Generator

A circle unrolled into a sine wave

A generator in the analysis library, called 43 times across 8 essays. Below: what it draws with nothing chosen and at each mode an essay asks for, what it checks while drawing, and everywhere it is used.

circle-to-sine 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

A circle unrolled into a sine wave. On the left a radius turns through an angle; on the right the height of its tip is plotted against the angle, tracing a sine curve.

Products of widening sinc waves, all with area π/2 but the last

Products of widening sinc waves, all with area π/2 but the last. Graphs of the products of sinc(x/(2k+1)) for k = 0 to n, n = 0, 1, 3, 7, on 0 ≤ x ≤ 24.

Why the pattern breaks: the steps finally reach past the window

Why the pattern breaks: the steps finally reach past the window. Densities of sums of 1 to 7 uniform steps of half-widths 1/3, 1/5, …, 1/15, with maximal reaches 0.333, 0.533, 0.676, 0.787, 0.878, 0.955, 1.022, against the window [−1, 1].

The Borwein integrals, exactly

The Borwein integrals, exactly. 1 factor: 1; 2 factors: 1; 3 factors: 1; 4 factors: 1; 5 factors: 1; 6 factors: 1; 7 factors: 1; 8 factors: 1 − 1.471e-11; 9 factors: 1 − 1.192e-8; 10 factors: 1 − 9.337e-8.

The reach of the steps passes 1 at seven and 2 at fifty-six

The reach of the steps passes 1 at seven and 2 at fifty-six. Partial sums of 1/(2k+1) for k up to 64; first exceeding 1 at k = 7 and 2 at k = 56; the cosine family's shortfall at 56 is about 10^-137.8.

Which patterns of waves keep the area at π/2

Which patterns of waves keep the area at π/2. sinc(x/k), k = 2, 3, 4, …: fails at 4 factors; sinc(x/(2k + 1)), k = 1, 2, …: fails at 8 factors; sinc(x/k²), k = 2, 3, …: never fails; sinc(x/2ᵏ), k = 1, 2, …: never fails.

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.

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.

Analysis

A sine wave is a circle seen from the side

Sine is introduced as a ratio in a right triangle, which is true and explains nothing about why its graph is a wave. There is a better picture.

Analysis

Seven times half of π, and then not

The area under sin x / x, out to infinity, is π/2. Multiply in sin(x/3)/(x/3) and the area is still π/2; multiply in the waves for 5, 7, 9, 11 and 13 and it is still π/2, exactly. Add the wave for 15 and the area falls short — by about 2.3 × 10⁻¹¹. Nothing about the curves changes at the eighth factor. What changes is that 1/3 + 1/5 + … + 1/15 has just passed 1.

Analysis

The angle that is really an area

On a unit circle the angle t is the length of arc the point has walked, and it is also twice the area of the slice it has swept. The two readings agree on the circle and part company on the hyperbola — where arc length leads nowhere and area leads straight to cosh, sinh, and the exponential.

Dynamics

The 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.

Analysis

The sine of a figure eight

Walk round a circle and read the height against the distance walked, and the reading is the sine. Do the same on Bernoulli's figure eight and the reading is a new function, with an arc integral that has t⁴ where the circle's has t², a length Gauss found inside an average, two periods instead of one — and exactly the same list of equal divisions that compass and straightedge can draw.

Analysis

The sine, rebuilt from its zeros

A polynomial is a product of factors, one for each root. Euler treated the sine the same way — one factor for each place the wave crosses the axis — and the product he wrote down is correct, though the zeros alone do not justify it. Multiplied out, it hands over the sum of the reciprocal squares, π²/6, and every even power after it.

Analysis

The tangent counts zigzags

Arrange the numbers 1 to n so that they go up, down, up, down. There are 1, 1, 2, 5, 16, 61, 272 such arrangements, and those are, multiplied by factorials, the Taylor coefficients of tan x and sec x — the odd lengths from the tangent, the even from the secant. Désiré André proved it in 1879 by splitting a zigzag at its largest number, and the split turns out to be the derivative of the tangent written out term by term.

Analysis

When two circular motions come home

Drive a point across with one sine wave and up and down with another. If the two frequencies are in a whole-number ratio the point retraces a closed figure whose crossings can be counted in advance — 2pq − p − q of them — and if they are not, it never comes back and fills the square, spending twenty times longer in the corners than in the middle.

The whole library · What the figures prove