## The Category of Representations of a Group

Sorry for missing yesterday. I had this written up but completely forgot to post it while getting prepared for next week’s trip back to a city. Speaking of which, I’ll be heading off for the week, and I’ll just give things here a rest until the beginning of December. Except for the Samples, and maybe an I Made It or so…

Okay, let’s say we have a group . This gives us a cocommutative Hopf algebra. Thus the category of representations of is monoidal — symmetric, even — and has duals. Let’s consider these structures a bit more closely.

We start with two representations and . We use the comultiplication on to give us an action on the tensor product . Specifically, we find

That is, we make two copies of the group element , use to act on the first tensorand, and use to act on the second tensorand. If and came from actions of on sets, then this is just what you’d expect from linearizing the product of the -actions.

Symmetry is straightforward. We just use the twist on the underlying vector spaces, and it’s automatically an intertwiner of the actions, so it defines a morphism between the representations.

Duals, though, take a bit of work. Remember that the antipode of sends group elements to their inverses. So if we start with a representation we calculate its dual representation on :

Composing linear maps from the right reverses the order of multiplication from that in the group, but taking the inverse of reverses it again, and so we have a proper action again.

[...] 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. [...]

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