# Roadmap to study Atiyah-Singer index theorem

I am a physics undergrad and want to pursue a PhD in Math (geometry or topology). I study it almost completely by myself, as the program in my country offers very less flexibility to take non departmental courses, thought I will be able to take them is a couple of semesters. Anyways, So I was thinking about taking a mini-project of sorts which I can self-study or maybe ask a prof. I had heard about this theorem while studying topological solitons in Physics.

What are the prerequisites for studying the Atiyah-Singer theorem. I had a look online, but I couldn't figure out exactly, in what field this is, or what are the prerequisites. Wikipedia says it is a theorem in differential geometry, but obviously what differential geometry I know is insufficient. I know about manifolds, differential forms, lie derivatives, lie groups, and some killing vectors stuff. A look at the material online also talks about some operators in DG which I have never come across. Are there an algebraic topology prerequisites?

Please could you recommend me some references which can take me there?

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You should get a copy of Bertlmann's Anomalies in QFT textbook. See books.google.com/books/about/… .As a physics student it should appeal to you and there is a great swath of math covered at a very speedy pace. – James S. Cook Feb 5 '13 at 2:45
Seminar on the Atiyah-Singer Index Theorem is certainly a good source. I didn't read it in full (I only read the first few chapters) and I remember it contains a very good account on Fredholm theory – Olivier Bégassat Feb 5 '13 at 2:50
@JamesS.Cook: Hey. That book looks really cool! Thanks. – user23238 Feb 5 '13 at 11:09

The Atiyah-Singer index theorem involves a mixture of algebra, geometry/topology, and analysis. Here are the main things you'll want to understand to be able to know what the index theorem is really even saying.

1. Algebra: The most important concept here is Clifford algebras. For example, Dirac operators arise from combining covariant differentiation and Clifford multiplication. You'll also want to learn about the associated spin groups.

2. Geometry/Topology: The most fundamental idea here is understanding vector bundles. The Atiyah-Singer index theorem is about elliptic differential operators between sections of vector bundles, so you won't get anywhere without a firm understanding of bundles. Next, you'll want to understand the basics of spin geometry and Dirac operators, especially if your interests are more physics-based. One nice form of the index theorem is the cohomological formula for the index in terms of characteristic classes. I would advise you to get familiar with cohomology and obtain a basic knowledge of characteristic classes (Chern-Weil theory is nice if you already have a geometry background). If you want to understand the original index theorem or how the cohomological formula for the index is derived from it, you'll need to learn some topological K-theory.

3. Analysis: Here the big topic is differential operators in the context of manifolds. Hence you'll want to know what a differential operator between vector bundles is. You'll need to know what the symbol of such a differential operator is, and also what it means for such a differential operator to be elliptic.

As for a reference, the classic text is Spin Geometry by Lawson and Michelsohn. An easier but less detailed introduction can be found in the relevant sections of Geometry, Topology, and Physics by Nakahara. There's a fair amount of other books but these are the two I know best. There are some good notes on spin geometry here. These notes by Nicolaescu may also be of interest to you.

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+1 Very detailed. Good references and links. – Jesse Madnick Feb 5 '13 at 3:08
@Henry: Nice answer! Do you know the books by Naber? Would you recommend them over Nakahara or Lawson? – user23238 Feb 5 '13 at 16:39
@ramanujan_dirac: Naber's two volumes of Topology, Geometry, and Gauge Fields (which I assume you are talking about?) don't cover any index theory. They do discuss some of the preliminaries I listed above, but the development is quite slow and elementary. I think you're better off with Nakahara for general background and a quick introduction and Lawson and Michelsohn for the full treatment. – Henry T. Horton Feb 5 '13 at 16:44
@HenryT.Horton: Ok Though I think Naber, is much more detailed about the elementary topics than Nakahara. I find Nakahara very superficial.BTW, +1 – user23238 Feb 6 '13 at 2:32

There are two main approaches to the index theorem: $K$-theory and heat kernels. The latter approach treats the index of Dirac operators, which is actually more general than it sounds since for common situations all operators (in terms of index theory) are twisted Dirac operators.

Based on your background I would recommend the heat kernel approach since it is more geometric, physicsy, and you'd have to learn a decent amount of algebraic topology for the $K$-theory approach.

There is some algebraic topology that enters into both approaches and this is the theory of characteristic classes. In the heat kernel approach, one takes the Chern-Weil viewpoint, which is pretty easy to pick up as long as you are familiar with de Rham cohomology.

I found the most useful books to be Berline, Getzler, and Vergne's Heat Kernels and Dirac Operators and John Roe's Elliptic Operators, Topology, and Asymptotic Methods.

For the $K$-theory approach (which I think is useful to at least get a bit of an understanding of) I like the original papers as well as expository works of Nigel Higson e.g. Lectures on Operator K-Theory and the Atiyah-Singer Index.

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Paul Loya's Notes are introductory and follow the heat kernel approach. – Robert Cardona Jan 29 '14 at 16:02

There is a new wonderful, big and deep book dealing precisely with everything you need (assuming knowledge of "basic" differential geometry):

These are the same authors of the following old classic, but the new book is NOT a new edition but a completely NEW title (also with modern TeX typeset, figures and additional topics and details):

You may also find in this other answer a very brief overview of the proof of Atiyah-Singer and why pseudo-differential operators are needed. There you will find links to lectures and other titles regarding the theorem.

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Sorry for an unrelated comment, but would you mind to help out a little? Here is a good question regarding the spectral theorem. Would be nice, if you could give some hints on it. math.stackexchange.com/questions/1745355/… – Valery Saharov May 1 at 7:42