Generator

The dot product as a shadow

A generator in the algebra library, called 42 times across 9 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.

projection 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

The dot product as a shadow. Two vectors and the shadow the first casts on the second. The shadow is 2.425 long and b is 4.123, so the dot product is 10.000.

Perpendicular to both, and as long as their parallelogram

Perpendicular to both, and as long as their parallelogram. The vectors (2, 0.4, 0.2) and (0.6, 1.8, 0.3), the parallelogram they span, and their cross product (−0.24, −0.48, 3.36), drawn perpendicular to both with a length of 3.403, which is the parallelogram's area.

The three numbers of a cross product are three shadows

The three numbers of a cross product are three shadows. Three panels projecting the parallelogram spanned by (1.6, 0.3, 1) and (0.2, 1.5, 0.8) onto the y–z, z–x and x–y planes. The signed areas of the shadows are −1.26, −1.08, 2.34, the components of the cross product.

Reflected, a cross product points the wrong way

Reflected, a cross product points the wrong way. Two vectors, their cross product, and a mirror. The reflection of the cross product is (0.58, 0.52, 1.84); the cross product of the reflected vectors is (−0.58, −0.52, −1.84), pointing the opposite way.

Three is where a plane and an arrow need the same numbers

Three is where a plane and an arrow need the same numbers. For each dimension from 2 to 8, the count of numbers an arrow needs, the count a plane needs, and the dimension of what is perpendicular to a plane. The first two meet only at 3, where the third is 1.

A normal does not move the way an arrow moves

A normal does not move the way an arrow moves. A parallelogram patch after a shear and a squash, its true normal (−0.39, 0.08, 2.24), and the old normal pushed through the same map, (−1.13, −0.48, 1.23), which misses the true normal by 37.1°.

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.

Algebra

A plane disguised as an arrow

The cross product of two arrows is an arrow perpendicular to both, as long as the area of their parallelogram. Reflect everything in a mirror and it points the wrong way, because it was never an arrow: it is a plane, written as the one direction a plane in three dimensions leaves over.

Algebra

Moving a map across a product

The transpose looks like a fact about a matrix: reflect its entries in the diagonal. It is a fact about the inner product. Measure lengths and angles differently and the map that slides to the other side of the product is a different matrix, and a matrix that was symmetric stops being so.

Algebra

One subtraction clears a direction

A basis is a set of directions to measure along, and most bases are awkward because the directions get in each other's way. Removing one shadow at a time turns any basis into one where every coordinate is a shadow and nothing interferes.

Algebra

Symmetry forces a right angle

A matrix equal to its own reflection across the diagonal always has real stretches and always has perpendicular directions to stretch along. Neither is true of matrices in general, and both follow from one line of algebra.

Algebra

The dot product is a shadow

Multiply the matching coordinates and add them up. That rule explains nothing, and it hides the fact that the answer is a length — how far one arrow reaches along another, times how long that other one is.

Algebra

The nearest point of a flat thing

More equations than unknowns almost never have a solution. Asking instead for the point of a plane nearest to where the answer should have been turns an unanswerable question into a shadow, and the shadow is what a line of best fit is.

Algebra

The square that cannot be negative

Cauchy–Schwarz is the load-bearing inequality of the whole subject and is nearly always stated without proof. It is one line away from a fact nobody would argue with, and the line is a parabola with no room to cross the axis.

Algebra

What a map throws away

A linear map redraws the grid, and the determinant measures how much it stretches area. When that measurement comes out zero the map has flattened the plane onto a line — and the question worth asking is not how much was lost but how much survived, because the two always add to what there was.

Analysis

Where the coefficients come from

The recipe for a square wave has a four over pi in front and a one over three on the second term, and the essay that built a square wave from sines used them without saying where they came from. They come from multiplying by one harmonic and taking the area.

The whole library · What the figures prove