Generator

A product of 3 polynomials, and what its coefficients count

A generator in the discrete library, called 28 times across 5 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.

genfun 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 product of 3 polynomials, and what its coefficients count. The coefficients of a product of small polynomials, with the combinations of choices that reach one marked total written out beneath it.

Multiplying two series is summing a diagonal

Multiplying two series is summing a diagonal. A multiplication table of the coefficients of two series, with one anti-diagonal marked: its sum is the coefficient of the corresponding power in the product.

1 / (1 − x − x²), and the objects its coefficients count

1 / (1 − x − x²), and the objects its coefficients count. Two rows of coefficients: those of a generating function, and a direct count of the objects it is meant to count. The rows agree at every power.

Where the descent polynomial of 10 vanishes

Where the descent polynomial of 10 vanishes. A logarithmic number line from −10^-3 to −10^3 with the 9 real negative roots of the Eulerian polynomial for n = 10 marked, paired by reciprocals.

Descents of 10 as a sum of 9 hidden coins

Descents of 10 as a sum of 9 hidden coins. Bars of 9 coin probabilities beside a bar chart of the descents distribution for n = 10, with dots giving the coin-sum distribution landing on every bar.

Descents settle onto the bell curve

Descents settle onto the bell curve. Three bar charts of the descent distribution for n = 4, 8, 16, each with a normal curve of matching mean and variance; the largest cumulative gaps are 0.019, 0.011, 0.005.

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.

Discrete

A polynomial that counts

Hang a counting sequence on the powers of a variable and the two ways of combining choices — this and that, this or that — become multiplication and addition, so a recursion turns into an equation and the equation can be solved.

Discrete

Coins hidden in the roots

The polynomial that counts permutations by their descents has no product formula, and nothing in the definition of a descent is a coin toss. But every root of the polynomial is real and negative, and a polynomial like that is a product of coins in disguise: each root r is a coin landing heads with chance 1/(1 − r). The descent count of a random permutation is exactly a sum of independent coins nobody can point to — which is why it is bell-shaped, and why its coefficients obey inequalities the inversion count breaks.

Number

Every partition, hidden in a product

Multiply out one factor for each part size and the coefficient of q to the n is the number of partitions of n. Nothing is being approximated: the product is a bookkeeping device that does the counting by multiplying.

Discrete

The coefficient that is a polynomial

Add a second variable to track a statistic and each coefficient stops being a number. Set the new variable to one and the old count comes back untouched; leave it in and the mean of the statistic is a derivative rather than an average.

Discrete

The product that deals the labels

Multiplying two counting series pairs one choice with another. When the things being counted carry labels, the labels have to be dealt out as well, and the only series that survive the extra bookkeeping are the ones divided by n factorial.

The whole library · What the figures prove