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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) …
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 …
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 $\Gamma_1(N)$ …
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. …
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.
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 …
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 …
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 …
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 …
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.
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 …
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 …
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 $\operatorname{GL}_2 / K$ …
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 …
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 …