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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 …
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 …
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 …
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 …
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 …
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 …
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 …
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 …
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} …
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 …
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 …