8 rectangles under a curve
riemann 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
Four stages of the four-corner Cantor dust
The shadows of the Cantor dust in two directions, stage by stage
Why the shadow in the direction (1, 2) never shrinks
How the dust's shadow depends on the direction
Favard length of the Cantor dust, stage by stage
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.
- the integral gives 0! exactly ×11
- the area under x^0e^(−x) is 0! ×8
- at 1 steps the length is still 2 ×7
- the shadow in the direction (1, 2) keeps its full length at stage 0 ×7
- the area under 1/x^0.9 from 1 to 10 matches its closed form ×6
- the integral of sin to the power 2 is (2 − 1)/2 times the one for power 0 ×5
- the area under 1/x^1.25 beyond 1 is 1/(p − 1) ×4
- the horn to 10 holds π(1 − 1/T) ×4
- and it equals the far-end area of the reflected exponent 1.75 ×3
- on block 1 the integrand gains exactly 1 ×3
- the area under 1/√x from 1 to 10 matches its closed form ×3
- the area under 1/x from 1 to 10 matches its closed form ×3
- the area under 1/x² from 1 to 10 matches its closed form ×3
- the staircase of 2 steps is exactly 2 long ×3
- between 256 and 8192 samples ×2
- the area under 1/x^0.25 between 0 and 1 is 1/(1 − p) ×2
- √π lies inside every pair ×1
- ∫ dx/(x ln x) from e^e to 10^50 is ln ln x between the ends ×1
- ∫ from 1 to ∞ of x^(−3/2) is 2 ×1
- ∮ x dy and the half-sum ½∮(x dy − y dx) give the same area ×1
- ½∮(x dy − y dx) is π(b² + a²/2) for the limaçon ×1
- a circle's perimeter is 2π and a flattened ellipse's tends to 4a ×1
- a fine enough polygon reaches the curve's length ×1
- a known increasing curve over a real stretch ×1
- adding corners never shortens the inscribed polygon ×1
- after 20,000 lines the estimate is within its own error band of the true length ×1
- and (−½)! = √π ×1
- and are within 1/N of it after N factors ×1
- and by this stage it is already small ×1
- and every one of them falls short of the curve's own length ×1
- and is nearly nothing by the end ×1
- and its length is nowhere near the curve's ×1
- and so does the polynomial ×1
- and the fat one keeps a length in the limit ×1
- and the finest is strictly longer than the coarsest, so the family is climbing ×1
- at least one triangle counts negative, so cancellation is visible ×1
- between 20 and 20,000 lines ×1
- between 6 and 40 strips are drawn ×1
- between three and five corner counts, each up to 64 ×1
- between two and six divisions ×1
- both parts only ever increase ×1
- doubling the corners divides the gap by four ×1
- each area is n times the one before ×1
- each curve has the same length ×1
- each curve's mean crossing count is 2L/(πd), whatever its shape ×1
- each divergent integrand ends above the convergent ones at its own level ×1
- each inscribed polygon is shorter than the ellipse, and doubling the corners shortens the gap ×1
- each integral is still growing at the far end ×1
- each panel is at least as close as the one before it ×1
- each stage is met by fewer lines than the one before ×1
- each staircase runs closer to the curve than the last ×1
- every horizontal line crosses the curve going up as often as coming down ×1
- every integrand in the chain still has a finite integral to infinity ×1
- every parametrisation traces the same point set ×1
- every wiggle eventually breaks log-convexity ×1
- for a flat ellipse, P − 4a is (2b²/a)(ln(4a/b) − ½) to within a hundredth of the bracket ×1
- for an ellipse the error falls like 1/n² ×1
- for nearly circular ellipses the error of each formula falls like a power of h: the first power for Kepler, the average and Euler, the third and fifth for Ramanujan's ×1
- for the wavy curve too the error falls like 1/n², with a larger constant ×1
- halving the wiggle doubles how long it survives ×1
- in a general direction the shadow shrinks ×1
- lines is read only by the crofton view ×1
- log of the wiggled curve bends the wrong way over stretches of every unit interval ×1
- log Γ bends one way everywhere ×1
- middle quarter, halving: no cell lies wholly inside the set, so the lower sum is nought ×1
- middle quarter, halving: refining the partition never raises the upper sum ×1
- middle quarter, halving: the upper sum never falls below the set's own length ×1
- middle thirds: no cell lies wholly inside the set, so the lower sum is nought ×1
- middle thirds: refining the partition never raises the upper sum ×1
- middle thirds: the upper sum never falls below the set's own length ×1
- more bars means less error ×1
- no width, no area ×1
- out and back covers the image a whole number of times ×1
- out, back, out again covers the image a whole number of times ×1
- refining a partition never lowers the variation ×1
- seed is read only by the crofton, favbuffon view ×1
- sin p is the fraction's error p − qπ, to first order ×1
- so in the limit the middle-thirds set has no length at all ×1
- so the first upper sum is on its way down to nothing ×1
- stage is read only by the cantorlen view ×1
- straight through, accelerating covers the image a whole number of times ×1
- straight through, once covers the image a whole number of times ×1
- the accumulation only ever climbs, because f is positive ×1
- the area between the two shrinks every time the steps are halved ×1
- the area left of the curve up to b ×1
- the area read from the boundary matches the cells counted inside ×1
- the area under 1/√x between 0 and 1 is 1/(1 − p) ×1
- the area under the curve up to a ×1
- the arithmetic–geometric mean and the trapezoid rule agree to twelve places ×1
- the arithmetic–geometric mean settles to the last digit within seven steps ×1
- the boundary integral equals the area weighted by winding number ×1
- the Cantor, noodle and Koch views draw their own curves ×1
- the counts increase ×1
- the curve is one of quarter, parabola, wiggle ×1
- the curve lies inside the disc the lines are thrown at ×1
- the directions (2, 1) and (1, 2) keep the full shadow ×1
- the drawing keeps a few points per column rather than every term ×1
- the drawn partition has between 24 and 240 cells ×1
- the error shrinks as points are added ×1
- the fat construction's first middle is between a third and an eighth ×1
- the finest one is within a twentieth of the curve everywhere ×1
- the finest partition is a power of two between 64 and 16384 ×1
- the four corners project to 0, 3, 6 and 9 quarters along x + 2y ×1
- the function is one of root, square, smooth ×1
- the function is one the figure knows ×1
- the function moves, so the decomposition is not trivial ×1
- the functions are among root, square, smooth ×1
- the gap shrinks like √π/(2n) ×1
- the inner loop is wound round twice ×1
- the inscribed polygon's length, measured, matches the closed form ×1
- the interval runs from nought to at most 2 ×1
- the limaçon has an inner loop: 0 < b < a ×1
- the maps differ in length or in speed, so the panels are comparing something ×1
- the maps run at different speeds ×1
- the middle-thirds approximation's length is two-thirds to the power of the stage ×1
- the panels really are in increasing order of rectangles ×1
- the parametrisations are among once, back, thrice, slow ×1
- the partial products rise towards π/2 from below ×1
- the polynomial's ratio does not ×1
- the printed total is the area of the bars drawn ×1
- the quoted exact area is the area under this curve ×1
- the series needs a handful of terms for a mild ellipse and hundreds for a flat one ×1
- the set is approximated at between 5 and 14 stages ×1
- the share of random lines that meet the stage agrees with the shadows' average ×1
- the share of the straight line, w, is between 0 and 1 ×1
- the shoelace sum equals the area counted inside ×1
- the signed area is within 1/T of π/2 ×1
- the signed triangles add up to the shoelace sum ×1
- the slope of the accumulated area is the height of the curve ×1
- the slowest divergent drawn is still above the fastest-converging drawn at the far end ×1
- the speed integral and the polygon agree for out and back ×1
- the speed integral and the polygon agree for out, back, out again ×1
- the speed integral and the polygon agree for straight through, accelerating ×1
- the speed integral and the polygon agree for straight through, once ×1
- the squares' shadows along x + 2y tile the interval end to end ×1
- the stage drawn is 1 to 7 ×1
- the stage-10 polygon is within 0.02 of the limit ×1
- the stage-k Koch curve has length 1.8 × (4/3)^k ×1
- the staircase is built against quarter or diagonal ×1
- the staircase of 1 step is exactly 2 long ×1
- the step counts are between 2 and 5 whole numbers up to 512 ×1
- the sum to ten million is 30.3145 ×1
- the sums run to between ten thousand and twenty million terms ×1
- the sweep runs over between three and eight partition sizes ×1
- the sweep runs up to between 4 and 16384 steps, or an interval end for the variation view ×1
- the term at n = 355 is about 24.6 ×1
- the third block ends at a number of a few hundred digits ×1
- the too-well-approximated number's sum races away ×1
- the tracer arm's length times the wheel's roll is the area enclosed ×1
- the tracer is within reach of the arms ×1
- the two areas together are at least ab ×1
- the two regions fill the big rectangle less the small one ×1
- the unsigned area exceeds (2/π) ln T ×1
- the variation of t sin(1/t) climbs by as much at the finest refinement as at the coarsest ×1
- the variation of t² climbs less and less, so it settles ×1
- the variation of t² sin(1/t) climbs less and less, so it settles ×1
- the view is one the family draws ×1
- the volume is below π and the surface above 2π ln T ×1
- the wiggle even keeps the recurrence ×1
- the wiggle keeps every factorial ×1
- the wiggle's ratio to Γ repeats with period one ×1
- their difference is the function ×1
- two to four exponents between 0 and 3 ×1
- w is read only by the cantorlen view ×1
- while the second has already settled on the set's own length ×1
- with equality when b = f(a) ×1
- x runs up to 10 to a power from 20 to 300 ×1
- Γ lies between the two products ×1
- Σ 1/(n (ln n)²) has almost stopped growing ×1
- Σ 1/(n ln n) − ln ln N has settled to a constant ×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 dust that almost every line misses
Keep the four corner squares of a square, then the four corners of each of those, and so on. What is left has a length, in the sense that measures length, and yet almost every straight line misses it entirely. Stage by stage the chance that a random line hits the dust falls — to 48% by the eighth stage — and how fast it falls to nothing is one of the few questions about a simple picture that is still open.
AnalysisA length counted by the lines that cross it
Throw straight lines at random across a curve and count how often they cross it. The average count, times π times the radius of the target, is the curve's length — for a wiggly closed curve, a spiral or a snowflake alike, with no following of the curve and no derivative anywhere. It is Crofton's formula of 1868, and it measures length the way a map-reader's ruled transparency does.
AnalysisA rectangle cut by a curve
Integration by parts is taught as the product rule run backwards. It is also a picture: an increasing curve cuts a rectangle into two pieces, one of them the area under the curve and the other the area beside it, and the formula says only that the pieces fill the rectangle. Run repeatedly, the same cut produces the factorials and Wallis's product for π.
AnalysisA rectangle grown on two sides
A product of two changing quantities is the area of a rectangle whose sides both move. The extra area is two strips and a corner, and the whole of the product rule is the observation that the corner is negligible and the strips are not.
AnalysisA series that waits on π
Add 1/(n³ sin² n) for n = 1, 2, 3, … and the terms are mostly tiny, except where n is almost a multiple of π and sin n is almost nought. Ten million terms add to 30.3145, four-fifths of it from the single term at n = 355. Whether the sum is finite depends on how closely fractions can approach π — on a number called its irrationality measure — and the best proof available says only that the measure is below 7.1, where the series needs it below 2.5.
AnalysisAdding up rectangles until they stop being rectangles
The integral is defined as a limit of sums of rectangles. The definition is exact, the picture is honest about what it costs, and the gap between them is the whole subject.
AnalysisAn area measured by walking round it
A surveyor's instrument from 1854 measures the area of any shape on a map by having its pointer steered once round the boundary; a small wheel rolls and slides, and its reading, times the length of one arm, is the area. Nothing touches the inside. The reason is that area can be written as an integral over the boundary — Green's theorem — and the instrument is that integral built in brass. Walk a curve that crosses itself and the same integral counts some regions twice.
AnalysisAn endless region with a finite area
A region that runs off to infinity can still have a finite area, and for the curves 1/xᵖ the exponent that makes the far end finite is exactly the one that makes the end at zero infinite. 1/x fails at both, no power succeeds at both, and a horn can hold less than π while needing infinite paint.
AnalysisAn error with an unknown in it
Taylor's theorem does not say a partial sum is close to anything. It says the error is one more derivative evaluated somewhere nobody can name, and everything the theorem is worth comes from what happens when that somewhere is replaced by the worst case.
NumberAn integral that cannot be a whole number
Niven's proof that π is not a fraction is the same squeeze as the one for e, with a much harder multiplier. A polynomial supplies the whole number; its own smallness supplies the contradiction; and both halves are computable.
AnalysisArea is the undoing of slope
Two operations invented for unrelated reasons — measuring a region and measuring a rate — turn out to be inverse. The picture is two panels sharing one axis, and the claim is that the lower curve's steepness is the upper curve's height.
AnalysisCovering a set from outside
To say how long a set is, cover it with intervals and add their lengths, then take the smallest total any covering achieves. That definition is short, obviously right for an interval, and gives the rationals a length of nothing.
AnalysisNo integrand sits on the border
The area under 1/x out to infinity is infinite and the area under 1/x² is finite, so it is natural to look for the dividing line. There is none. Divide any divergent integrand by its own running integral and it still diverges, more slowly; divide any convergent one by the square root of its tail and it still converges, more slowly. Paul du Bois-Reymond proved in 1873 that no single integrand can separate the two families — and the slow ones are slow beyond imagining.
AnalysisNo interval in it, and length to spare
The middle-thirds set has no length because the removed pieces add to one. Remove shrinking middles instead and they add to a half — leaving a set that still contains no interval anywhere, and still has half the length it started with.
AnalysisOnly one curve through the factorials bends the right way
Infinitely many smooth curves pass through 1, 1, 2, 6, 24, 120, and some of them even obey the factorial's own rule, f(x + 1) = (x + 1)·f(x), at every x. Ask that the logarithm of the curve bend upwards everywhere and exactly one survives — Euler's integral. A wiggle of any size breaks the condition somewhere, the smaller the wiggle the further out, and the proof that nothing else survives is a squeeze that computes √π along the way.
AnalysisThe area that names the number
The number e can be defined without mentioning slopes at all. Slide right along the curve 1/x until the area underneath reaches exactly one, and stop. That is where e is, and the reason logarithms turn multiplication into addition is visible in the same picture.
AnalysisThe length belongs to the journey
Three maps from an interval with exactly the same image, and three different lengths. The picture of a curve is the set of points it passes through, and that set does not determine how far anything travelled along it.
AnalysisThe length round an ellipse
An ellipse has an area anyone can write down, πab, and a perimeter nobody can: it is an integral with no expression in the functions of school mathematics. Kepler guessed it, Euler guessed better, and in 1914 Ramanujan wrote down a formula whose error falls like the fifth power of how far the ellipse is from a circle — off by four hundredths of a per cent even for a segment. Gauss had already found the way to compute it exactly, doubling the correct digits at every step.
AnalysisThe length the derivative never sees
The Cantor function climbs from 0 to 1 with a slope of zero almost everywhere, so the formula ∫√(1 + f′²) dx says its graph has length 1 — the length of a flat line. The inscribed polygons say 2. Mix it half and half with the diagonal and the length becomes exactly the golden ratio, while the formula still reports only the part the slope can see.
AnalysisThe slope of a single point
A slope needs two points. A derivative is the slope at one. The construction that bridges the gap is a sequence of secants, and the whole difficulty of calculus is in what "the limit of that sequence" is allowed to mean.
AnalysisThe staircase that is not the diagonal
A staircase can be made to follow a quarter circle as closely as anyone likes. Its length is 2 at every stage and the arc's length is 1.5708, and no amount of refinement closes the gap — which is a fact about length rather than about staircases.
AnalysisWhich curves have a length at all
A length is defined as a supremum over inscribed polygons, which behaves because every refinement is longer than the last. It is also sometimes infinite — and the condition separating the two cases is a sum of absolute differences that either settles or does not.
AnalysisWhich functions can be added up
Riemann's integral works when the upper and lower sums close on each other. The exact condition for that, found once measure existed to state it in, is that the points where the function jumps have measure zero — which some nowhere dense sets fail.