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7 votes
Accepted

Intuition behind centralizers of Langlands parameters

The simplest non-trivial case for this is for $G = SL_2$. If you take an unramified principal series representation of $\tilde G = GL_2$ and restrict it to $G$, then it will almost always be irreducib …
David Loeffler's user avatar
4 votes

Physical Applications of Locally Symmetric Spaces

There is a very interesting interaction between some aspects of the Langlands program and high-energy physics, involving elliptic polylogarithms (and other related generalisations of the classical pol …
David Loeffler's user avatar
8 votes
Accepted

Roadmap for studying Galois deformation theory/modularity theorems from a modern perspective

A fantastic place to start would be Toby Gee's notes from the 2013 Arizona Winter School. This gives a nice overview of the theory as it then existed -- things have of course moved on further since th …
David Loeffler's user avatar
2 votes
Accepted

Example of a non-odd motive appearing in cohomology of intermediate degree

How about the following construction? Let $A$ be a principally-polarised abelian surface over $\mathbf{Q}$ which is "generic", i.e. $End_{\overline{\mathbf{Q}}}(A) = \mathbf{Z}$. Then the Galois acti …
David Loeffler's user avatar
15 votes
Accepted

Loss of cuspidality by Langlands tranfer

You are quite correct that the Langlands transfer map does not preserve cuspidality in general. E.g. if you take a modular form of CM type, coming from a Groessencharacter $\psi$ of some imaginary qua …
David Loeffler's user avatar
13 votes

What kind of non-cuspidal automorphic representation are not isobaric sums?

EDIT. A colleague wrote to me to point out that my original answer to this question was actually completely wrong: I had confused "isobaric" representations with "pure" representations (which are not …
David Loeffler's user avatar
3 votes
Accepted

Motive associated to a cuspidal representation of $GSp_{4}$

The formula you quote from Harris defines a Galois representation, not a motive. We expect that there is a motive whose etale realisation is Harris' space, but that is not immediate. The problems are: …
David Loeffler's user avatar
10 votes
Accepted

Non-existence of "higher" Artin map

There is no way of reformulating local Langlands for $n > 1$ in terms of such a map. Local Langlands is a bijection between irreducible smooth representations of $\operatorname{GL}_n(K)$, and $n$-dime …
David Loeffler's user avatar
12 votes
Accepted

What's the status of Arthur's announced classification for GSp(4)?

This question is answered pretty definitively by the following recent paper: Gee, Toby; Taïbi, Olivier, Arthur’s multiplicity formula for $\mathrm{GSp}_4$ and restriction to $\mathrm{Sp}_4$, J …
David Loeffler's user avatar
2 votes
Accepted

Variants of the classical Satake classfication

(1) Borel's article in the Corvallis proceedings does this slightly differently: he chooses a specific Frobenius element $\sigma$, and then looks at the subset $\widehat{G} \times \{\sigma\}$ of ${}^L …
David Loeffler's user avatar
4 votes
Accepted

Eigenvarieties and functoriality

You have asked a lot of questions at once, and it is impossible to give more than a hint at a small subset of these questions. I think the general theme here is: the existence of eigenvarieties doesn' …
David Loeffler's user avatar
2 votes
Accepted

Understand the $p$-adic local Langlands correspondence with examples

Let's look at the case of representations associated to modular forms. I'm going to switch the roles of $\ell$ and $p$, because I find $\ell$-adic Hodge theory disturbing; so I'm going to look at $\rh …
David Loeffler's user avatar
6 votes
Accepted

Two different local Langlands parameters for quadratic extension

This came up in a paper of mine not so long ago, and my coauthors and I were surprised that it wasn't made explicit in the standard references, so we wrote it out ourselves: Dembélé, Lassina; Loeffler …
David Loeffler's user avatar
10 votes

Symmetric powers of Ramanujan tau-function

It is indeed true that substantially more is known for holomorphic cusp forms than for general automorphic representations, as a consequence of modularity lifting theorems. The strongest result so f …
David Loeffler's user avatar
3 votes

branching laws for $p$-adic representations of reductive groups

If you're asking about admissible p-adic Banach space representations in the sense of Schneider--Teitelbaum, then I think virtually nothing is known in this setting about branching laws, even in the s …
David Loeffler's user avatar

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