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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
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
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 …
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
26 votes
Accepted

Relation between Hecke Operator and Hecke Algebra

The fact that Hecke operators (double coset stuff coming from $SL_2(\mathbf{Z})$ acting on modular forms) and Hecke algebras (locally constant functions on $GL_2(\mathbf{Q}_p)$) are related has nothin …
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
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 …
Kevin Buzzard's user avatar
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
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
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
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
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

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