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Linear representations of algebras and groups, Lie theory, associative algebras, multilinear algebra.
19
votes
4
answers
5k
views
Unitary representations of SL(2, R)
I've heard that irreducible unitary representations of noncompact forms of simple Lie groups, the first example of such a group G being SL(2, R), can be completely described and that there is a discre …
14
votes
1
answer
4k
views
How to understand character sheaves
There's a well-known series of articles by Lusztig about Character Sheaves. They have important connections to many things in (geometric) representation theory, e.g. 0904.1247
How to understand these …
13
votes
3
answers
1k
views
Decomposition of k[G]
There's a well-known decomposition of $L^2(G)$, a regular representation of compact complex group Lie $G$, called Peter-Weyl theorem.
Turns out for some reason I automatically think that there is a …
12
votes
Number of irreducible representations
It's not entirely trivial, but here's the sketch of the proof if you would like to finish it yourself. Note that people usually denote complex numbers by $\mathbb C$:
(1) Every irreducible represe …
9
votes
1
answer
2k
views
Explanation for Satake correspondence
Some time ago I was told there's an interesting classical Satake correspondence which I will write as
$$[\mathop{\mathrm{disk}} \Rightarrow G] \,\backslash\, [\mathop{\mathrm{disk}^\times} \Rightarr …
7
votes
3
answers
3k
views
Beilinson-Bernstein and Koszul duality
For geometric representation theorists down here.
Consider the Beilinson-Bernstein theorem:
Functor of global sections establishes
the correspondence between twisted
D-modules with fixed twi …
5
votes
Number of faithful representations of a finite group
I haven't heard this question before, but I would approach it in a following way: you know some basic relations between irreducible representations of a groups — e.g. their # is the # of conjugacy cl …
5
votes
0
answers
320
views
Real representations of G = those of Langlands dual and maps of a cylinder
There is a result about the real representations of a simple Lie group $G$ which is known as Soergel conjecture or Vogan duality. We'll focus on the formulation that for a fixed character of $G$
$\cup …
4
votes
3
answers
562
views
Functions on hyperbolic space and modular curves
The decomposition of $L^{2}\left(S^{2}\right)$ under $SO\left(3,\mathbb{R}\right)$ is well-known.
Focus now on the hyperbolic plane $H$ presented as the quotient $SL\left(2,\mathbb{R}\right)/SO\left( …
3
votes
sl(2)-modules...
You can't assume the module is irreducible, since many aren't!
However, if you want to learn something about modules of $sl_2$ it helps to make the following observations:
Each module is a sum of i …
3
votes
1
answer
419
views
Categorifying the group representations
I've heard about this construction on the lecture about higher representation theory:
Given a Lie algebra $g$, one constructs $\mathcal A$, a category whose $K_0$ is the universal enveloping algeb …
3
votes
Learning about Lie groups
There are many courses, including something about Lie groups at J.Milne's page: jmilne.org
2
votes
How to compute irreducible representation of Lie algebra in the framework of BBD
There could be different ways to give meaning to the phrase "explicit construction".
In an algebro-geometric sense, an expicit construction comes from more classical Borel-Weil-Bott theorem of which …
1
vote
Homology of algebraic varieties in Okounkov's paper on enumerating algebraic curves
It appears that you want the "rough, introductory answers", so feel fine to ask more detailed questions if anything is unclear/potentially wrong. Also, I only have very general knowledge about enumera …
1
vote
1
answer
272
views
Character theory over integers
This question comes from my notes, heavily edited, thus slightly unusual structure.
For Lie groups one can reformulate character theory as saying that
C ⊗ K(G\ pt) = C[T/W] = C[ X* ]W
where …