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Fun with y^2=x^p-x
*May 3, 2012*

*Posted by David Speyer in Algebraic Geometry, characteristic p, Number theory.*

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Here’s a basic example that comes up if you work with elliptic curves: Let be a prime which is . Let be the elliptic curve over a field of characteristic . Then has an endomorphism . It turns out that, in the group law on , we have . That is to say, plus copies of is trivial.

I remember when I learned this trying to check it by hand, and being astonished at how out of reach the computation was. There are nice proofs using higher theory, but shouldn’t you just be able to write down an equation which had a pole at and vanished to order at ?

There is a nice way to check the prime by hand. I’ll use for equivalence in the group law of . Remember that the group law on has and has whenever , and are collinear.

We first show that

**Proof of :** We want to show that , and add up to zero in the group law of . In other words, we want to show that these points are collinear. We just check:

as desired. .

**Use of :** Let be a point on . Applying twice, we get

.

Now, the horizontal line crosses at three points: , and . (Of course, , since we are in characteristic three.) So and we have

as desired. .

I was reminded of this last year when Jared Weinstein visited Michigan and told me a stronger statement: In the Jacobian of , we have , where is once again the automorphism .

Let me first note why this is related to the discussion of the elliptic curve above. (Please don’t run away just because that sentence contained the word Jacobian! It’s really a very concrete thing. I’ll explain more below.) Letting be the curve , and letting be , we have a map sending , and this map commutes with . I’m going to gloss over why checking on will also check it on , because I want to get on to playing with the curve , but it does.

So, after talking to Jared, I was really curious why acted so nicely on the Jacobian of . There are some nice conceptual proofs but, again, I wanted to actually see it. Now I do.

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Rationality of the zeta function mod p
*December 12, 2011*

*Posted by David Speyer in Algebraic Geometry, characteristic p, Number theory.*

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Here’s a neat argument about counting points that you could present at the end of a second course in number theory. I’m sure it’s not original, but, hey, that’s what blogs are for!

Let be a smooth hypersurface in , over the field with elements. The Weil conjectures are conjectures about the number of points of over . Specifically, they say that there should be some matrix such that

and that the eigenvalues of should be algebraic integers of norm .

Here I am using the Lefschetz hyperplane theorem to know what is for .

This is, of course, a famously hard theorem. The claim about the eigenvalues is the hardest part, but simply the existence of a matrix for which this formula holds is already quite hard; the first proof was due to Dwork.

What I am going to show you is that there is a much easier proof of the above formula modulo ; a proof of the sort that could be appear in Ireland and Rosen. Many of the terms above disappear mod , so our goal is just to show that there is some matrix such that

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The Weil Conjectures: The approach via the Standard Conjectures
*May 3, 2010*

*Posted by David Speyer in Algebraic Geometry, characteristic p, Number theory.*

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The aim of this post is to outline a strategy for proving the Weil conjectures, proposed by Grothendieck and others. This strategy is incomplete; at various stages, we will need to assume conjectures which are still open today.

Our aim is to prove:

**Theorem:**

Let be a smooth projective variety, over a field of any characteristic. Let be a “reasonable” cohomology theory. Let be the hyperplane class for a projective embedding of . Let be an automorphism of , such that . Then the eigenvalues of are algebraic numbers and, when interpreted as elements of , have norm .

In previous posts, we gave for a proof in characteristic zero and a proof in the case that is a curve. I also explained why I need to say how I am embedding these eigenvalues into . Our proof requires all the ideas of these previous posts, plus some new ones.

(more…)

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The Weil conjectures : Curves
*April 19, 2010*

*Posted by David Speyer in Algebraic Geometry, characteristic p.*

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Our goal for today is to prove the following theorem:

**Theorem 1:** Let be a projective algebraic curve of genus and an endomorphism of degree . Let be a reasonable cohomology theory. Then the action of on has eigenvalues which are algebraic integers, with norm .

For those who know the term, “reasonable cohomology theory” means “Weil cohomology theory”.

The consequence of this theorem, which does not mention cohomology, is

**Theorem 2:** Let be a projective algebraic curve of genus and an endomorphism of degree . Then there are algebraic integers , , …, with norm such that

.

In a previous post, we established this in characteristic zero, by putting a positive definite hermitian structure on such that became unitary. But, as I discussed last time, we can’t define when has characteristic . Instead, will be defined over some other field of characteristic zero, like . We will therefore need to know that the eigenvalues of are algebraic integers before we can even make sense of the statement that they have norm .

It is possible to take the proof I present here and strip it down to its bare essentials, to give a proof of Theorem 2 which doesn’t even mention cohomology. See Hartshorne Exercise V.1.10. I am going to do the opposite; I will go slowly and focus on what each step is proving about . The essential argument here is Weil’s, although I have modernized the presentation.

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Characteristic zero analogues of the Weil conjectures: higher dimension
*April 5, 2010*

*Posted by David Speyer in Algebraic Geometry, characteristic p, complex analysis.*

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In our previous post, we proved

**Theorem** Let be a smooth projective curve over and an endomorphism of degree . The eigenvalues of on have norm .

Today, we would like to generalize this to varieties of higher dimension. The obvious guess is

**Nontheorem** Let be a smooth projective variety, over , of dimension . Let be an endomorphism of of degree . The eigenvalues of on have norm .

This is not a theorem! I believe it is Serre who first figured out how to fix and prove this result. That is the topic of today’s post.

(more…)

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Characteristic zero analogues of the Weil conjectures: Curves
*April 2, 2010*

*Posted by David Speyer in Algebraic Geometry, characteristic p, complex analysis.*

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The quest to prove the Weil conjectures drove algebraic geometry throughout the middle of the twentieth century. It was understood very early that a proof should involve creating a theory of cohomology for varieties in characteristic . This theory, known as étale cohomology, was developed by Grothendieck and his collaborators. Near the end, there was a period where étale cohomology was established but the hardest of the conjectures, the Riemann hypothesis, was not proved. Several mathematicians* proposed a path which would require proving results that were new even in the complex setting; results now known as the standard conjectures. That route was not taken; instead, Deligne found a different proof with its own insights and innovations.

This is the first of a series of posts sketching how the route through the standard conjectures would have gone. There is of course nothing original here; the usual sources are Kleiman, Grothendieck and Serre. I will try to write in a very classical way; I won’t even leave characteristic zero for the first two posts. And there will be tensor categories before the end!

In today’s post, we will prove the following theorem.

**Theorem 1** Let be a smooth projective curve over and an algebraic endomorphism of degree . The eigenvalues of on obey .

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Christol’s theorem and the Cartier operator
*February 11, 2010*

*Posted by David Speyer in Algebraic Geometry, characteristic p, Number theory.*

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Let’s suppose that we want to compute , and we have already been given written out in base as . Here is a small prime and we want to do this conversation repeatedly for many ‘s.

Remember that and thus . So, start with , multiply it by , then by , then by and so forth. When you get to the end, read off .

We can precompute the effect on of multiplying by , for . Then we can compute just by scanning across the base representation of and applying these precomputed maps to the finite set .

The precise way to say that this is a simple, one-pass, process is that it is a computation which can be done by a finite-state automaton. Here is the definition: let , and be finite sets (**input**, **states** and **output**), and (the **start**). For each , let be a map . We also have a map (**readout**). Given a string in , we compute . So our input is a string of characters from , and our output is in .

We can say that our example above shows that is computable by a finite-state automaton. (In our example, the sets , and all have cardinality , but I do not want to identify them.)

This is a special case of an amazing result of Christol et al: Let be a sequence of elements of . Then can be computed by a finite-state automaton if and only if the generating function is algebraic over !

We have just explained the case . The reader might enjoy working out the cases (the Fibonacci numbers) and (the Catalans).

In this post, I will use Christol’s theorem as an excuse to promote the Cartier operator, an amazing tool for working with differential forms in characteristic .

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Matthew Emerton is smart and helpful
*April 22, 2009*

*Posted by David Speyer in Algebraic Geometry, characteristic p.*

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In the comments to my previous post, John Mangual raised a number of questions about the relationship between etale and topological cohomology. Matthew Emerton has posted very thorough answers. If you are interested in the topic, and haven’t read his comments yet, you should.

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More F_un
*November 30, 2008*

*Posted by Ben Webster in Algebraic Geometry, characteristic p, crazy ideas, Number theory.*

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Incidentally, I hope you’ve all been reading F_un mathematics. Even if you aren’t all that interested in the field with one element, it’s a beautifully designed site and might give you some ideas about pushing Web 2.0 in mathematics a bit further than just blogs. While I like our blog, with all its messy diversity (as my collaborators can tell you, messy diversity is a core component of my mathematical style), F_un mathematics has a much more organized focused feel, which I think maybe more promising for getting actual mathematics done. I also think the division of the posts into “outreach,” “undergraduate,” “graduate,” and “research” has some interesting potential and sort of makes me feel like we should be doing a better job of indicating the background level for our post.

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Uniform Position in Characteristic *p*
*March 8, 2008*

*Posted by David Speyer in Algebraic Geometry, characteristic p.*

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Let be an irreducible degree curve in . Some hyperplanes will be tangent to , or pass through singularities of , or even perhaps contain . But most hyperplanes won’t do any of these things. Let be the space of hyperplanes which do not exhibit any of these bad behaviors; these hyperplanes are said to be transverse to . Every hyperplane in meets in points. As we move through , these points move around . If navigates a loop in , before returning to itself, then these points are permuted. Last week, in his class on Algebraic Curves, Joe Harris proved the following theorem.

**The Uniform Position Principle:** Let and be two hyperplanes in , with and . Then, for any permutation , there is a path from to within such that traveling along this path takes to .

In this proof, it was crucial that we were working with a field of characteristic zero. The key lemma was that there was a little loop in such that traveling around that loop swapped exactly two points. The proof, in sketch, is to find a hyperplane which is not transverse to , but just barely; so that is tangent to at one point and meets transversely at other points. If we then wiggle in a little disc, then the tangency point of and will separate into two distinct points, while the other points will wiggle around a little. The boundary of that disc will be a loop in and (exercise!) as you travel around that loop, the two perturbations of the tangency point switch with each other.

In characteristic , there are curves for which there are no such tangent planes . As I’ll show you soon, there exist curves where every single point is a flex, meaning that the tangent line to at touches with degree . So the proof falls apart. We got into a discussion in Harris’ class about whether the result also falls apart. I’ve done some computations and the answer is “yes”. Moreover, the monodromy groups we get are very pretty. (more…)