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.
At its defaults
show: "contract"
show: "stability"
show: "composed"
show: "rate"
show: "density"
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.
- bin 1 holds the share the density says it should ×60
- every point over 1 comes back after 1 doublings ×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
- both maps have the same number of points of period dividing 1 ×5
- 0.6154 is fixed at r = 2.6 ×4
- the number of steps is a whole number between 2 and 400 ×4
- the numerator of the starting point is a whole number between 1 and 10000000 ×3
- at r = 2.4 the error shrinks by the slope at the fixed point each step ×2
- the number of steps drawn is a whole number between 3 and 20 ×2
- the orbit at r = 2.4 converges far enough to measure a rate ×2
- the parameter is between 0 and 2 ×2
- √2 is fixed by the Newton step ×1
- a new crossing has a period above one that divides 2 ×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 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
- between 6 and 60 steps are taken ×1
- between one and three logistic parameters, each with an attracting fixed point ×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
- 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 start reaches the same point ×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
- its denominator is a whole number between 3 and 10000000 ×1
- one point drawn per step, and the start ×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 change of coordinate fixes the left end ×1
- the change of coordinate is increasing, so it is reversible ×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 denominator is odd, or the exact orbit reaches zero as surely as the float one does ×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 longest cycle counted is a whole number between 2 and 8 ×1
- the longest word counted is a whole number between 2 and 8 ×1
- the longest word counted is between two and eight ×1
- the map is one the family knows ×1
- the map is one this figure knows ×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 bins is a whole number between 8 and 80 ×1
- the number of drawn steps is a whole number between 2 and 400 ×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 stays inside the unit interval ×1
- the point a repeating word names has that word as its itinerary ×1
- the point found by bisection is fixed ×1
- the quadratic curve is on or off ×1
- the slope at the fixed point is 2 − r ×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 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 view is one the family draws ×1
- the words of each length and the points of that period are equally many ×1
- two logistic parameters above one ×1
- two points are at most k times as far apart after the map as before ×1
Where it is called
Changing this generator changes every figure on this list. That is what makes the list worth publishing rather than keeping in a check script.
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 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.
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 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.