## The Weyl Group of a Root System

Let’s take a root system in the inner product space . Each vector in gives rise to a reflection in , the group of transformations preserving the inner product on . So what sorts of transformations can we build up from these reflections? The subgroup of generated by the reflections for all is called the Weyl group of the root system. It’s pronounced “vile”, but we don’t mean that as any sort of value judgement.

Anyway, we can also realize as a subgroup of the group of permutations on the vectors in . Indeed, by definition each sends each vector in back to another vector in , and so shuffles them around. So if has vectors, the Weyl group can be realized as a subgroup of .

In particular, is a finite group, as a subgroup of another finite group. In fact, we even know that the number of transformations in divides . It may well (and usually does) have elements which are not of the form , but there are still only a finite number of them.

The first thing we want to take note of is how certain transformations in act on by conjugation. Specifically, if leaves invariant, then it induces an automorphism on that sends the generator to — which (it turns out) is the generator — for all . Further, it turns out that for all .

Indeed, we can calculate

Now, every vector in is of the form for some , and so sends it to the vector , which is again in , so it leaves invariant. The transformation also fixes every vector in the hyperplane , for if is orthogonal to , then the above formula shows that is left unchanged by the transformation. Finally, sends to .

This is all the data we need to invoke our lemma, and conclude that is actually equal to . Specifying the action on the generators of is enough to determine the whole automorphism. Of course, we can also just let act on each element of by conjugation, but it’s useful to know that the generating reflections are sent to each other exactly as their corresponding vectors are.

Now we can calculate from the definition of a reflection

Comparing this with the equation above, we find that , as asserted.