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An automorphic form is a well-behaved function from a topological group $G$ to the complex numbers (or complex vector space) which is invariant under the action of a discrete subgroup $\Gamma \subset G$ of the topological group. Automorphic forms are a generalization of the idea of periodic functions in Euclidean space to general topological groups.

6 votes

L and epsilon factors of Gelbart-Jacquet lifts

The function $s \mapsto \epsilon(s, \pi)$ has the form $s \mapsto A e^{Bs}$ for some constants $A, B$, so it vanishes nowhere on $\mathbb{C}$. That has nothing to do with being a GJ lift, it's a gener …
David Loeffler's user avatar
4 votes
Accepted

Global symplectic (orthogonal) type of automorphic representation compels its type to all it...

Here is a proof of the claim using results from Arthur's monograph The Endoscopic Classification of Representations: Orthogonal and Symplectic Groups. Let $N = 2n$ be an even integer, and $\pi$ a cusp …
David Loeffler's user avatar
1 vote

Antiholomorphic cusp forms of negative weight

I am not entirely sure what Clozel is trying to do here. He doesn't make it terribly precise what he means, and as you yourself have realised, if you read the article literally it seems that one case …
David Loeffler's user avatar
1 vote

Existence of a Hilbert modular form of parallel weight 6

There is functionality for computing Hilbert modular forms in MAGMA, based on the algorithms of Dembele, Donnelly, Greenberg, and Voight. If you don't have access to a copy, try the online MAGMA calcu …
David Loeffler's user avatar
4 votes
Accepted

On the Cartan decomposition of unitary group

Theorem: Let $G$ be a reductive algebraic group over a local field $F$, let $K$ be any maximal compact subgroup of $G(F)$, and let $Z = Z(G)$. Then $K \cap Z(F)$ is the unique maximal compact subgroup …
David Loeffler's user avatar
10 votes
Accepted

Is there a known construction of Cuspidal representations of GL(3) isomorphic to their own t...

Let $E / F$ be the cyclic cubic extension corresponding to $\chi$ by class field theory. Let $\sigma$ be a generator of $\operatorname{Gal}(E / F)$, and let $\psi$ be a character of $E^\times \backsla …
David Loeffler's user avatar
5 votes

What is the meaning of the $L$-group?

I find this question somewhat strange; you ask "what is the meaning of the L-group?", but the survey article of Casselman which you link to is largely devoted to explaining the historical and conceptu …
David Loeffler's user avatar
12 votes
Accepted

What is the relationship between (g,K)-module and Maass forms?

So you've seen that there are essentially three types of (g, K)-modules: finite-dimensional ones; principal series; and discrete series. The finite-dimensional ones don't interest us, since they are n …
David Loeffler's user avatar
11 votes
Accepted

modular form Fourier coefficients and associated automorphic representation

Jared Weinstein and I wrote a paper on how to compute $\pi_p$: see here. As Olivier says, $a_p$ will often be zero, and in fact if the central character is trivial (or has conductor coprime to $p$) …
David Loeffler's user avatar
9 votes
Accepted

Exceptional primes

No, this does not work: for any modular form of weight $k \ge 2$, the image of the projective representation $\tilde\rho_{f, \lambda}$ is infinite for every prime $\lambda$. Proof: if $\tilde{\rho}_ …
David Loeffler's user avatar
2 votes

Fourier expansion at inequivalent cusps

A chunk of the Princeton PhD thesis of Dan Collins is devoted to this problem. See Collins' preprint here: https://arxiv.org/abs/1802.09740, and the accompanying Sage code.
David Loeffler's user avatar
3 votes

Jacquet module and Frobenius reciprocity

In general, all we can say from "general abstract nonsense" is that if $\sigma$ is a subrepresentation of $Ind_P^G(\pi)$, then $\pi$ is a quotient of $J_N(\sigma)$; but you don't immediately get any f …
David Loeffler's user avatar
5 votes

Galois representations attached to a cusp form for different primes

At the most basic level, $\rho_p$ and $\rho_q$ are "nothing to do with each other". E.g. the kernels of $\rho_p \bmod p$ and of $\rho_q \bmod q$ cut out two finite Galois extensions of $\mathbf{Q}$ wh …
David Loeffler's user avatar
2 votes
Accepted

Part of some generic representation is also generic?

Let $\pi$ be the irreducible generic unramified representation of $Sp(W) $ that is a subquotient of $Ind(\chi_1, \dots, \chi_n)$. I think the key here is to realise that this does not exist for all …
David Loeffler's user avatar
6 votes
Accepted

Relation between $\xi$-cohomological and discrete series

This condition comes up because of $(\mathfrak{g}, K)$-cohomology, which is an extremely important invariant of automorphic representations. If $\xi$ is an algebraic rep, then $\xi$ defines a locally- …
David Loeffler's user avatar

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