The dot product as a shadow
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
Perpendicular to both, and as long as their parallelogram
The three numbers of a cross product are three shadows
Reflected, a cross product points the wrong way
Three is where a plane and an arrow need the same numbers
A normal does not move the way an arrow moves
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.
- basis vectors 1 and 2 are perpendicular ×12
- the polynomials of degree 0 and 1 are exactly perpendicular ×10
- the polynomial of degree 0 is monic ×5
- basis vector 1 has length one ×4
- the degree-0 result is the Legendre polynomial scaled to be monic ×4
- vector 1 is rebuilt from its own row of the table ×4
- moving coefficient 0 by -0.25 makes the total worse ×3
- moving coefficient 0 by 0.25 makes the total worse ×3
- the 1th vector uses no direction built after it ×3
- the residual is perpendicular to the column of x^0 ×3
- vector 1 is two finite coordinates no larger than twelve ×2
- a non-zero vector has positive weighted length ×1
- a plane and an arrow need the same number of coordinates only in three dimensions ×1
- a plane in seven dimensions needs twenty-one numbers ×1
- a rational is two whole numbers ×1
- a squared length is never negative ×1
- a, b and a × b, in that order, form a right-handed frame ×1
- a·b is |a||b| cos θ ×1
- a·b is the shadow's length times |b| ×1
- an ordinary fit is compared only against a weighted one ×1
- and holds in three ×1
- and in the second ×1
- and is not perpendicular in the ordinary sense unless the weights agree ×1
- and it is linear in its first slot ×1
- and its squared length is what Cauchy–Schwarz leaves over ×1
- and only there is the perpendicular to a plane a single line ×1
- and so has the second ×1
- and so it lies along the adjoint's kernel ×1
- and the weighted fit does worse under the ordinary one ×1
- and to the second ×1
- b is reachable exactly when it casts no shadow on the kernel ×1
- between one and three stages ×1
- between three and forty points ×1
- between two and five columns ×1
- between two and four vectors go in ×1
- both weights are positive and no larger than twenty-five ×1
- degree between one and five ×1
- each column is three finite coordinates ×1
- each of the three pairs multiplies to the fourth unit ×1
- every neighbouring point of the plane is further from the target ×1
- every pair of distinct units lies on exactly one line ×1
- every point is a finite pair ×1
- every point of the drawn curve has length one under the product ×1
- every point of the drawn curve has weighted length one ×1
- every vector has the same number of finite coordinates ×1
- every ε is between nought and one ×1
- everything the map reaches is perpendicular, under the product, to what the adjoint kills ×1
- its length is the parallelogram's area, measured in the parallelogram's own plane ×1
- its square and the square of the dot product add to |a|²|b|² ×1
- Lagrange's identity holds for every pair tried ×1
- moving the map across the product leaves the number where it was ×1
- multiplying the reflections gives minus the reflection of the product ×1
- no division by nought ×1
- no sampled value falls below the vertex ×1
- on the smallest ε drawn, the classical order loses orthogonality outright ×1
- removing a shadow never lengthens a vector ×1
- so the two arrows point into opposite half-spaces ×1
- some pair of vectors tells the transpose apart from the adjoint under this product ×1
- the adjoint of the adjoint is the map that started ×1
- the adjoint sends the drawn direction to nought ×1
- the camera angles are within a half turn ×1
- the classical order is never the better of the two ×1
- the cofactor matrix carries the old normal to the new one exactly ×1
- the count runs to between 4 and 10 dimensions ×1
- the degree is 1, 2 or 3 ×1
- the drawn direction is an eigenvector, in the first coordinate ×1
- the dropped line meets b at a right angle ×1
- the eigen-directions are named by two short strings, or not at all ×1
- the eigenvectors are perpendicular under the product exactly when the map is its own adjoint under it ×1
- the first basis vector has been scaled to length one ×1
- the first edge is three finite coordinates no larger than six ×1
- the first vector is not zero ×1
- the first vector is seven whole numbers under ten ×1
- the first vector is three finite coordinates no larger than six ×1
- the first vector is two finite coordinates no larger than twelve ×1
- the identity holds for every pair tried, not only for the drawn one ×1
- the inequality holds for the weighted product too ×1
- the inner product is positive definite, so every non-zero vector has a positive length ×1
- the inner product is three finite numbers [p, q, r] no larger than twenty-five ×1
- the Jacobi identity fails in seven dimensions ×1
- the least value is zero exactly when the two vectors are parallel ×1
- the map applied to the old normal is visibly not the new normal ×1
- the map has rank one, so what it reaches is a line ×1
- the map has two real eigenvalues ×1
- the map is four finite entries no larger than twelve ×1
- the map is invertible, so the patch stays a patch ×1
- the map is three rows of three finite entries no larger than four ×1
- the matrix being inverted is invertible ×1
- the mirror is a coordinate plane, named by the axis it reverses ×1
- the mode of the projection family is one of shadow, signs, schwarz, gram, qr, least, columns, weight, orthloss, legendre, adjoint, selfadjoint, ranges, cross, shadows, mirror, normal, planes, seven ×1
- the modified order stays near the arithmetic's own precision ×1
- the new normal is perpendicular to the first new edge ×1
- the normal equations have a solution ×1
- the nudged fit has one coefficient per column ×1
- the nudged line is worse than the fitted one ×1
- the ordinary fit does worse under the weighted product ×1
- the pair still spans the same area, so it spans the same plane ×1
- the parabola's least value is |a|² − (a·b)²/|b|² ×1
- the plotted span is between 0.2 and 6 ×1
- the product is perpendicular to the first factor ×1
- the product is perpendicular to the first vector ×1
- the quadratic cannot have two distinct real roots ×1
- the remainder is perpendicular to the line the map reaches ×1
- the residual is perpendicular to the first column ×1
- the second drawn vector is perpendicular in the weighted sense ×1
- the second edge is three finite coordinates no larger than six ×1
- the second vector is seven whole numbers under ten ×1
- the second vector is three finite coordinates no larger than six ×1
- the second vector is two finite coordinates no larger than twelve ×1
- the shadow on the x–y plane has the signed area of the product's third component ×1
- the shadow on the y–z plane has the signed area of the product's first component ×1
- the shadow on the z–x plane has the signed area of the product's second component ×1
- the sign of a·b follows the angle ×1
- the squares of the three shadows add to the square of the area itself ×1
- the tangent at each eigenvector's tip runs parallel to the other eigenvector ×1
- the target is three finite coordinates ×1
- the target is two finite coordinates no larger than twelve ×1
- the target's squared length splits into the projection's and the residual's ×1
- the three pairs span three planes with no direction in common ×1
- the two columns are independent, or they span a line rather than a plane ×1
- the two eigenvalues are different, so the directions are determined ×1
- the two vectors are not parallel ×1
- the two vectors are not parallel, or there is nothing left after the subtraction ×1
- the two vectors are not parallel, so there is a plane to write down ×1
- the vector being bounded is two finite coordinates no larger than twelve ×1
- the vector it is measured against is two finite coordinates no larger than twelve ×1
- the vector projected onto is not zero ×1
- the vectors are independent, so nothing vanishes at its own step ×1
- the vectors span a plane ×1
- the weighted product does not care about the order ×1
- the weights are one positive number per point, no larger than a hundred ×1
- there are at least as many coordinates as vectors ×1
- there are more points than coefficients, or nothing is being fitted ×1
- two columns span the plane ×1
- two vectors go in ×1
- under the ordinary product the adjoint is the transpose ×1
- what is left after the subtraction is perpendicular to the first vector ×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 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.
AlgebraMoving 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.
AlgebraOne 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.
AlgebraSymmetry 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.
AlgebraThe 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.
AlgebraThe 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.
AlgebraThe 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.
AlgebraWhat 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.
AnalysisWhere 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.