A circle unrolled into a sine wave
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
Products of widening sinc waves, all with area π/2 but the last
Why the pattern breaks: the steps finally reach past the window
The Borwein integrals, exactly
The reach of the steps passes 1 at seven and 2 at fifty-six
Which patterns of waves keep the area at π/2
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 product to 1 vanishes at 0 ×17
- the product to 1 vanishes at -1 ×14
- the 1:1 figure crosses itself 2pq − p − q times ×11
- at phase 1.178 the 1:2 figure has the generic crossing count ×7
- the chords of the regular 3-gon from one corner multiply to 3 ×6
- both vanish at 0 ×4
- both vanish at -3 ×3
- the doubled arc from 0.25 of a quarter ends where Fagnano's formula says ×3
- the gap after 10 terms lies between 1/(N + 1) and 1/N ×3
- 1 + 1/2² + 1/3² + … is π²/6 ×1
- 1 + 1/2⁴ + 1/3⁴ + … is π⁴/90 ×1
- 1 + 1/2⁶ + 1/3⁶ + … is π⁶/945 ×1
- 5 and 17 pass and 7 fails ×1
- a coprime pair up to five ×1
- a generic Lissajous figure crosses itself 2pq − p − q times ×1
- a million terms and the tail 1/N give π²/6 ×1
- a step up to 1 and two to six sectors ×1
- and they stay on the hyperbola ×1
- between three and seven offsets ×1
- cosh and sinh are the even and odd halves of e^t ×1
- every slice between neighbouring points has the same area ×1
- every station lies on the figure eight ×1
- more factors bring the product closer at x = ½ ×1
- sl blows up beside each marked pole ×1
- sl repeats along (1 + i)ϖ, imaginary part ×1
- sl repeats along (1 + i)ϖ, real part ×1
- sl repeats along 2ϖ ×1
- sl vanishes at each marked zero ×1
- the catenary's parameter is between 0.2 and 2 ×1
- the circle's parameter is between 0 and 2π ×1
- the circle's quarter arc is π/2 ×1
- the circular sector's area is half its parameter ×1
- the correct digits roughly double each step ×1
- the drawn arrangement is a zigzag ×1
- the drawn list has the zigzag number of members ×1
- the figure eight, walked as chords, is 2ϖ long ×1
- the figure eight's quarter arc is π over twice the arithmetic–geometric mean of 1 and √2 ×1
- the first station is one equal share of arc from the centre ×1
- the folding phases really fold ×1
- the four families fail at 4, 8, never, never ×1
- the gap after N factors is about π/(8N) ×1
- the grid runs to between 2 and 5 ×1
- the hyperbola's parameter is between 0 and 2.2 ×1
- the hyperbolic sector's area is half its parameter ×1
- the length is between 3 and 6 ×1
- the offset is generic — the curve does not fold back on itself ×1
- the parabola and the catenary really differ ×1
- the point is on the circle ×1
- the point is on the hyperbola ×1
- the point's height is the wave's height there ×1
- the product's x² coefficient is the Taylor series' ×1
- the product's x⁴ coefficient is the Taylor series' ×1
- the product's x⁶ coefficient is the Taylor series' ×1
- the range is between 1 and 3.2 ×1
- the ratio is between one and three ×1
- the relative error is within the next pole's 3^-(n+1) ×1
- the row shows both a folded and an open figure ×1
- the second is not odd ×1
- the steps pass 1 at the seventh and 2 at the fifty-sixth ×1
- the time spent in each cell is close to the product of two arcsine densities ×1
- the two coordinates are a point of the unit circle ×1
- the view is one of sector, squeeze, waves, catenary, lissajous, lissgrid, lissphase, lissdense, lemwalk, lemlength, lemdouble, lemagm, lemcomplex, lemdivide, prodpartial, prodsame, polychords, basel, wallis, prodcoef ×1
- twenty thousand factors give sin(π/2) to four places ×1
- two whole frequencies up to six with no common factor ×1
- Wallis's partial products climb towards π/2 from below ×1
- x + y is e to the parameter ×1
- π divided by the mean is ϖ ×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 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.
AnalysisSeven 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.
AnalysisThe 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.
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.
AnalysisThe 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.
AnalysisThe 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.
AnalysisThe 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.
AnalysisWhen 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.