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Algebraic number fields, Algebraic integers, Arithmetic Geometry, Elliptic Curves, Function fields, Local fields, Arithmetic groups, Automorphic forms, zeta functions, $L$-functions, Quadratic forms, Quaternion algebras, Homogenous forms, Class groups, Units, Galois theory, Group cohomology, Étale cohomology, Motives, Class field theory, Iwasawa theory, Modular curves, Shimura varieties, Jacobian varieties, Moduli spaces

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 …
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
27 votes

"Understanding" $\mathrm{Gal}(\bar{\mathbb{Q}}/\mathbb{Q})$

I am pretty sure that when different number theorists say "one of the main goals of number theory is to understand Gal(Q-bar/Q)" they may well mean different things. One example of what someone might …
Kevin Buzzard's user avatar
26 votes
2 answers
2k views

Are there any Hecke operators acting on an elliptic curve with additive reduction that I don...

I could have made this question very brief but instead I've maximally gone the other way and explained a huge amount of background. I don't know whether I put off readers or attract them this way. The …
Kevin Buzzard's user avatar
15 votes
1 answer
843 views

components of E[p], E universal in char p.

I have just realised that a group scheme I've known and loved for years is probably a bit wackier than I'd realised. In this question, in Charles Rezk's answer, I erroneously claim that his construct …
Kevin Buzzard's user avatar
13 votes

$A_5$-extension of number fields unramified everywhere

Oh, I know how I would try and build examples. First I would write down a random $A_5$ extension $K$ of $\mathbf{Q}$, ramified at some primes (in fact I would look in a table, e.g. in Buhler's thesis …
Kevin Buzzard's user avatar
13 votes

Why do congruence conditions not suffice to determine which primes split in non-abelian exte...

OK how's about this to finish (I don't think either argument posted so far deals with this case). Say $K/\mathbf{Q}$ is finite and (away from a finite set of exceptions) $p$ splits completely in $K$ i …
Kevin Buzzard's user avatar
13 votes
1 answer
729 views

Non-trivial class number at some finite level in the cyclotomic $\mathbf{Z}_p$-extension of ...

An MSc student asked me if I knew an example of a prime $p$ and some finite layer $K_n$ in the cyclotomic $\mathbf{Z}_p$-extension of $\mathbf{Q}$ (so $[K_n:\mathbf{Q}]=p^n$) which had non-trivial cla …
Kevin Buzzard's user avatar
12 votes

Conjugacy classes in the absolute galois group

Firstly, Frobenius elements aren't even conjugacy classes, as you know. So you had better look at quotients $Gal(K/\mathbf{Q})$ of the Galois group which are unramified outside some set $S$. Now you h …
Kevin Buzzard's user avatar
12 votes
Accepted

Insolvable number fields ramified only at one (small) prime

Minhyong's comments indicate the issue here. If I want to come up with an extension unramified outside $p$ then why not look at the 2-dimensional mod $p$ representation attached to the $\Delta$ functi …
Kevin Buzzard's user avatar
10 votes
Accepted

Type of place versus type of unitary group

Things are perhaps a bit messier than you hope. In particular it is not true that the unitary group is non-quasi-split if and only if $v$ ramifies. Disclaimer: I did not know the answer to this questi …
Kevin Buzzard's user avatar
8 votes

Does there exist a general theory of "arithmetic complexity"/"arithmetic height"?

In practice what you'll do is look for linear relations with integer coefficients between powers of $\alpha$ (starting with $\alpha^0=1$). If $\alpha$ is algebraic then suddenly you'll find a relation …
Kevin Buzzard's user avatar
6 votes
Accepted

Hilbert Symbols, Norms, and p-adic roots of unity

I think I can construct an explicit counterexample with $a\in\mathbb{Q}_p$. Choose a compatible sequence $\zeta_{p^m}$ of $p^m$th roots of unity in $\overline{\mathbb{Q}}_p$. Write $q=p^n$ with $n\g …
Kevin Buzzard's user avatar
5 votes

Erratum for Cassels-Froehlich

Ok so it looks like I misjudged this and the community seem happy to have the question here, at least at present. So I figured I'd pass on the comments which Serre sent the LMS. p.135, part b) of L …
4 votes
Accepted

Are Fredholm hypersurfaces affinoid?

No they're not in general affinoid. The problem is that the zero locus of the power series is computed within a space which is almost never affinoid -- for example in the modular curve case the ambien …
Kevin Buzzard's user avatar

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