Categories

## Cartier Divisors

As an obvious first blog post, easily understandable and very relevant to cryptography (/s), here a description of Cartier Divisors, because Thai asked for it on Twitter.

For this, first some history: A while ago, I taught a Google internal course about the mathematics of elliptic curves. It would probably make sense to start with that content, but I’m going to assume that I’ll come back to it and fix the order later.

Anyways, the objects we are looking at are Divisors and Principal Divisors. The come up when studying curves as a way to describe the zeroes and poles of functions. Over the projective line \mathbb{P}_K^1 (also known as just the base field plus a point at infinity), a rational function (the quotient of two polynomials) can have any selection of zeroes and poles it so pleases, with the only constraint being that there must be (with multiplicity) the same number of zeroes and poles. We can see that by looking at

\frac{(X-a_1)(X-a_2)\dots (X-a_n)}{(X-b_1)(X-b_2)\dots (X-b_n)}

for a function with zeroes at a_1, a_2, \dots, a_n and poles at b_1, b_2, \dots, b_n. If a_i or b_i is \infty, then we ignore the corresponding term, and get a zero/pole at infinity.

On more general curves, we do not have this amount of freedom. The lack of freedom we have in choosing zeroes and poles is tied surprisingly deeply to the curve in question, so describing it turns out to be very useful.

A Weil divisor is a formal sum of points of the curve, that is, we assign an integer to every number of the curve, with all but finitely many points getting assigned the integer zero. The degree of a divisor is the sum of all these integers. The divisor of a function \operatorname{div}f is the divisor we get by assigning the order of the function in that point to the point, i.e. setting it 1 for simple zeroes, -1 for simple poles, and so on. If a divisor is a divisor of a function, we call the divisor a principal divisor.

With these definitions out of the way, we can get to Thai’s question. It turns out that the thing we are interested in is the divisors of degree 0 modulo the principal divisors. This group in some sense measures how restricted we are in our choice for placing zeroes and poles. It turns out, that for Elliptic curves, all divisors are equal to a divisor of the form P - O, with O being the point at infinity (or really any fixed (“marked”) point on the curve) up to a principal divisor (equal up to principal divisor is also called linearly equivalent). So what Thai is asking is that while we can think of principal divisors as a description of rational functions, what are the other divisors? The simple answer to that is that they are just what we said, formal sums of points, just some points with some integer weights. For elliptic curves, they are conveniently in a 1:1 correspondence with the points of the curve itself, which is why we usually gloss over the whole divisor thing and just pretend to add points of the curve themselves. But this answer is kind of unsatisfying, and it does generalize well in higher dimensions or for curves with singularities in them, so a better concept is needed.

Enter Cartier Divisors. In order to explain these, we’re technically going to need sheaves, but sheaves are a bit much, so I’ll try to handwave some things. The basic idea is, since we want to describe zeroes and poles, why don’t we just use zeroes and poles for that? Of course we can’t use a full function that is defined everywhere for that, that would only give us the principal divisors. But locally, we can use a function to describe zeroes or poles. Now what does locally mean? In algebraic geometry, the topologies we’re using are kind of weird. Here, we are using the Zariski topology, which for curves just means that when we say locally, we mean the whole curve with a finite number of points removed. We use this to remove the any zeroes or poles we don’t want for our divisor from our local representative.

All in all that means a Cartier divisor on a curve C is a covering (U_i), i.e. a collection of open sets (curve minus finite amounts of points) such that their union is the whole curve, and a rational function f_i per U_i, defined on U_i. This function’s zeroes and poles are what we understand as the divisor. Obviously, we now need to make sure all these partial functions work well as a whole. We do that by looking at U_i \cap U_j and the functions f_i and f_j restricted to that intersection. If we want this construction to define a consistent divisor, then f_i/f_j can not have any zeroes or poles in U_i \cap U_j. We write this as

f_i/f_j \in \mathcal{O}^\times (U_i \cap U_j)\;\;.

This now describes a consistent global object with zeroes and poles as we want them, getting quite close to describing divisors in a completely different way! We just have one problem, there are way too many functions with a specific pattern of zeroes and poles on our local neighborhood U_i, we need to get rid of all the extra behavior that isn’t just zeroes and poles! To do that, we need to look at two functions f_i and g_i on U_i that have the same pattern of zeroes and poles. What happens when we take f_i/g_i? Well we, as above, get a function without zeroes or poles on U_i. So if we want to forget all that extra structure, we need to take f_i modulo the set of functions without zeroes or poles on U_i. And that’s it.

If we write \mathcal{M}^\times(U_i) for the rational functions that are not equal to zero (so the rational functions that have a multiplicative inverse) and write \mathcal{O}^\times (U_i) for the functions without zeroes or poles on U_i, we can now describe a Cartier divisor as a covering (U_i) together with an element f_i \in \mathcal{M}^\times(U_i)/\mathcal{O}^\times(U_i) such that f_i/f_j\in\mathcal{O}^\times(U_i \cap U_j). A principal Cartier divisor is a Cartier divisor that can be described with just using just the entire curve C as the only element of the covering.

For extra bonus points (which I will not describe in detail here, because this blog post is already way too long and completely incomprehensible), we can look at what happens if we now take these Cartier divisors modulo principal Cartier divisors. It turns out, that the result can be described again with a covering U_i, but this time, instead of going through all that choosing of rational functions per set, we just use the intersections, and choose an element f_{ij}\in \mathcal{O}^\times (U_i \cap U_j), without even looking at rational functions in U_i at all, with some sheafy/cohomological rules for when two of those things are equal.