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Linear representations of algebras and groups, Lie theory, associative algebras, multilinear algebra.

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
9 votes
4 answers
1k views

Structure of cuspidal Bernstein components—do non-commutative endomorphism rings ever really...

Let $F$ be a finite extension of $\mathbf{Q}_p$ with integers $\mathscr{O}$, let $\mathbb{G}$ be a connected reductive group over $F$ and let $G=\mathbb{G}(F)$ be its $F$-points. Let $X(G)=\operatorna …
18 votes
6 answers
2k views

Explicit formula for the trace of an unramified principal series representation of $GL(n,K)$...

Let $K$ be a non-arch local field (I'm only interested in the char 0 case), let $\mathbb{G}$ be a connected reductive group over $K$ and let $G=\mathbb{G}(K)$. If $V$ is a smooth irreducible complex r …
30 votes
Accepted

What's the point of a Whittaker model?

This question is a bit like saying "what's the point of the theory of bases for vector spaces -- this just gives you an isomorphism of your space with $\mathbb{R}^n$. What is the point of defining thi …
Pig's user avatar
  • 809
19 votes
2 answers
3k views

Uniqueness of local Langlands correspondence for connected reductive groups over real/comple...

In Langlands' notes "On the classification of irreducible representations of real algebraic groups", available at the Langlands Digital Archive page here, Langlands gives a construction which is now r …
8 votes
3 answers
1k views

When does a unitary Hilbert space rep of a reductive Lie group decompose into a direct sum o...

I'm giving some lectures on the trace formula. Here's something I proved in the last lecture. Let $G$ be a locally compact Hausdorff unimodular topological group (e.g. a reductive Lie group), let $\Ga …
19 votes
2 answers
2k views

Clifford theory: behaviour of a very general irreducible representation under restriction to...

Let $G$ be a group and let $H$ be a subgroup of finite index. Let $V$ be an irreducible complex representation of $G$ (no topology or anything: $V$ is just a non-zero complex vector space with a line …
29 votes
5 answers
2k views

Does $S_4$ inject into $SL(2,R)$ for some commutative ring $R$?

$\newcommand{\Z}{\mathbf{Z}}$ Given a nice infinite collection of groups, for example the symmetric groups, one can ask whether any finite group is a subgroup of one of them. Of course any finite grou …
6 votes
Accepted

Finite dimensional automorphic representations of a definite quaternion with prime discrimin...

1) Yes, I think that's true. I guess it follows relatively easy from the statement that an automorphic representation of the algebraic group $D^\times$ is finite-dimensional iff it's 1-dimensional and …
Kevin Buzzard's user avatar
14 votes
Accepted

Any finite dimensional admissible(smooth) irreducible representation of GL(2,Q_p) is 1-dim

What does "admissible" mean for you? Does it imply smoothness (stabilisers are open)? If not then I think the statement might be false (choose some hopelessly discontinuous injection from $\mathbf{Q}_ …
Kevin Buzzard's user avatar
4 votes
1 answer
463 views

Injection between non-isomorphic irreducible Hilbert space reps?

I must be missing something trivial here. Let $G$ be, say, a reductive Lie group (or more generally any locally compact Hausdorff unimodular topological group). A unitary Hilbert space representation …
11 votes
1 answer
1k views

Simple explicit example of local Jacquet-Langlands theorem for inner forms of GL(n), and con...

This one will be very easy for the experts. Let $F$ be a nonarch local field, let $n\geq1$ be an integer, choose $0\leq d<n$ and let $D$ be the central simple algebra over $F$ with invariant $d/n$ in …
2 votes

what information of a representation was killed by Jacquet functor?

EDIT: I assumed the OP was asking about reductive groups over non-arch local fields. Emerton raises the possibility that the question is actually about groups over R or C, and he's probably right. So …
Kevin Buzzard's user avatar
20 votes
Accepted

When do the sizes of conjugacy classes and squares of degrees of irreps give the same partit...

My standard rant about "what can we say about $G$": what we can say about $G$ is that the two partitions are the same. If the questioner doesn't find that a helpful answer then they might want to cons …
Kevin Buzzard's user avatar
13 votes

Definition of L-function attached to automorphic representation

I think you're slightly misled. $\pi$ doesn't have an $L$-function "in abstracta". An unramified $\pi_v$ at a finite place $v$ gives rise, by Langlands' interpretation of the Satake isomorphism, to a …
Kevin Buzzard's user avatar

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