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Questions about modular forms and related areas

4 votes

Finiteness and bounds for elliptic curves realizing a given galois representation

The set $\mathcal{L}_{\rho}$ is either empty, or a singleton finite. This follows from Faltings' isogeny theorem, which states that for any two elliptic curves (or, more generally, abelian varieties) …
David Loeffler's user avatar
5 votes
Accepted

Uniqueness of the $J$ invariant

Any meromorphic modular function of weight $0$ for $\mathrm{SL}(2,\Bbb Z)$ is a rational function of $j$, say $P(j)$. Since your function is holomorphic, $P$ is a polynomial. Since your function has a …
David Loeffler's user avatar
18 votes

Are some congruence subgroups better than others?

This question already has multiple nice answers, but I am going to add one more thing which isn't quite covered by the existing posts. One distinctive advantage of the $\Gamma_0(N)$ and $\Gamma_1(N)$ …
David Loeffler's user avatar
12 votes

Lacunary weight one modular forms

Serre proved in 1985 that weight 1 cusp forms are always lacunary; see Theorem 19 of Serre's article Serre, Jean-Pierre, Quelques applications du théorème de densité de Chebotarev, Publ. Math., Inst. …
David Loeffler's user avatar
3 votes
Accepted

$\pi$-adic Galois representations of attached to newforms at $p \nmid N$ are crystalline

Blasius and Rogawski's paper "Motives for Hilbert modular forms" (1993) proves a more general result for Hilbert modular forms over any totally-real field, which includes this as a special case.
David Loeffler's user avatar
3 votes
Accepted

Why can Hecke operators be regarded as finite flat cohomological correspondence?

The first half of the question has been answered in the comments, so let me address the second half of the question. We want to define Hecke operators on the complex $R\Gamma(X, \omega^k)$, because th …
David Loeffler's user avatar
4 votes
Accepted

What is the image of the Hecke operator $U_p$?

The statement, as claimed, is false. Let $p = 2, N = 11$, and let $f_0$ be the unique normalised eigenform in $S_2(\Gamma_0(11))$; and set $f(\tau) = f_0(8\tau)$. Then $f \in M_2(\Gamma_0(Np^3))$, but …
David Loeffler's user avatar
10 votes
Accepted

Definition of modular curve associated to $\Gamma(N)$

This is a subtle issue (which has come up before on this site several times, see e.g. is the modular curve X(N) defined over Q? for a related question). Your $S(N)$ is naturally a scheme over $\mathbb …
David Loeffler's user avatar
9 votes

$p$-adic analogue of modular forms, upper half-plane, and $L$-functions

The subjects of "p-adic L-functions" and "p-adic modular forms" are so closely intertwined that it's virtually impossible to talk about either one without immediately running into the other. Applicati …
David Loeffler's user avatar
3 votes
Accepted

Upper bound of the analytic rank of the modular Jacobian varieties $J_1(N)$

I remember discussing this with Emmanuel Kowalski not long ago. The short answer is that generalising the result to $J_1(N)$ is an open problem, and seems to be very difficult.
David Loeffler's user avatar
10 votes
Accepted

How to get the dimension of Atkin-Lehner eigenspace or do you have any data already obtained?

You can compute these dimensions using modular symbols (an auxiliary space which has the same Hecke action as modular forms, but is easier to compute). Here's a Sage example for weight 4 cusp forms of …
David Loeffler's user avatar
2 votes
Accepted

Overconvergent modular forms and the level at $p$

The curve $X_1(Np^n)$ is connected, but the ordinary locus in this curve is not: if you remove the residue discs of the supersingular points, what's left "falls apart" into a disjoint union of several …
David Loeffler's user avatar
9 votes
Accepted

Non-modular elliptic curves

It is a widely believed conjecture that all elliptic curves, over any number field $K$, are modular (in the sense that there exists an automorphic representation [*] $\pi$ of $\operatorname{GL}_2 / K$ …
David Loeffler's user avatar
2 votes

Explicit expressions for "weakly holomorphic" modular forms of weight 1

I'm wary of speaking about "the trivialisation of $\omega$", since the unit group $\mathcal{O}(Y(N))^\times$ (the group of modular units) is a pretty big group, and your trivialization will only be we …
David Loeffler's user avatar
4 votes

Motive of CM elliptic curve and modular forms

Since this question has come alive again, let me point out that the Hecke operators cannot give a splitting of $h^1(E)$ into two pieces over $F$, since the Hecke correspondences on a modular curve are …
David Loeffler's user avatar

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