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Questions about the branch of algebra that deals with groups.

73 votes
9 answers
9k views

What are "classical groups"?

Unlike many other terms in mathematics which have a universally understood meaning (for instance, "group"), the term classical group seems to have a fuzzier definition. Apparently it originates with …
Jim Humphreys's user avatar
46 votes
2 answers
7k views

Definition of "finite group of Lie type"?

The list of finite simple groups of Lie type has been understood for half a century, modulo some differences in notation (and identifications between some of the very small groups coming from differen …
Jim Humphreys's user avatar
35 votes
3 answers
2k views

Is there any need to study Coxeter systems (W,S) with S infinite?

In their treatise Groupes et algebres de Lie, Bourbaki (no doubt heavily influenced by Tits) devoted Chapter IV (1968) to the general theory of what they dubbed "Coxeter systems" $(W,S)$ along with "T …
Jim Humphreys's user avatar
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
28 votes
Accepted

About unpublished lecture notes of Philip Hall

It may help to have some explicit bibliographic references, though some items are by now out of print and may be difficult to locate even through libraries. First, the 1966 paper by Gorenstein is lo …
Jim Humphreys's user avatar
24 votes
2 answers
609 views

Uniform proof that a finite (irreducible real) reflection group is determined by its degrees?

Given a finite (irreducible real) group $G$ generated by reflections acting on euclidean $n$-space, it was shown by Chevalley in the 1950s that the algebra of invariants of $G$ in the associated polyn …
Jim Humphreys's user avatar
23 votes

Isomorphic general linear groups implies isomorphic fields?

The answer to the question is yes, though I don't have all the old literature at my fingertips. This kind of question for various classes of linear groups has a long history in the study of homomorphi …
Jim Humphreys's user avatar
22 votes
4 answers
4k views

How did "Ore's Conjecture" become a conjecture?

The narrow question here concerns the history of one development in group theory, but the broader context involves the sometimes loose use of the term "conjecture". This goes back to older work of …
Jim Humphreys's user avatar
20 votes
Accepted

Small-index subgroups of SL(3,Z)

To fill in the comments, there are basically two serious issues involved. 1) You want to know that every subgroup of finite index in $\mathrm{SL}(3,\mathbb{Z})$ contains some congruence kernel: the k …
Jim Humphreys's user avatar
18 votes
0 answers
516 views

Applications of the surjectivity of Brauer's decomposition map over arbitrary fields?

Recently I've been going over some of Serre's reformulation of Brauer theory with a student, following the influential treatment in Part III of Serre's lectures (revised 1971 French edition) later pub …
Jim Humphreys's user avatar
15 votes
Accepted

Motivation behind the construction of Deligne and Lusztig

It's not easy to explain the motivation without being one of the authors, but in fact Lusztig has provided some helpful perspective on the writing of his joint paper with Deligne (1976) and his earlie …
15 votes
Accepted

Affine Weyl groups as Coxeter groups

In the abstract Bourbaki set-up, the affine Weyl group is defined to be a semidirect product of an irreducible Weyl group with its coroot lattice. This is naturally a Coxeter group, characterized in t …
Jim Humphreys's user avatar
13 votes
Accepted

Failure of a basic fact from Representation Theory

To reinforce Geoff's answer, I'd emphasize that the "fairly standard exercise" mentioned in the question is only standard when you consider characteristic 0 irreducible representations as is usually d …
Jim Humphreys's user avatar
13 votes
1 answer
1k views

Convenient reference for subgroups of a finite semidirect product?

Given a finite group $G= H \ltimes N$ (with no particular constraints on $H, N$), it's probably been known for a long time how to describe efficiently the possible subgroups of $G$. A graduate stude …
Jim Humphreys's user avatar
13 votes
4 answers
3k views

What is a "block" in an abelian category?

In the literature and in some posts here, there has been variation in the undefined use of the term "block" for a category of modules over a ring, or more abstractly an abelian category (all of which …
Jim Humphreys's user avatar

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