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Questions about modular forms and related areas

49 votes
Accepted

Are there Maass forms where the expected Galois representation is $\ell$-adic?

Here's some piece of the bigger picture. Maass forms and holomorphic modular forms are both automorphic representations for $GL(2)$ over the rationals. An automorphic representation is a typically hug …
Kevin Buzzard's user avatar
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
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
20 votes
2 answers
685 views

Can something finite over $\mathbb{C}(q)$ be a modular form?

If $f\in\mathbf{C}[[q]]$ is non-constant, and algebraic over $\mathbf{C}[q]$ (in the sense that it is a root of a polynomial with coefficients in in $\mathbf{C}[q]$) then can $f$ be the $q$-expansion …
27 votes
2 answers
2k views

How to explicitly compute lifting of points from an elliptic curve to a modular curve?

Say $E$ is an elliptic curve over the rationals, of conductor $N$. There's a covering of $E$ by the modular curve $X_0(N)$, and if you rig it right then you can define this map over $\mathbf{Q}$: ther …
10 votes
1 answer
532 views

Example of a non-smooth irreducible component of the generic fibre of a Hida family?

Is there a known example of a non-smooth irreducible component of the rigid generic fibre of a Hida family? Let me explain some of the context around this question (but I'm not going to explain Hida …
13 votes
Accepted

Does anyone want a pretty Maass form?

[these are comments, not an answer, but there were too many for the comments box] Hey---I wrote that code too! I did it to teach myself "practical Maass forms". I wrote in pari, not sage. I didn't do …
Kevin Buzzard's user avatar
8 votes

Number of modular lifts with prescribed parameters

I can give you a "formula" in the sense that I can give you an algorithm to compute the number in any given case. If $\ell\not=p$ is prime then an old result of Carayol and Livn\'e says that the condu …
Kevin Buzzard's user avatar
5 votes

Image of complex conjugation by modular representations in characteristic 2

Joel -- it's difficult to work out what you're asking. Of course both possibilities can occur, as Wanax said. Furthermore both possibilities can occur even for the same modular form. For example, if y …
Kevin Buzzard's user avatar
7 votes
Accepted

Effective detection of CM modular forms

If the form is CM then it will be isomorphic to a quadratic twist of itself. So I think what I'd do with a form which I suspect is or is not CM is to just twist by all the (finitely many ) possible qu …
Kevin Buzzard's user avatar
18 votes

Why does the definition of modularity demand weight 2?

There has been a lot written already about this question. but here is a simple answer. The Hodge--Tate weights of the Tate module of an elliptic curve are 0 and 1. The Hodge--Tate weights of the Galoi …
Kevin Buzzard's user avatar
17 votes
Accepted

Why is there a weight 2 modular form congruent to any modular form

By "level $\ell$" I assume you mean "level $\Gamma_1(\ell)$". Here's a proof. By the Eichler-Shimura theorem, the system of eigenvalues associated to the modular form shows up in $H^1(SL(2,\mathbf{Z} …
Kevin Buzzard's user avatar
9 votes
Accepted

If $f$ is an $p$-nonordinary eigenform of weight $k\leqslant p+1$ are there always two eigen...

It's deeper than theta cycles, I think. I am going to assume that $N$ is prime to $p$ -- you don't say this in your question but most of my answer assumes this in a very serious way. If $f$ is ordin …
Kevin Buzzard's user avatar
19 votes
6 answers
2k views

weight 4 eigenforms with rational coefficients---is it reasonable to expect they all come fr...

A weight 2 modular form which happens to be a normalised cuspidal eigenform with rational coefficients has a natural geometric avatar---namely an elliptic curve over the rationals. It seems to be a su …
15 votes

Hecke algebra generated by a single element

[I took the time to chase this up so may as well post it as an answer.] There is a (cuspidal) modular (eigen)form of level $\Gamma_0(512)$ and weight 2, which if I remember correctly was shown to me …
Kevin Buzzard's user avatar

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