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Questions about modular forms and related areas

18 votes

Why are modular forms (usually) defined only for congruence subgroups?

The main point is that the basic definitions work fine but the link with arithmetic is much more "vague". Look at early papers of Tony Scholl. There are Galois representations attached to certain non- …
Kevin Buzzard's user avatar
3 votes

How many L-values determine a modular form?

I think the answer to your second question is "no". For example if $k=2$ and $f$ and $g$ correspond to elliptic curves over $Q$ with positive rank, then the only critical point is $s=1$ and (at least …
Kevin Buzzard's user avatar
13 votes
Accepted

Can a the q-expansion of a p-adic modular form be a non-constant polynomial?

It is. I want to argue the following way: if the polynomial is non-constant then after scaling it has integral coefficients and so the reduction of the p-adic form mod p^n will be a classical form who …
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
4 votes

SL(2,Z/N)-decomposition of space of cusp forms for Gamma(N)

If you think about this question in terms of automorphic representations then it sort of becomes trivial. The space $Sk(\Gamma(N))$ can be re-interpreted as the direct sum of $\pi^{U(N)}$, where $\pi$ …
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
11 votes
Accepted

Hilbert Modular Newforms

If I've understood your question correctly, you're right that $C(q,f)\not=0$ always and there is a natural representation-theoretic proof of this result (before I start let me say that I don't know ho …
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 …
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
15 votes
Accepted

Galois representations attached to newforms

The right way to do this sort of question is to apply Saito's local-global theorem, which says that the (semisimplification of the) Weil-Deligne representation built from $D_{pst}(\rho_{f,p})$ by forg …
Kevin Buzzard's user avatar
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
10 votes
Accepted

Level raising by prime powers

Presumably you want the form (let me call it g) of level Np^3 to be new at p, otherwise it's trivial. Let me also assume ell isn't p. If the form g is new at p, and has level Gamma0(p^3) at p, then …
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
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
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

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