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Prime numbers, diophantine equations, diophantine approximations, analytic or algebraic number theory, arithmetic geometry, Galois theory, transcendental number theory, continued fractions

124 votes
Accepted

Etale cohomology -- Why study it?

$\DeclareMathOperator{\gal}{Gal}$ Here's a comment which one can make to differential geometers which at least explains what etale cohomology "does". Given an algebraic variety over the reals, say a s …
Kevin Buzzard's user avatar
86 votes
8 answers
13k views

What are the local Langlands conjectures nowadays, for connected reductive groups over a $p$...

Let me stress that I am only interested in $p$-adic fields in this question, for reasons that will become clear later. Let me also stress that in some sense I am basically assuming that the reader kno …
Kevin Buzzard's user avatar
72 votes

What are "perfectoid spaces"?

Here is a completely different kind of answer to this question. A perfectoid space is a term of type PerfectoidSpace in the Lean theorem prover. Here's a quote from the source code: structure perfe …
Kevin Buzzard's user avatar
68 votes
Accepted

Smooth proper scheme over Z

Hey Bjorn. Let me try for a counterexample. Consider a hypersurface in projective $N$-space, defined by one degree 2 equation with integral coefficients. When is such a gadget smooth? Well the partial …
Kevin Buzzard's user avatar
68 votes
7 answers
5k views

Open project: Let's compute the Fourier expansion of a non-solvable algebraic Maass form.

OK so let's see if I can use MO to explicitly compute an example of something, by getting other people to join in. Sort of "one level up"---often people answer questions here but I'm going to see if I …
Kevin Buzzard's user avatar
59 votes
Accepted

Fermat's Last Theorem for Gaussian Integers ( excluding $\mathbb{Z}$ or $i\mathbb{Z}$ )

This is still way open, I should think. "Elementary" methods won't even solve the analogous problem over $\mathbf{Z}$, so you need to use "modular form" methods. The problem is that even if the result …
Kevin Buzzard's user avatar
50 votes

If $2^x $and $3^x$ are integers, must $x$ be as well?

This question came up recently on the NMBRTHRY mailing list and I can't resist paraphrasing a comment I made there. So, perhaps surprisingly, this question has links to automorphic forms! For if $x$ i …
50 votes
13 answers
6k views

Erratum for Cassels-Froehlich

Edit 25 April 2010: I have a physical copy of the new printing of the book. I can only assume the LMS is now selling it (but have no details). IMPORTANT EDIT: THE RESULTS ARE IN! Ok, the deadline has …
49 votes

Are there mistakes in the proof of FLT?

No there are not any mistakes in these papers of any interest. In the 1990s there were a bazillion study groups and seminars across the world devoted to these papers; I personally read all three of th …
Kevin Buzzard's user avatar
44 votes
4 answers
3k views

Are there motives which do not, or should not, show up in the cohomology of any Shimura vari...

Let $F$ be a real quadratic field and let $E/F$ be an elliptic curve with conductor 1 (i.e. with good reduction everywhere; these things can and do exist) (perhaps also I should assume E has no CM, ev …
Kevin Buzzard's user avatar
40 votes
Accepted

How did Birch and Swinnerton Dyer arrive at their conjecture?

For what it's worth, here are some historical comments. Both Birch and S-D spoke in Cambridge a few weeks ago, on the history of their conjecture. To my surprise, both of them emphasized the role not …
Kevin Buzzard's user avatar
34 votes
Accepted

Integers not represented by $ 2 x^2 + x y + 3 y^2 + z^3 - z $

EDIT: Hendrik Lenstra emailed me a proof of Conjecture 2. I'll append it below. So Jagy's question is now solved. OK so I think that Jagy wants to make the following conjecture: CONJECTURE 1: an i …
Kevin Buzzard's user avatar
34 votes
2 answers
3k views

The work of E. Artin and F. K. Schmidt on (what are now called) the Weil conjectures.

I was reading Dieudonne's "On the history of the Weil conjectures" and found two things that surprised me. Dieudonne makes some assertions about the work of Artin and Schmidt which are no doubt correc …
Kevin Buzzard's user avatar
32 votes
Accepted

Why is one interested in the mod p reduction of modular curves and Shimura varieties?

The Eichler-Shimura relation doesn't just prove the Hasse-Weil conjecture for modular curves. It e.g. attaches Galois representations to modular forms of weight 2. More delicate arguments (using etale …
Kevin Buzzard's user avatar
31 votes

Intuition behind the Eichler-Shimura relation?

Let me highlight some issues that Emerton doesn't: 1) you seem to hint that you don't know that modular forms can be viewed as a product of a bunch of local terms. So there is an adelic story, where …
Kevin Buzzard's user avatar

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