## 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.

[…] from this, we find that the Weyl group of not only acts on itself, but on . Indeed, induces a homomorphism that sends the generator […]

Pingback by The Category of Root Systems « The Unapologetic Mathematician | January 22, 2010 |

You write that:

It’s clear that this defines a homomorphism from the Weyl group into . But it doesn’t seem trivially obvious that this is injective — a priori, couldn’t there be two funny products of ‘s which aren’t equal but agree on elements of ?

Comment by Peter LeFanu Lumsdaine | January 24, 2010 |

That’s a good question, Peter. But there can’t be, because spans the whole vector space. So if any two linear transformations agree on , they must agree on the whole vector space, and thus must be the same transformation.

On the other hand, you might be thinking that some linear transformation in can be written as a product of the in two different ways. This is possible, but it leads to a

relationin the Weyl group. Notice that I never said that was freely-generated (beyond the obvious fact that each generator is an involution).Does that make sense?

Comment by John Armstrong | January 25, 2010 |

Ah! I’d forgotten that spans the space. OK, it’s obvious 🙂 (It was indeed your first interpretation that I meant, not your second.)

In fact, I guess even if we don’t assume that spans the space, they agree on the span of as you say, and they also act trivially on the orthogonal complement of (since each does), so still agree on the whole space.

Comment by Peter LeFanu Lumsdaine | January 25, 2010 |

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[…] that any combination of the serves to permute the coefficients of a given vector. That is, the Weyl group of the system is naturally isomorphic to the symmetric group . Possibly related posts: […]

Pingback by Construction of A-Series Root Systems « The Unapologetic Mathematician | March 2, 2010 |

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Pingback by The Automorphism Group of a Root System « The Unapologetic Mathematician | March 11, 2010 |

[…] a collection of reflections, and these reflections generate a group of transformations called the Weyl group of the root system. It’s one of the most useful tools we have at our disposal through the […]

Pingback by Root Systems Recap « The Unapologetic Mathematician | March 12, 2010 |

“pronounced “vile”, but we don’t mean that as any sort of value judgement”, lovely really enjoyed reading this blog.

Comment by James Pearson | April 2, 2013 |