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The Weyl group of a root system is a subgroup generated by reflections through the hyperplanes orthogonal to the roots.

17 votes

Polynomial invariants of the exceptional Weyl groups

Keeping in mind that a generating set of invariant polynomials having the required degrees is not unique, various computations have been recorded in the literature. Those I was aware of before 1990 …
Jim Humphreys's user avatar
16 votes
Accepted

About the intrinsic definition of the Weyl group of complex semisimple Lie algebras

Probably the earliest intrinsic definition of Weyl group occurs in section 1.2 of the groundbreaking paper "Representations of Reductive Groups Over Finite Fields" by Deligne and Lusztig (Ann. of Math …
Jim Humphreys's user avatar
16 votes

Longest element of Weyl groups

EDIT: This is a belated attempt (motivated by a question from Yongjun Xu) to answer the question more precisely than I did at first, with more emphasis on careful choice of a Coxeter element when the …
Jim Humphreys's user avatar
12 votes
0 answers
416 views

Reference for class of involutions containing longest element of finite Coxeter group?

Consider a finite (say irreducible) Coxeter group $W$ with a fixed generator set $S$ and rank $n$. This is the same thing as a finite real reflection group, generated by a set of “simple” reflectio …
Jim Humphreys's user avatar
11 votes
Accepted

Does -I belong to Weyl group?

As Koen S points out, the longest element of an irreducible Weyl group is treated in an earlier question (in fact, it comes up in several questions). The question asked here presupposes a standard li …
Jim Humphreys's user avatar
9 votes
2 answers
687 views

Reference for embeddings of reflection groups (related to folding ADE Coxeter graphs)?

There are a couple of indirect methods, using Lie theory or Springer's general theory of regular elements in (real, complex) reflection groups, to construct natural embeddings among certain Weyl group …
Jim Humphreys's user avatar
8 votes

Longest element of a Weyl group

The role of the longest element in $W$ emerges only gradually in the Chevalley structure theory. This is developed similarly but in slightly different styles in the three books with the same title Li …
Jim Humphreys's user avatar
8 votes

Cells in affine Weyl groups

You are asking a number of related questions here, most of which require more reading of Lusztig's papers. See the reference list in my conference paper here, for example. But note first that the n …
Jim Humphreys's user avatar
7 votes

Weyl groups of $E_6$ and $E_7$

It's important to emphasize that the simple group here is typically isomorphic to the rotation subgroup of $W$, which has index 2 and doesn't contain the reflections. So you need to look at the prec …
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
3 answers
977 views

Occurrences of a simple reflection in the longest element of a Weyl group?

While looking at a preprint I've just bumped into a question about the longest element $w_0$ of a Weyl group $W$ (say irreducible of a Lie type $A$ - $G$ and of rank $n>1$, to simplify). Suppose t …
Jim Humphreys's user avatar
5 votes

Conjugacy of Regular Semisimple Subalgebras

It would help to recall Dynkin's definition of "regular semisimple subalgebra" of a complex semisimple Lie algebra (say $\mathfrak{g}$), which has not become standard in later literature. This just …
Jim Humphreys's user avatar
4 votes

Weyl group elements fixing a set of simple roots

I'm not sure whether there is an efficient way to answer your question (or a written reference), but it's possible to analyze the situation case-by-case. It's probably best to start with an irreducib …
Jim Humphreys's user avatar
4 votes

Reference for embeddings of reflection groups (related to folding ADE Coxeter graphs)?

Rather than overload the question with side remarks, I'll add some background here in community-wiki format to indicate what can be gotten from Springer's relatively elementary treatment of regular el …
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

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