My All 2020 Mathematics A to Z: Quadratic Form


I’m happy to have a subject from Elke Stangl, author of elkemental Force. That’s a fun and wide-ranging blog which, among other things, just published a poem about proofs. You might enjoy.

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Quadratic Form.

One delight, and sometimes deadline frustration, of these essays is discovering things I had not thought about. Researching quadratic forms invited the obvious question of what is a form? And that goes undefined on, for example, Mathworld. Also in the textbooks I’ve kept. Even ones you’d think would mention, like R W R Darling’s Differential Forms and Connections, or Frigyes Riesz and Béla Sz-Nagy’s Functional Analysis. Reluctantly I started thinking about what we talk about when discussing forms.

Quadratic forms offer some hints. These take a vector in some n-dimensional space, and return a scalar. Linear forms, and cubic forms, do the same. The pattern suggests a form is a mapping from a space like R^n to R or maybe C^n to C . That looks good, but then we have to ask: isn’t that just an operator? Also: then what about differential forms? Or volume forms? These are about how to fill space. There’s nothing scalar in that. But maybe these are both called forms because they fill similar roles. They might have as little to do with one another as red pandas and giant pandas do.

Enlightenment comes after much consideration or happening on Wikipedia’s page about homogenous polynomials. That offers “an algebraic form, or simply form, is a function defined by a homogeneous polynomial”. That satisfies. First, because it gets us back to polynomials. Second, because all the forms I could think of do have rules based in homogeneous polynomials. They might be peculiar polynomials. Volume forms, for example, have a polynomial in wedge products of differentials. But it counts.

A function’s homogenous if it scales a particular way. Evaluate it at some set of coordinates x, y, z, (more variables if you need). That’s some number (let’s say). Take all those coordinates and multiply them by the same constant; let me call that α. Evaluate the function at α x, α y α z, (α times more variables if you need). Then that value is αk times the original value of f. k is some constant. It depends on the function, but not on what x, y, z, (more) are.

For a quadratic form, this constant k equals 4. This is because in the quadratic form, all the terms in the polynomial are of the second degree. So, for example, x^2 + y^2 is a quadratic form. So is x^2 + 2xy + y^2 ; the x times the y brings this to a second degree. Also a quadratic form is xy + yz + zx . So is x^2 + y^2 + zw + wx + wy .

This can have many variables. If we have a lot, we have a couple choices. One is to start using subscripts, and to write the form something like:

q = \sum_{i = 1}^n \sum_{j = 1}^n a_{i, j} x_i x_j

This is respectable enough. People who do a lot of differential geometry get used to a shortcut, the Einstein Summation Convention. In that, we take as implicit the summation instructions. So they’d write the more compact q = a_{i, j} x_i x_j . Those of us who don’t do a lot of differential geometry think that looks funny. And we have more familiar ways to write things down. Like, we can put the collection of variables x_1, x_2, x_3, \cdots x_n into an ordered n-tuple. Call it the vector \vec{x} . If we then think to put the numbers a_{i, j} into a square matrix we have a great way of writing things. We have to manipulate the a_{i, j} a little to make the matrix, but it’s nothing complicated. Once that’s done we can write the quadratic form as:

q_A = \vec{x}^T A \vec{x}

This uses matrix multiplication. The vector \vec{x} we assume is a column vector, a bunch of rows one column across. Then we have to take its transposition, one row a bunch of columns across, to make the matrix multiplication work out. If we don’t like that notation with its annoying superscripts? We can declare the bare ‘x’ to mean the vector, and use inner products:

q_A = (x, Ax)

This is easier to type at least. But what does it get us?

Looking at some quadratic forms may give us an idea. x^2 + y^2 practically begs to be matched to an = r^2 , and the name “the equation of a circle”. x^2 - y^2 is less familiar, but to the crowd reading this, not much less familiar. Fill that out to x^2 - y^2 = C and we have a hyperbola. If we have x^2 + 2y^2 and let that = C then we have an ellipse, something a bit wider than it is tall. Similarly \frac{1}{4}x^2 - 2y^2 = C is a hyperbola still, just anamorphic.

If we expand into three variables we start to see spheres: x^2 + y^2 + z^2 just begs to equal r^2 . Or ellipsoids: x^2 + 2y^2 + 10z^2 , set equal to some (positive) C , is something we might get from rolling out clay. Or hyperboloids: x^2 + y^2 - z^2 or x^2 - y^2 - z^2 , set equal to C , give us nice shapes. (We can also get cylinders: x^2 + z^2 equalling some positive number describes a tube.)

How about x^2 - xy + y^2 ? This also wants to be an ellipse. x^2 - xy + y^2 = 3 , to pick an easy number, is a rotated ellipse. The long axis is along the line described by y = x . The short axis is along the line described by y = -x . How about — let me make this easy. xy ? The equation xy = C describes a hyperbola, but a rotated one, with the x- and y-axes as its asymptotes.

Do you want to take any guesses about three-dimensional shapes? Like, what x^2 - xy + y^2 + 6z^2 might represent? If you’re thinking “ellipsoid, only it’s at an angle” you’re doing well. It runs really long in one direction, along the plane described by y = x . It runs medium-size along the plane described by y = -x . It runs pretty short along the z-axis. We could run some more complicated shapes. Ellipses pointing in weird directions. Hyperboloids of different shapes. They’ll have things in common.

One is that they have obviously important axes. Axes of symmetry, particularly. There’ll be one for each dimension of space. An ellipse has a long axis and a short axis. An ellipsoid has a long, a middle, and a short. (It might be that two of these have the same length. If all three have the same length, you have a sphere, my friend.) A hyperbola, similarly, has two axes of symmetry. One of them is the midpoint between the two branches of the hyperbola. One of them slices through the two branches, through the points where the two legs come closest together. Hyperboloids, in three dimensions, have three axes of symmetry. One of them connects the points where the two branches of hyperboloid come closest together. The other two run perpendicular to that.

We can go on imagining more dimensions of space. We don’t need them. The important things are already there. There are, for these shapes, some preferred directions. The ones around which these quadratic-form shapes have symmetries. These directions are perpendicular to each other. These preferred directions are important. We call them “eigenvectors”, a partly-German name.

Eigenvectors are great for a bunch of purposes. One is that if the matrix A represents a problem you’re interested in? The eigenvectors are probably a great basis to solve problems in it. This is a change of basis vectors, which is the same work as doing a rotation. And it’s happy to report this change of coordinates doesn’t mess up the problem any. We can rewrite the problem to be easier.

And, roughly, any time we look at reflections in a Euclidean space, there’s a quadratic form lurking around. This leads us into interesting places. Looking at reflections encourages us to see abstract algebra, to see groups. That space can be rotated in infinitesimally small pieces gets us a kind of group named a Lie (pronounced ‘lee’) Algebra. Quadratic forms give us a way of classifying those.

Quadratic forms work in number theory also. There’s a neat theorem, the 15 Theorem. If a quadratic form, with integer coefficients, can produce all the integers from 1 through 15, then it can produce all positive numbers. For example, x^2 + y^2 + z^2 + w^2 can, for sets of integers x, y, z, and w, add up to any positive number you like. (It’s not guaranteed this will happen. x^2 + 2y^2 + 5z^2 + 5w^2 can’t produce 15.) We know of at least 54 combinations which generate all the positive integers, like x^2 + y^2 + 2z^2 + 14w^2 and x^2 + 2y^2 + 3z^2 + 5w^2 and such.

There’s more, of course. There always is. I spent time skimming Quadratic Forms and their Applications, Proceedings of the Conference on Quadratic Forms and their Applications. It was held at University College Dublin in July of 1999. It’s some impressive work. I can think of very little that I can describe. Even Winfried Scharlau’s On the History of the Algebraic Theory of Quadratic Forms, from page 229, is tough going. Ina Kersten’s Biography of Ernst Witt, one of the major influences on quadratic forms, is accessible. I’m not sure how much of the particular work communicates.

It’s easy at least to know what things this field is about, though. The things that we calculate. That they connect to novel and abstract places shows how close together arithmetic and dynamical systems and topology and group theory and number theory are, despite appearances.


Thanks for reading this. Today’s and all the other 2020 A-to-Z essays should be at this link. Both the All-2020 and past A-to-Z essays should be at this link. And I am looking for letter S, T, and U topics for the coming weeks. I’m grateful for your thoughts.

Author: Joseph Nebus

I was born 198 years to the day after Johnny Appleseed. The differences between us do not end there. He/him.

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