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More Lie algebra constructions
*July 28, 2009*

*Posted by Akhil Mathew in algebra, representation theory.*

Tags: bilinear forms, Lie algebras, Lie groups, representation theory

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Tags: bilinear forms, Lie algebras, Lie groups, representation theory

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The ultimate aim in the series on Lie algebras I am posting here is to cover the representation theory of semisimple Lie algebras. To get there, we first need to discuss some technical tools—for instance, invariant bilinear forms.

** Generalities on representations **

Fix a Lie algebra . Given representations , we clearly have a representation ; given a morphism of representations , i.e. one which respects the action of , the kernel and image are themselves representations.

Proposition 1The category of finite-dimensional representations of is an abelian category.

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Lie’s Theorem II
*July 27, 2009*

*Posted by Akhil Mathew in algebra, representation theory.*

Tags: eigenvalues, Lie algebras, Lie's theorem, linear algebra, representation theory, solvability

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Tags: eigenvalues, Lie algebras, Lie's theorem, linear algebra, representation theory, solvability

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Yesterday I was talking about Lie’s theorem for solvable Lie algebras. I went through most of the proof, but didn’t finish the last step. We had a solvable Lie algebra and an ideal such that was of codimension one.

There was a finite-dimensional representation of . For , we set

We assumed for some by the induction hypothesis. Then the following then completes the proof of Lie’s theorem, by the “fundamental calculation:”

Lemma 1If , then .

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Lie’s Theorem I
*July 26, 2009*

*Posted by Akhil Mathew in algebra, representation theory.*

Tags: derived series, Lie algebras, Lie's theorem, solvability

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Tags: derived series, Lie algebras, Lie's theorem, solvability

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I talked a bit earlier about nilpotent Lie algebras and Engel’s theorem. There is an analog for *solvable* Lie algebras, and the corresponding Lie’s theorem.

So, first the definitions. Solvability is similar to nilpotence in that one takes repeated commutators, except one uses the *derived series* instead of the lower central series.

In the future, fix a Lie algebra over an algebraically closed field of characteristic zero.

Definition 1Thederived seriesof is the descending filtration defined by . The Lie algebra issolvableif for some .

For instance, a nilpotent Lie algebra is solvable, since if is the lower central series, then for each .

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The enveloping algebra
*July 25, 2009*

*Posted by Akhil Mathew in algebra.*

Tags: algebra, enveloping algebras, Lie algebras, tensor algebras, universal properties

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Tags: algebra, enveloping algebras, Lie algebras, tensor algebras, universal properties

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As we saw in the first post, a representation of a finite group can be thought of simply as a module over a certain ring: the group ring. The analog for Lie algebras is the enveloping algebra. That’s the topic of this post.

** Definition **

The basic idea is as follows. Just as a representation of a finite group was a group-homomorphism for a vector space, a representation of a Lie algebra is a Lie-algebra homomorphism . Now, is the Lie algebra constructed from an associative algebra, —just as is the group constructed from taking invertible elements.

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Engel’s Theorem and Nilpotent Lie Algebras
*July 23, 2009*

*Posted by Akhil Mathew in algebra, representation theory.*

Tags: algebra, Engel's theorem, Lie algebras, linear algebra, nilpotent

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Tags: algebra, Engel's theorem, Lie algebras, linear algebra, nilpotent

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Now that I’ve discussed some of the basic definitions in the theory of Lie algebras, it’s time to look at specific subclasses: nilpotent, solvable, and eventually semisimple Lie algebras. Today, I want to focus on nilpotence and its applications.

** Engel’s Theorem **

To start with, choose a Lie algebra for some finite-dimensional -vector space ; recall that is the Lie algebra of linear transformations with the bracket . The previous definition was in terms of matrices, but here it is more natural to think in terms of linear transformations without initially fixing a basis.

Engel’s theorem is somewhat similar in its statement to the fact that commuting diagonalizable operators can be simultaneously diagonalized.

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Lie algebras II
*July 20, 2009*

*Posted by Akhil Mathew in algebra.*

Tags: algebra, general theory, Lie algebras, linear algebra, quotients

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Tags: algebra, general theory, Lie algebras, linear algebra, quotients

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I’m going to get back eventually to the story about finite-dimensional modules, but for now, Lie algebras are more immediate to my project, so I’ll talk about them here.

From an expository standpoint, jumping straight to basically right after defining Lie algebras was unsound. I am going to try to motivate them here and discuss some theorems, to lead into more of the general representation theory.

** Derivations **

So let’s consider a not-necessarily-associative algebra over some field . In other words, is a -vector space, and there is a -bilinear map , which sends say , but it doesn’t have to either be commutative or associative (or unital). A Lie algebra with the Lie bracket would be one example.

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Representations of sl2, Part II
*July 18, 2009*

*Posted by Akhil Mathew in algebra, representation theory.*

Tags: highest weights, Lie algebras, linear algebra, representation theory, sl2

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Tags: highest weights, Lie algebras, linear algebra, representation theory, sl2

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This post is the second in the series on and the third in the series on Lie algebras. I’m going to start where we left off yesterday on , and go straight from there to classification. Basically, it’s linear algebra.

** Classification **

We’ve covered all the preliminaries now and we can classify the -representations, the really interesting material here. By Weyl’s theorem, we can restrict ourselves to irreducible representations. Fix an irreducible .

So, we know that acts diagonalizably on , which means we can write

where for each , i.e. is the -eigenspace.

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Representations of sl2, Part I
*July 17, 2009*

*Posted by Akhil Mathew in algebra, representation theory.*

Tags: algebra, Jordan decomposition, Lie algebras, representation theory, semisimplicity, sl2

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Tags: algebra, Jordan decomposition, Lie algebras, representation theory, semisimplicity, sl2

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is a special Lie algebra, mentioned in my previous post briefly. It is the set of 2-by-2 matrices over of trace zero, with the Lie bracket defined by:

The representation theory of is important for several reasons.

- It’s elegant.
- It introduces important ideas that generalize to the setting of semisimple Lie algebras.
- Knowing the theory for is useful in the proofs of the general theory, as it is often used as a tool there.

In this way, is an ideal example. Thus, I am posting this partially to help myself learn about Lie algebras.

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Lie algebras: fundamentals
*July 16, 2009*

*Posted by Akhil Mathew in algebra, representation theory, Uncategorized.*

Tags: algebra, Lie algebras, representation theory

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Tags: algebra, Lie algebras, representation theory

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The following topic came up in the context of my project, which has been expanding to include new areas of mathematics that I did not initially anticipate. Consequently, I have had to learn about several new areas of mathematics; this is, of course, a common experience at RSI. For me, the representation theory of Lie algebras has been one of those areas, and I will post here about it to help myself understand it. Right here, I’ll aim to cover the groundwork necessary to get to the actual representation theory in a future post.

** Lie Algebras **

Throughout, we work over , or even an algebraically closed field of characteristic zero.

Definition 1ALie algebrais a finite-dimensional vector space with aLie bracketsatisfying:

The bracket is -bilinear in both variables.for any .. This is theJacobi identity.

To elucidate the meaning of the conditions, let’s look at a few examples. (more…)