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Questions about the branch of algebra that deals with groups.

24 votes
Accepted

Does every finite nontrivial group have two distinct irreducible representations over the co...

It seems that the answer is yes. A MathSciNet search brought up the paper Y. Berkovich, D. Chillag, and M. Herzog, Finite groups in which the degrees of the nonlinear irreducible characters are di …
Faisal's user avatar
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5 votes

Permutation representation inner product

Here's another proof. If $G$ acts transitively on $X$, then the permutation representation of $G$ is induced from the permutation representation of the stabilizer $S$ of an arbitrary point. As a resu …
Faisal's user avatar
  • 10.3k
11 votes
Accepted

Is SL_n of an order in a number ring finite-index in SL_n of the number ring?

Yes. If $f = (\mathcal O : R)$, then $f\mathcal O \subset R$ and consequently $\text{SL}_n(R)$ contains $\ker (\text{SL}_n(\mathcal O) \to \text{SL}_n(\mathcal O/f\mathcal O))$, which has finite index …
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11 votes
Accepted

Many p,q-Sylow subgroups

The answer is no: see Corollary 1.3 in Robert M. Guralnick; Gunter Malle; Gabriel Navarro, Self-normalizing Sylow subgroups, Proc. Amer. Math. Soc. 132 (2004), 973-979.
Faisal's user avatar
  • 10.3k
21 votes

Lie group examples

Here are three examples. A finite field extension of $\mathbb R$ must be quadratic. For if $\mathbb R^n$ carries the structure of a field, then its group of units $\mathbb R^n - \{0\}$ is an abelian …
8 votes

Locally compact abelian groups

Corollary 7.54 in Hoffman and Morris, The Structure of Compact Groups, seems to be what you want.
Faisal's user avatar
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20 votes
Accepted

Unitary representations of the ax+b group: an accessible presentation

I know of two clean approaches to classifying the unitary irreps of the $ax+b$ group. The first is to write the group as a semidirect product $\mathbb R \ltimes \mathbb R_{>0}$. There is a theory (due …
Faisal's user avatar
  • 10.3k
5 votes

Are there any nontrivial ring homomorphisms $M_{n+1}(R)\rightarrow M_n(R)$?

We can also rule out the case of commutative $R$ by appealing to the Artin–Procesi theorem: an Azumaya algebra of constant rank $(n+1)^2$ (e.g. $M_{n+1}(R)$) satisfies all the $\mathbb Z$-multilinear …
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13 votes

determinant of the table of characters

If $A$ is the character table and $A^\ast$ is its conjugate transpose, then the orthogonality relations tell us that $A A^\ast = \text{diag}\{|C_G(g)|\} $, where the enties run over a fixed choice of …
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6 votes
Accepted

Product of conjugacy classes - is there an analog of Tanaka-Krein reconstruction ?

The answer to your first question is negative. For a concrete example, you can show that the conjugacy class rings of the nonisomorphic groups $Q_8$ and $D_8$ are isomorphic, via an isomorphism that p …
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26 votes
Accepted

Is a reductive adelic group a Type I group?

I believe the answer is yes. Let's begin by recalling that if one wants to show that a locally compact group $G$ is of type I, it suffices to show that $G$ contains a "large" compact subgroup $K$, in …
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