## Update on representation theory notes

After a long hiatus, I have just put up online an updated version of my lecture notes on representation theory. The delay was psychologically interesting: after a long period where I added material more or less in the order I wanted it to appear in the text, I started in June to proceed in much more chaotic (or random?) manner, with an explanation of the Larsen alternative for unitary groups coming before the Peter-Weyl theorem, and so son. Inly in the last few days did the text regain at least some coherence. (In particular, it took me a long time to finally sit up and write an account of the Peter-Weyl theorem that I felt to be at least somewhat motivated.)

There are still things missing before the notes contain all that I’d like, in particular at least a few pages of survey concerning the representation theory of some locally compact, non compact, groups. There are still a few weeks before the beginning of the new semester, however, and hopefully I will have time to do some work on this part in the coming weeks…

As usual, any remarks or corrections will be very appreciated!

## Who was Peter?

The fundamental fact about the representation theory of a compact topological group $G$ is the Peter-Weyl Theorem, which can be described as follows: the regular representation of $G$ on $L^2(G)$ (defined using the probability Haar-measure on $G$) decomposes as an orthogonal Hilbert direct sum of the spaces of matrix coefficients of the finite-dimensional irreducible representations of $G$. (Books, for instance the one of Knapp on semisimple Lie groups, often include more in what is called Peter-Weyl Theory, but this is the statement that is proved in the original paper.)

As I am currently preparing to write down a proof of the Peter-Weyl theorem for my notes on representation theory, I had a look at this paper. Although I probably won’t follow it quite completely, I found it very interesting — it is quite subtly different from all modern treatments I have seen, in an interesting way, and without being much more complicated than what is found, e.g., in Knapp’s book. (For a short masterful online account, this post of Terry Tao is very good; from a search in the same Göttingen archive web site, it seems that — maybe — the modern treatment dates back to Pontryagin, in 1936.)

In any case, the question for today is: Who was Peter? Only the initial “F.” and the affiliation “in Karlsruhe” identifies this coauthor on the original paper (even the “F.” is misread as “P.” on the PDF cover page…) It seem he was a student of Weyl, but note that there is no Peter on the math genealogy page for Weyl. (A joke here at ETH, is that when the lecture room known as the Hermann Weyl Zimmer is renovated this summer, some unfortunate skeletons will be found in the closets and under the floor, or behind the blackboards…)

## Representation theory course

The Spring semester at ETH is starting next week, and I will be teaching an introductory course on representation theory (for third-year and later students). I am looking forward to it, since this is the first time I teach a class identified as “algebra” (except for linear algebra, of course).

My lecture notes will be available as I prepare them (the first chapter will be ready soon, at least in draft form) and it will be seen that (partly because of my own bias) I think of a representation of a group as a homomorphism $G\rightarrow \mathrm{GL}(E)$, and not as modules over the group algebra. I certainly intend to mention the latter approach at some point (indeed, I have in preparation a long “Variants” chapter, where I mention also, e.g., topological representations, unitary representations, permutation representations, projective representations, Lie algebra representations, etc, with basic examples), but I find it useful to go through the elementary theory of representations completely “by hand”, for a course at this level. In some sense, this is because this is what I can do best, with my own knowledge; but also, going through these basic facts purely with the tools and language of representations-as-homomorphisms does provide excellent opportunities to start seeing various ways of using representation theory (I will for instance prove Burnside’s irreducibility criterion and the linear independence of matrix coefficients using my understanding of Burnside’s original arguments). And I do intend to use this course to introduce some functorial language to the students, and I feel that abstract nonsense will be quite appealing after trying to make sense of the confusion that may arise from proving the transitivity formula for induction strictly using the “subset of functions on the group” model for induced representations…

Here’s a question about the module-over-the-group-algebra approach: what is the first (or simplest, or most fundamental) argument where it is better? The one I can think of for the moment — after going through the first chapter of my notes — is that it seems to give the simplest way to argue that a semisimple representation always contains an irreducible subrepresentation. (And of course this is only relevant for infinite-dimensional representations, since otherwise one can argue using dimension.)

[Update (23.2.2011): the first chapters are now online.]

## More exponential sums

Here’s an update on the front of exponential sums…

(1) The conference I co-organized at FIM on exponential sums over finite fields

ended about two weeks ago. It was quite nice (at least from my point of view…). As usual here, the excellent organization made it possible to enjoy the mathematics with no extra issues to take care of. Most talks were on blackboard but, with authorization, here are the files for those that were on beamer (I think I may miss one or two, which I will add later):

(2) The second part of my class on exponential sums (cohomological methods) continues. I only barely started the lecture notes I was planning to write, the first few weeks of the semester having been simply too busy. As a result, although they are available and will be updated regularly from now on, they are quite incomplete. Most damagingly, the beginning material (introduction and motivation for going towards describing algebraic exponential sums using traces of Frobenius on suitable “Galois” representations of fundamental groups) is mostly missing for the moment. Still, the end parts of Chapter III are written and the contents, from Chapter IV onwards, will hopefully be kept in sync with the class, and no further gaps will appear. Since Chapter IV will start introducing the Grothendieck-Lefschetz trace formula, the étale cohomology groups, and then go on towards the formalism of weights and Deligne’s general version of the Riemann Hypothesis, this might still be of interest even before I find the time (next semester, probably) for filling up the gaps.

My plan next year, when (and if) this second part is done nicely enough, will be to put it together with the first part of the class (on elementary methods) to produce a proper book on exponential sums over finite fields. I’ll be happy to receive even before any feedback on the way the text shapes up, especially from the point of view of people in analytic number theory or in other fields where exponential sums are applied.

## Positions in Zürich

Now seems a good time for the annual update on the postdoctoral positions at the Mathematics Department at ETH. Basic information, and a link to the online application form is here. While there’s not much to add this year to what I wrote two years ago about these positions, I’ll just say that the fairly confusing ticket machines in the tramway stations are being replaced with newer ones which are much more self-explanatory (and take credit cards in addition to cash)… The first deadline for application is November 22.

While I’m at it, here is a link to the lectureship/postdoc positions at the University of Zürich. In particular, one position there is funded by the Swiss National Science Foundation project of Ulrich Derenthal, and is therefore in the direction of arithmetic geometry.

And finally, there are two assistant professor positions (non tenure-track) currently advertised at ETH. One of them is in pure mathematics (deadline December 31), the other in applied mathematics (deadline December 15).