# Learning Roadmap for Borel - Weil - Bott Theorem

Next semester I may study a course where the ultimate goal is to get to the Borel - Weil - Bott (BWB) Theorem, if not at least try to understand it in the case that we have $G = \text{SL}_n$. I have studied some representation theory of Lie groups from Brian Hall's book Lie groups, Lie algebras and Representations. I am ok with highest weight theory for $\mathfrak{sl}_n$ and I have also studied some Schur - Weyl duality and classification of irreps of $\mathfrak{sl}_n$ from Fulton and Harris.

My question is: What would a learning roadmap for understanding the BWB Theorem be?

I was told by the lecturer we would probably start out by looking at line bundles over $\Bbb{P}^1$. Now I don't mind if there is no single reference/book to read linearly that I have to look into. Though, I don't know how one would build up one's background to get to the theorem. I don't mind if I have to learn things like sheaf cohomology and the like on the way.

If it helps, I have also studied differential geometry and am familiar with the material in chapters 1-5,7,8,11,14,16 of Lee's Smooth Manifolds, second edition.

The two, broadly defined things you need to know are Lie theory and (complex) differential geometry. The specific things from each topic are

Lie groups/algebras

Highest weight theory of compact Lie groups/complex semisimple Lie groups. One needs to build up to the theorem that irreducible complex representations of a Lie algebra are parametrized by dominant positive weights.

Most books (including Brain Hall's) build up to this. I personally like Compact Lie Groups by Sepanski if one already understand the basics of manifolds, as its very concise, has good exercises, and concludes with a proof of the Borel-Weil theorem.

Differential geometry

One needs to know what a holomorphic vector bundle over a complex manifold is and their Dolbeault cohomology. Thus the following general differential geometry background is needed, all of which the last can be found in Lee's Smooth Manifolds

• Vector bundles over a manifold (I think Lee mostly talks about real vector bundles but complex ones are just replacing $\mathbb R$ with $\mathbb C$).

• Differential forms.

• de Rham theory.
• Connections on vector bundles (maybe not necessary but I think it's useful).

After having a solid differential geometry background, in my opinion the best place to learn the necessary complex geometry is part 1 of the freely available notes by Moroianu, titled Lectures on Kahler Geometry.

For the Borel-Weil-Bott theorem proper, I would see Sepanski's text and also the paper Representations in Dolbeault Cohomology by Zierau.

• Thanks for your answer. I went to the library today to borrow Sepanski's book on Lie groups. I will also read up on vector bundles from my copy of Lee's book. – user38268 Jun 7 '13 at 15:32