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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 …
Ilya Nikokoshev's user avatar
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 …
Ilya Nikokoshev's user avatar
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 …
Ilya Nikokoshev's user avatar
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 …
Ilya Nikokoshev's user avatar
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 …
Ilya Nikokoshev's user avatar
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 …
Ilya Nikokoshev's user avatar
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 …
Ilya Nikokoshev's user avatar
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 …
Ilya Nikokoshev's user avatar
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( …
Ilya Nikokoshev's user avatar
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 …
Ilya Nikokoshev's user avatar
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 …
Ilya Nikokoshev's user avatar
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 …
Ilya Nikokoshev's user avatar
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 …
Ilya Nikokoshev's user avatar
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 …
Ilya Nikokoshev's user avatar

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