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Linear representations of algebras and groups, Lie theory, associative algebras, multilinear algebra.
3
votes
Accepted
On a characterization of the Gårding subspace of the left regular representation of reals
Yes, your desired equality is true: regarding the left regular representation as
$$
\operatorname{Ind}_{\{0\}}^{\mathbf R}1,
$$
it becomes a special case of the characterization of smooth vectors in i …
2
votes
"Nice" basis for highest-weight irreducible module of a simple Lie algebra
I am not an expert, but it seems that Geck's constructions specialize the Lusztig-Kashiwara "canonical" or "crystal" bases of highest weight modules. These also have a Littelmann "path model", related …
5
votes
Accepted
The annihilator of a Borel subalgebra being its nilpotent radical
As in Chriss-Ginzburg p. 130, pick a regular semisimple $h\in\mathfrak h$ (Cartan subalgebra) and write $\mathfrak g_a$ for the eigenspace of $\text{ad}_h$ belonging to eigenvalue $a$. Invariance of t …
2
votes
Accepted
Is it necessary for $\pi:H\to U(\mathcal{H}_{\pi})$ to be a homomorphism in order for $\text...
I don't believe that weak continuity is "the only place in which $\pi$ being a homomorphism plays a role".
In fact, before you can even talk about weak continuity of the resulting representation, you …
0
votes
Representations of the two dimensional non-abelian Lie algebra
Quoth Dixmier, Enveloping algebras, p. xii: "But a deeper study reveals the existence of an enormous number of irreducible representations of [the 3-dimensional Heisenberg algebra]. It seems that thes …
5
votes
Accepted
Measurable representations of semi simple Lie groups
This is true and due to Béla von Szőkefalvi-Nagy, Über meßbare Darstellungen Liescher Gruppen (1936). Generalized to finite-dimensional representations of locally compact groups in A. Weil, L'intégrat …
1
vote
How does one show the existence of discrete and complementary series for SL(2,R)?
For complementary series, I'd recommend §V.4 of Sugiura (available on the internet) for a very careful description of their embedding in nonunitary principal series. In particular, Prop. 4.6 explains …
12
votes
Accepted
What is the name of the following theorem: dimension of complex irreducible representation d...
Georg Frobenius, Über die Primfactoren der Gruppendeterminante, Sitzungsber. Akad. Berlin (1896) 1343-1382. The theorem is announced at the beginning, p. 1344:
Der Grad $f$ ist ein Divisor der Ord …
5
votes
Accepted
Fell topology vs. convergence of matrix coefficients
In your inequality you need to evaluate the $\pi$'s somewhere!
Unless I am mistaken unpacking Fell (1962), Theorem 2.2 and Remark following, $[\pi_j]\to[\pi]$ means that for every choice of an $\vare …
8
votes
Introduction to representation theory of algebraic groups
I would suggest Procesi's Lie Groups, as a text that introduces algebraic groups with minimal prerequisites. Chapter 7 "Algebraic Groups" is
a quick introduction to algebraic groups. In this chapt …
28
votes
Why all irreducible representations of compact groups are finite-dimensional ? [EDIT: Subtle...
(Addressing only the title question.) There is a short proof avoiding Peter-Weyl and the theory of compact operators. It is due to Nachbin and is reproduced in Hewitt-Ross, Abstract Harmonic Analysis …
2
votes
Unitary representations of SO(1,4) and SO(2,3)
The unitary dual of SO(1,4) was computed by Dixmier (1961); that of SO(2,3), or the locally isomorphic Sp(2,R), by Angelopoulos (1981) and Nzoukoudi (1983).
3
votes
Accepted
Induced representation of locally compact groups
Such a $\Phi$ is not going to exist — nor is $H$ going to have nonzero $\mu(H)$ — unless $H$ is open as well as closed in $G$. This is the very case treated in Mackey (1951, Part II): then $G/H$ is di …
1
vote
Plancherel expansion for Spin(n-1,1)
Knapp (1986, p. 736) attributes the Plancherel formula for real-rank-one groups to Okamoto (1965), Hirai (1966), and Harish-Chandra (1966). More details in Sally-Warner (1973), Miatello (1979).
11
votes
When are two subvarieties of matrices conjugate?
I may be completely off-base, and would be happy to be proved wrong, but I believe you are veering close to problems that are reputedly intractable. Namely, to simplify your simplest example even furt …