# Can we realize Weyl group as a subgroup?

Given a semisimple Lie group G, let T be a maximal torus, W be the Weyl group defined as the quotient N(T)/C(T), where N(T) denotes the normalizer of T and C(T) denotes the centralizer.

Two questions are:

1. How many ways are there we can realize W as a subset of G?

2. Can we always realize W as a subgroup of G?

• This question (the second, more interesting question) will probably need some clarification before it can be cleanly answered. Is $G$ a semisimple real Lie group? Is $T$ a maximal split torus? – Marty Jan 27 '10 at 15:21
• It's worth noting that the questions here make equal sense and have mostly the same answers when the group is assumed to be a (connected) semisimple algebraic group over an arbitrary algebraically closed field. In any case, smaller fields need more discussion. – Jim Humphreys Apr 16 '10 at 18:45

## 1 Answer

In general it is not possible to embed the Weyl group $$W$$ in the group $$G$$: already you can see this for $$SL_2(\mathbb C)$$, where the Weyl group has order $$2$$: if the torus fixes the lines spanned by $$e_1$$ and $$e_2$$ respectively, you want to pick the linear map taking $$e_1$$ to $$e_2$$ and $$e_2$$ to $$e_1$$, but this has determinant $$-1$$. A lift of $$W$$ to $$N(T)$$ must be an element of order $$4$$ not $$2$$, say $$e_1 \mapsto -e_2$$ and $$e_2 \mapsto e_1$$.

In fact, Tits has shown (Normalisateurs de Tores I. Groupes de Coxeter Étendus (Journ. Alg. 4, 1966, pp. 96-116) that this is essentially the only obstruction: the Weyl group can always be lifted to a group $$\tilde{W}$$ inside $$G$$ which is an extension of $$W$$ by an elementary abelian $$2$$-group of order $$2^l$$ where $$l$$ is the number of simple roots. If I recall correctly, this lift is then unique up to conjugation.

• One should point out that Tits' result is for G simply connected. If we use a smaller Lie group, then the rank of the $2$-group drops. I think I recall that, if G is the adjoint form of its Lie algebra, then W always lifts. – David E Speyer Jan 27 '10 at 16:37
• You're right I meant to take $G$ simply connected, but are you sure about the adjoint form case? For $E_8$ for example, the adjoint form is simply connected, so the Tits group can't get any smaller. – Kevin McGerty Jan 27 '10 at 17:03
• My recollection was incorrect. As Kevin McGerty points out, the adjoint and simply connected forms match for E_8; Allen Knutson points out (via e-mail) that SO_5 is another counter-example. – David E Speyer Jan 28 '10 at 18:47
• There is interesting later work on this kind of question for compact Lie groups and analogues, for example by Dwyer and Wilkerson. A follow-up to the paper by Tits ("part I" still searching for its "part II"), with useful references: MR2174268 (2006f:55015) Dwyer, W. G.; Wilkerson, C. W. Normalizers of tori. Geom. Topol. 9 (2005), 1337--1380 – Jim Humphreys Mar 17 '10 at 14:43
• For a bigger picture still, look at: MR2373153 (2009a:55012) 55R35 (20F55 22E40 55P35 57S15) Andersen, K. K. S. (DK-ARHS); Grodal, J. (1-CHI); Møller, J. M. (DK-CPNH); Viruel, A. (E-MAL) The classification of p-compact groups for p odd. Ann. of Math. (2) 167 (2008), no. 1, 95–210 – Jim Humphreys Mar 17 '10 at 14:50