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Questions about the branch of algebra that deals with groups.

3 votes

Group cohomology of modular representations for finite groups of Lie type

As Derek Holt comments, cohomology has complications even for fimite general linear groups. Probably you are using the term "reductive" too casually and should replace it by "simple" or perhaps "semis …
Jim Humphreys's user avatar
4 votes

Diagonal automorphisms for twisted Chevalley groups

First of all, I'd inquire what role the characteristic of the field plays here. It's true that the finite twisted groups rely on characteristics 2, 3 especiallu, but infinite twisted groups include …
Jim Humphreys's user avatar
7 votes
Accepted

For $G$ an adjoint Chevalley group, are all of $G(\mathbb Z)$'s finite-index subgroups congr...

Chapter VI of my old Springer Lecture Notes in Mathematics 789 Arithmetic Groups (1980) is in English and gives a version of H. Matsumoto's and C. Moore's arguments for the subgroups of $\operatorname …
Jim Humphreys's user avatar
3 votes

How to prove that Chevalley groups over $\mathbb R$ have no compact factors

There are probably multiple ways to see that $G(\mathbb{R})$ is non-compact when $G$ is a Chevalley group (in either the narrow sense of Chevalley or the broader sense of Steinberg's lectures). One …
Jim Humphreys's user avatar
2 votes

Character of a semisimple connected Lie groups

In the original sense, Chevalley groups are generated by copies of the additive group of the field and are in fact simple as abstract groups if the field is not too small. (This was the motive for …
Jim Humphreys's user avatar
2 votes

Coordinates of the Weyl vector of $E_8$ (and the 135 classes of $W(E_8)/W(D_8)$)

Concerning the first question in the header (and some of your preparatory remarks), it's useful to keep in mind the Planche VII for $E_8$ at the end of Chapters 4-6 of Bourbaki's treatise Groupes et a …
Jim Humphreys's user avatar
2 votes

Is the size of a conjugacy class in a finite classical group a polynomial?

I agree that the question needs a better formulation. In any case, an approach by Demetris Deriziotis might be useful because it's based on a different kind of analysis: see here (freely availab …
Jim Humphreys's user avatar
1 vote

Regular semisimple elements in $SL(n,q)$

This is just a comment but in community wiki format. Most studies of semisimple (or reductive) algebraic groups and finite groups of Lie type emphasize counting the number of classes of various el …
4 votes
Accepted

Conjugacy classes of symplectic group $\mathrm{Sp}(4,q)$

Keep in mind that it's tricky to assemble a character table for all finite groups of Lie type in a family, and in particular the thesis work by Srinivasan at Manchester (supervised by J.A. Green) excl …
Jim Humphreys's user avatar
5 votes

Module with indecomposable and decomposable reductions mod $p$

[EDIT] Since my various edits got quite long, maybe I can answer the question more directly and refer to previous versions for elaboration. A key elementary result can be found in Feit's 1982 monogra …
Jim Humphreys's user avatar
7 votes
1 answer
470 views

Centralizer of longest element in a finite irreducible Weyl group: related to folding of ADE...

Say $(W,S)$ is a finite Coxeter group, such as a Weyl group (which satisfies an additional crystallographic condition). Assume also that $W$ is irreducible. Then it has a longest element $w_o$ r …
Jim Humphreys's user avatar
6 votes

Dimensions of irreducible representations of $GL(n,F_q)$ are polynoms in q having roots ONLY...

You are asking many questions here, about which much is written down in the rather large literature of the subject. Maybe I can point to some of the answers. In the 1955 paper of J.A. Green on fini …
Jim Humphreys's user avatar
3 votes

Reference Request: Derived group of $\mathscr R_u(B)$

Let me add a few comments in community-wiki format. There doesn't seem to be a convenient reference, apart from the one in Digne-Michel which Jay Taylor cites. But even here, the authors don't give …
6 votes

Irreducible representations of $\text{SL}(2, \mathbb{F}_q)$ which don't exist in decomposition?

I'll add an overlong comment to provide more perspective, in community-wiki format. The question is probably a bit misguided, even taken as a purely pedagogical one (the older mathematics involved h …
30 votes
0 answers
997 views

Follow-up to Steinberg's problem (12) in his 1966 ICM talk?

Steinberg's lecture at the 1966 ICM in Moscow here surveyed his work on regular elements of semisimple algebraic groups, while also formulating a number of then-open questions as "problems" (with posi …
Jim Humphreys's user avatar

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