## Some Representations of the General Linear Group

Sorry for the delays, but it’s the last week of class and everyone came back from the break in a panic.

Okay, let’s look at some examples of group representations. Specifically, let’s take a vector space and consider its general linear group .

This group comes equipped with a representation already, on the vector space itself! Just use the identity homomorphism We often call this the “standard” or “defining” representation. In fact, it’s easy to forget that it’s a representation at all. But it is.

As with any other group, we have dual representations. That is, we immediately get an action of on . And we’ve seen it already! When we talked about the coevaluation on vector spaces we worked out how a change of basis affects linear functionals. What we found is that if is our action on , then the action on is by the transpose — the dual — of . And this is exactly the dual representation.

Also, as with any other group, we have tensor representations — actions on the tensor power for any number of factors of . How does this work? Well, every vector in is a linear combination of vectors of the form , where each . And we know how to act on these: just act on each tensorand separately. That is,

Then we just extend this action by linearity to all of .

Out of curiosity: where are you heading with this? Were you planning to discuss Schur-Weyl duality?

Comment by Todd Trimble | December 3, 2008 |

Actually, I’m going to mention it tomorrow. But I’m not going to actually prove it, or use it as such. What I

amgoing to do is introduce a particular representation that many readers may already be familiar with, but without picking a basis!Comment by John Armstrong | December 3, 2008 |

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