratio φ) ×9
- sampling finds every admissible word of length 1 (β = 1.8) ×9
- sampling finds every admissible word of length 1 (β = 5/2) ×9
- at level 1 the admissible intervals number F(3) = 2 ×8
- the number of steps is a whole number between 2 and 400 ×7
- every point over 1 comes back after 1 doublings ×6
- the trace is 1 + (1 + √2)ⁿ + (1 − √2)ⁿ at n = 1 ×6
- 0.00000 returns to itself after 2 steps ×5
- a full two-branch fold has 2^1 points fixed by its 1th power ×5
- after 0 doublings the overlap is within 1/2^2 of a sixth ×5
- both maps have the same number of points of period dividing 1 ×5
- 0.6154 is fixed at r = 2.6 ×4
- after 0 doublings the lowest frequency present is 1 ×4
- T2(cos θ) = cos 2θ ×4
- the random point visits quarter 1 a quarter of the time ×4
- the total energy of f composed 0 times is a quarter ×4
- in the window of three, at 3.8300, the estimate is log of the golden ratio ×3
- the folds of f^6 at r = 3.5, sampled and from the preimages of 1/2 ×3
- the longest word counted is a whole number between 3 and 10 ×3
- the numerator of the starting point is a whole number between 1 and 10000000 ×3
- the parameter is between 0 and 2 ×3
- at r = 2.4 the error shrinks by the slope at the fixed point each step ×2
- the longest iterate counted is a whole number between 6 and 22 ×2
- the number of bins is a whole number between 8 and 80 ×2
- the number of steps drawn is a whole number between 3 and 20 ×2
- the number of times the map is applied is a whole number between 2 and 5 ×2
- the orbit at r = 2.4 converges far enough to measure a rate ×2
- √2 is fixed by the Newton step ×1
- 0, 1/3 and 1 are a cycle of three ×1
- A covers A and B, B covers all three, C covers B and C ×1
- a new crossing has a period above one that divides 2 ×1
- a point with no two 1s together never enters the top quarter ×1
- after the computed orbit is dead the exact one is still moving ×1
- and carries the right end to the right end ×1
- and it never reaches zero, because an odd denominator cannot be halved away ×1
- and never grows ×1
- and not before ×1
- and shrinks every distance by a factor under one ×1
- and stays there, because zero is fixed ×1
- and the constant the ratio approaches is 1/(2√2) ×1
- and the repeat found really is a repeat ×1
- and then separate visibly, at a step the figure marks ×1
- at 1,000 digits more than 99% of strings are within 0.05 of a half ×1
- at r = 4 the folds double and the estimate is log 2 ×1
- below the end of the doublings the deeper estimate is the smaller ×1
- between 6 and 60 steps are taken ×1
- between one and six logistic parameters from 3 to 4 ×1
- between one and three logistic parameters, each with an attracting fixed point ×1
- deleting the first digit of an admissible word leaves it admissible ×1
- each density has total mass one ×1
- each letter of the itinerary is the next binary place of the starting point ×1
- each Newton error is the square of the last over twice the current guess ×1
- each point of the cycle is carried to another ×1
- each point of the cycle lies in its letter's piece ×1
- each step of a closed word's point lands in that word's piece ×1
- every bin is visited ×1
- every corner of the staircase lies on the curve or on the diagonal ×1
- every crossing of the map is also a crossing of the composed map ×1
- every exact iterate is a whole numerator over the same denominator ×1
- every orbit's share settles on the predicted frequency ×1
- every piece is stretched ×1
- every start reaches the same point ×1
- every word the map writes passes Parry's test ×1
- five Newton steps from 1.9 reach twelve decimal places ×1
- how many steps of the orbit are drawn is a whole number between 3 and 12 ×1
- in base the tribonacci constant, 1 = .111 ×1
- in base φ no two 1s are ever consecutive ×1
- in base φ the allowed words are counted by the Fibonacci numbers ×1
- in base φ the digit 1 appears with frequency 1/(1 + φ²) ×1
- in base φ, 1 = .11 ×1
- its denominator is a whole number between 3 and 10000000 ×1
- no point is named by two closed words ×1
- one point drawn per step, and the start ×1
- T₂∘T₃, T₃∘T₂ and T₆ agree everywhere ×1
- the 2 new crossings of period 2 are whole orbits of 2 points ×1
- the average of one long orbit is the average over the whole interval ×1
- the base has a finite expansion of 1, so its graph is finite ×1
- the base is one the family knows ×1
- the change of coordinate fixes the left end ×1
- the change of coordinate is increasing, so it is reversible ×1
- the column count is a whole number between 20 and 241 ×1
- the computed orbit reaches exactly zero inside the drawn window, having run out of binary places ×1
- the contraction is one the family knows ×1
- the coordinate change carries the tent map to the logistic map ×1
- the coordinate is one whose invariant density this figure knows ×1
- the counts grow by a factor of β per letter ×1
- the cycle closes after the word's length ×1
- the cycle is one the family draws ×1
- the deeper estimate never falls as r rises ×1
- the degree is a whole number between 2 and 5 ×1
- the denominator is odd, or the exact orbit reaches zero as surely as the float one does ×1
- the digits of 1 add back up to 1 ×1
- the digits written reconstruct the start to within β to the minus the number of steps ×1
- the error after n steps is inside k to the n times the error at the start ×1
- the exact orbit closes up inside the drawn window ×1
- the exact shares add to one ×1
- the fixed point attracts, so there is a rate to measure ×1
- the folds grow by the golden ratio at the last step ×1
- the frequency of nonzero digits is the density's mass on the right piece ×1
- the graph's growth rate is the base ×1
- the highest frequency drawn is a whole number between 16 and 96 ×1
- the left piece covers the right, and the right covers both ×1
- the longest cycle counted is a whole number between 2 and 8 ×1
- the longest word counted is between two and eight ×1
- the longest word is a whole number between 6 and 14 ×1
- the lower parameter is between 3 and 4 ×1
- the map is one the family knows ×1
- the map is one this figure knows ×1
- the map itself brings the point back ×1
- the map itself has a fixed point ×1
- the map sends the interval into itself ×1
- the number of binary places the starting point has is a whole number between 3 and 8 ×1
- the number of drawn steps is a whole number between 2 and 400 ×1
- the number of levels is a whole number between 4 and 10 ×1
- the number of stages is a whole number between 3 and 7 ×1
- the number of times the map is composed is a whole number between 2 and 4 ×1
- the orbit approaches the fixed point exactly when the slope is shallow ×1
- the orbit at r = 3.83 settles on a cycle of three ×1
- the orbit of 1/3 lives in the two middle quarters ×1
- the orbit of 1/3 never enters [0, ¼) ×1
- the orbit starts inside the interval, or at 1 ×1
- the orbit stays inside the unit interval ×1
- the orbit's histogram matches Parry's density away from the steps ×1
- the point a repeating word names has that word as its itinerary ×1
- the point found by bisection is fixed ×1
- the points returning after 4 steps are as many as the trace of the matrix power ×1
- the quadratic curve is on or off ×1
- the random point's running average reaches a quarter ×1
- the range runs upward ×1
- the share near a half grows with the length ×1
- the slope at the fixed point is 2 − r ×1
- the solved point comes back after exactly the word's steps ×1
- the starting point has between three and eight binary places ×1
- the starting point is between 0 and 1 ×1
- the starting point is inside the interval ×1
- the steps drawn is a whole number between 3 and 14 ×1
- the straight pieces of the composed map are the allowed words of 4 letters ×1
- the transported orbit is the orbit of the transported point ×1
- the transported orbit obeys the tent map's own recurrence ×1
- the two agree exactly for at least eight steps ×1
- the upper parameter is between 3 and 4 ×1
- the view is one the family draws ×1
- the word is not a shorter word repeated ×1
- the word is two to eight letters of L and R ×1
- the word, read round, never puts L after L ×1
- the words of each length and the points of that period are equally many ×1
- two depths, the deeper at most 20 ×1
- two logistic parameters above one ×1
- two points are at most k times as far apart after the map as before ×1
- x² and x² − 1 composed in the two orders differ ×1
Where it is called
Every figure on this list is drawn by the same rule, so a change to the rule changes all of them at once. That is why the list is published.
A constant that does not care which map
The gaps between successive period doublings shrink by a factor. Measure that factor for the logistic map and you get 4.669. Measure it for a completely different map and you get 4.669, and nobody expected that.
DynamicsA difference too small to draw
Two starting points a ten-thousandth apart, under the same rule, with nothing random anywhere. Within forty steps they have nothing in common — and the rule was not doing anything to them that it does not do to everything.
AnalysisA map that shrinks everything
One extra hypothesis — that every distance is shortened by at least a fixed factor — turns the existence of a fixed point into its uniqueness, an algorithm for finding it, and a bound on the error after any number of steps.
DynamicsA matrix that counts the returns
Draw a map straight through the cycle 0 → 1/3 → 1 and its two pieces carry each other in a fixed pattern: the left piece only across the right, the right across both. The orbits' words are then walks on a two-node graph, and the number of points that come back after n steps is the trace of that graph's matrix to the nth power — 1, 3, 4, 7, 11, 18 — each one checked by solving for the points exactly.
DynamicsA point that pulls, and a point that pushes
Every crossing of a curve with the diagonal is a value the rule leaves alone. Whether anything ever arrives there is decided by one number — the slope at the crossing — and the picture makes the reason obvious.
DynamicsA solvable chaos of every degree
The logistic map at four is chaotic and, through a change of coordinates, completely solvable: its orbits are cosines of doubling angles. The trick is not a one-off. For every whole number n there is a polynomial of degree n that multiplies angles by n instead of 2, and every one of them is exactly as solvable, has exactly nᵏ points of period k, and preserves the same distribution — and any two of them commute, which almost no two polynomials do.
DynamicsAlmost every orbit is fair
Double a number and keep the fractional part, and do it again, and again. Where the orbit goes is written in the number's binary digits, and for almost every starting point it spends exactly a quarter of its time in each quarter of the interval. The proof is a short argument about Fourier coefficients being pushed to infinity — and it leaves room for a set of exceptions with no length at all, which includes every fraction and, for all anyone can prove, every number anyone has ever named.
DynamicsCounting in a base that is not a whole number
Multiply by β and keep the fractional part, over and over, and the whole parts you throw away are the digits of the starting number in base β — even when β is the golden ratio. Which digit strings can ever appear is decided by one string alone: the way the number 1 is written in that base. In base φ it is .11, so 11 is the only thing forbidden; in base 1.8 it never ends, and no finite list of rules describes what is allowed.
DynamicsHow fast the staircase arrives
The slope at a crossing decides whether an orbit reaches it. The same number decides how fast — and when the slope is zero the arithmetic changes kind, from a fixed factor per step to a doubling of the correct digits.
DynamicsThe folds that measure chaos
Apply the logistic map six times and its graph goes up and down 38 times at r = 3.5 and 64 times at r = 4. How fast that number of folds multiplies with each further step is the map's topological entropy: zero through the whole cascade of period doublings, log of the golden ratio in the window of three, log 2 at the top — and it never decreases as r rises.
DynamicsThe histogram an orbit leaves
When no single step of an orbit is worth reporting, what is left is where it spends its time. That distribution is not uniform, it does not depend on where the orbit started, and it can be computed in closed form.
DynamicsThe orbit a computer draws
A chaotic orbit computed in floating point is not the orbit of the point it started from. Sometimes it is the true orbit of a nearby point, which is enough; sometimes the arithmetic simply runs out, and the picture is of the rounding.
DynamicsThe orbit written as a word
Cut the interval in two and record which half each step of an orbit lands in. The orbit becomes an infinite string of two letters, the map becomes the act of deleting the first letter, and questions about trajectories turn into questions about words.
DynamicsThe road paved with doublings
Turn one dial slowly and watch what a map settles into. It settles on a point, then on two points, then four, then eight — faster and faster, and the doublings run out at a parameter that is finite.
DynamicsThe same map in different coordinates
The tent map and the logistic map at four look nothing alike and are the same map, carried onto each other by a change of variable. Everything either one does the other does, and the change of variable is a sine squared.
DynamicsThe staircase that shows the whole orbit
Take a number, feed it to a rule, feed the answer back in. There is a way of drawing that on the rule's own graph which turns the entire future of a starting point into a shape — and the shape is legible.
DynamicsTwo lobes and no cycle
Three equations, three variables, and a trajectory that never crosses itself, never repeats, and never leaves a region of zero volume. The set it settles onto is not a point, not a loop, and not a surface.
DynamicsWhere a base-β orbit spends its time
Follow x ↦ βx mod 1 for a million steps and count how often the orbit visits each part of the interval. For the doubling map the answer is evenly; for the golden ratio it is a staircase with one step, spending 1.17 times the average near 0 and 0.72 times it near 1. The step sits exactly where the orbit of the number 1 lands, and in a base whose 1 never has a finite expansion the staircase has infinitely many steps.