Noncommutative Analysis

Souvenirs from San Diego

Every time that I fly to a conference, I think about the airport puzzle that I once read in Terry Tao’s blog. Suppose that you are trying to get quickly from point A to point B in an airport, and that part of the way has moving walkways, and part of it doesn’t. Suppose that you can either walk or run, but you can only run for a certain small amount of the time. Where is it better to spend that amount of time running: on the moving walkways or in between the moving walkways? Does it matter?

Another question that continues to puzzle me (and to which I still don’t have a complete answer to) is: why do I continue to inflict upon myself the tortures of international travel, such as ten hour jet lag or trans-atlantic flights? More generally, I spent a lot of time wondering: why do I continue going to conferences? Is it worth it for me? Is it worth the university’s money? Is it worth it for mankind? 

Last week I attended the Joint Mathematics Meeting in San-Diego. It was my first time in such a big conference. I will probably not return to such a conference for a while, since it is not so “cost effective”. I guess that I am a small workshop kind of person.

I spoke in and attended all the talks in the Free Convexity and Free Analysis special session, which was excellent. Here is the abstract and here are the slides of my talk (the slides).  I also attended some of the talks in the special sessions on Advances in Operator AlgebrasOperators on Function Spaces in One and Several Variables, and another one on Advances in Operator Theory, Operator Algebras, and Operator SemigroupsI also attended several plenary talks, which were all quite entertaining.

I am happy to report that the field of free analysis and free convexity is in really good shape! There was a sequence of talks in the first day (Hartz, Passer, Evert and Kriel) by three very young researchers on free convexity that really put me into high spirits! The field is blossoming and the competition is healthy and friendly. But the talk that got me most excited was the talk by Jim Agler, who gave a preliminary report on joint work with John McCarthy and Nicholas Young regarding noncommutative complex manifolds. Now, at first it might seem that nc manifolds will be hard to make sense of, because how can you take direct sums of points in a manifold, etc. Moreover, the only take on the free manifolds that I met before was Voiculescu’s construction of the free projective plane, which I found hard to swallow and kind of ruined my appetite for the subject.

However, it turns out that one can define a noncommutative complex manifold as topological space X that carries an atlas of charts (U,f) where U is an open subset of X and f : \Omega \to U is a homeomorphism form an nc domain \Omega onto U, such that given two intersecting charts (U,f), (U',f'), the map f^{-1} \circ f' going from f'(U \cap U') to f(U \cap U') is an nc biholomorphism. This definition is so natural and clear that I want to shout! Agler went on and showed us how one can construct a noncommutative Riemann surface, for example the Riemann surface corresponding to the noncommutative square root function. How can one not want to hear more of this? I am looking forward very enthusiastically to see what Agler, McCarthy and Young are up to this time; it looks like a very promising direction to study.

Among the plenary talks that I attended (see here for description), the one given by Avi Wigderson struck me the most. I went to the talk simply for mathematical entertainment (a.k.a. to broaden my horizons), but I was very pleasantly surprised to find completely positive maps and free functions in a talk that was supposed to be about computational complexity. I went to the first two talks but missed the third one because I had an opportunity to have lunch with a friend and collaborator, which in any respect was more important to me than the lecture. The above link (here it is again) contains links to a tutorial and papers related to Wigderson’s talks, and I hope to find time to study that, and at least catch up on what I missed in the third talk.

One more thing: there was one quite eminent operator theorist who is long retired, and came to several of the sessions that I attended. At some point I noticed that after every talk a came up to the speaker and said several words of encouragement or advice. Seeing such a pure expression of kindness and love of humanity was touching and inspiring. Upon later reflection, I noticed that such expressions were happening around me all the time, for example when another “celebrity” in our field arrived and a hugging (!) session began. This memory brings a smile to my face. Well, maybe going to San-Diego was worth it, after all.

Additional thoughts January 26: 

  1. The tutorial that you can find in “the above link” seems to cover all of Wigderson’s talk.
  2. I have had some more thoughts on “big conferences”. The good thing about them is that it gives an opportunity to interact with people people outside one’s own academic bubble, and attend high level talks by prominent mathematicians. The bad thing is that you fly far away, waste tons of grant money, and in the end have only a small time to discuss your research topic with experts. So: to go or not to go? I’ve found a solution! Attend local big conferences. Fly across the world only to meet with special colleagues or participate in focused and effective workshops or conferences on your subject of main interest. (And if they invite you to give a plenary talk at the ICM, then, OK, you should probably go).

The nightmare

In September 30 the mathematician Vladimir Voevodsky passed away. Voevodsky, a Fields medalist, is a mathematician of whom I barely heard earlier, but after bumping into an obituary I was drawn to read about him and about his career. His story is remarkable in many ways. Voevodsky comes out as brilliant, intellectually honest giant, who bravely and honestly confronted the crisis that he observed “higher dimensional mathematics” was in.

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I quit (from MathSciNet and ZbMath)

(This is the post that I wanted to write this weekend.)

Several months ago I informed both MathSciNet as well as Zentralblatt that I would like to stop reviewing papers for these repositories. If you don’t know what I am talking about (your PhD thesis advisor should be fired!), then MathSciNet and Zentralblatt are databases that index published papers in mathematics, contains some bibliographic information (such as a reference list for every paper, as well as a list of papers that reference it), and, significantly, has a review for every indexed paper. The reviews are written by mathematicians who do so voluntarily (they get AMS points or something). If the editors find nobody willing to review, then the abstract appears instead of a review. This used to a very valuable tool, and is still quite valuable.

I quit because:

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A comment on the sowa versus Gowers affair

I wanted to write about something else this weekend, but I got distracted and ended up writing this post. O well…

This is post is reply to (part of) a post by Scott Aaronson. I got kind of heated up by his unfair portrayal of the blog “Stop Timothy Gowers!!!“, and started writing a reply which got to be ridiculously long, so I moved it here.

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“A toolkit for constructing dilations on Banach spaces”, by Fackler and Gluck

About a week ago an interesting preprint appeared on the arxiv: “A toolkit for constructing dilations on Banach spaces“, by Stephan Fackler and Jochen Gluck. I have been studying various aspects of dilations for some years, but I haven’t really given much thought to dilation theory in general classes of Banach spaces. This paper – which is very clearly organized and written – was very refreshing for me, and in it a very general framework for proving existence of dilations in classes of Banach spaces is presented. The paper also contains a nice overview of the literature, and I was surprised by learning also about old results in, and application of, dilation theory, which I was not aware of and perhaps I should have been. The purpose of this post is to record my first impression of this paper and to put down some links to the references, which I would like to study better at some point.  Read the rest of this entry »

Tapioca on page 49

To my long camping vacation this year I took the book “Topological Vector Spaces” by Alex and Wendy Robertson. I “inherited” this book (together with a bunch of other classics) from an old friend after he officially decided to leave academic mathematics and go into high-tech. The book is a small and thin hard-cover, with pages of high quality that are starting to become a delicious cream color.

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A list of corrections to my book

My friend Daniel Reem has encouraged me to put up a webpage with corrections to my book “A First Course in Functional Analysis” (in fact, he even convinced me to insert a promise to put up such a page in the preface to my book, so that I don’t really have an option not to put up that corrigendum). Daniel was also kind enough to find a couple of mistakes! So thanks to Daniel, the page is up: here is the link.

Anyone who wishes to report mistakes can use the comment section in this page, or email me.

Hot morning for the Technion in arxiv math.OA

While I am spending my morning preparing for a two week vacation in the very hot Park Hayarden, it is was nice to browse the arxiv mailing list for math.OA (Operator Algebras) and find four entries by operator-people from the Technion. I don’t recall such a nice coincidence happening before.

There are two very interesting new submissions:

  1. Hyperrigid subsets of graph C*-algebras and the property of rigidity at 0“, by our PhD. student Guy Salomon.
  2. On fixed points of self maps of the free ball” by recently-become-ex postdoc Eli Shamovich.

There is also a cross listing (from Spectral Theory) to the paper “Spectral Continuity for Aperiodic Quantum Systems I. General Theory“, by Siegfried Beckus (a postdoc in our department) together with Jean Bellissard and Giuseppe De Nittis.

Finally, there is a new (and final) version of the paper “Compact Group Actions on Topological and Noncommutative Joins” by Benjamin Passer (another postdoc in our department) together with Alexandru Chirvasitu.


A review of my book A First Course in Functional Analysis

A review for my book A First Course in Functional Analysis appeared in Zentralblatt Math – here is a link to the review. I am quite thankful that someone has read my book and bothered to write a review, and that zBMath publishes reviews. That’s all great. Now I have a few words to say about it. This is an opportunity for me to bring up the subject of my book and highlight some things worth highlighting.

I am not too happy about this review. It is not that it is a negative review – actually it has a rather kind air to it. However, I am somewhat disappointed in the information that the review contains, and I am not sure that it does the reader some service which the potential readers could not achieve by simply reading the table of contents and the preface to the book (it is easy to look inside the book in the Amazon page; of course, it is also easy to find a copy of the book online).

The reviewer correctly notices that one key feature of the book is the treatment of L^2[a,b] as a completion of C([a,b]), and that this is used for applications in analysis. However, I would love it if a reviewer would point out to the fact that, although the idea of thinking about L^2[a,b] as a completion space is not new, few (if any) have attempted to actually walk the extra mile and work with L^2 in this way (i.e., without requiring measure theory) all the way up to rigorous and significant applications in analysis. Moreover, it would be nice if my attempt was compared to other such attempts (if they exist), and I would like to hear opinions about whether my take is successful.

I am grateful that the reviewer reports on the extensive exercises (this is indeed, in my opinion, one of the pluses of new books in general and my book in particular), but there are a couple of other innovations that are certainly worth remarking on, and I hope that the next reviewer does not miss them. For example, is it a good idea to include a chapter on Hilbert function spaces in an introductory text to FA? (a colleague of mine told me that he would keep that out). Another example: I think that my chapter on applications of compact operators is quite special. This chapter has two halves: one on integral equations and one on functional equations. Now, the subject of integral equations is well trodden and takes a central place in some introductions to FA, and one might wonder whether anything new can be done here in terms of the organization and presentation of the material. So, I think it is worth remarking about whether or not my exposition has anything to add. The half on applications of compact operators to integral equations contains some beautiful and highly non-trivial material that has never appeared in a book before, not to mention that functional equations of any kind are rarely considered in introductions to FA; this may also be worth a comment.

Souvenirs from Haifa


The “Multivariable operator theory workshop at the Technion, on occasion of Baruch Solel’s 65th birthday”, is over. Overall I think it was successful, and I enjoyed meeting old and new friend, and seeing the plan materialize. Everything ran very smoothly – mostly thanks to the Center for Mathematical Sciences and in particular Maya Shpigelman. It was a pleasure to have an occasion to thank Baruch, and I was proud to see my colleagues acknowledge Baruch’s contribution and wish him the best.

If you are curious about the talks, here is the book of abstracts. Most of the presentations can be found at the bottom of the workshop webpage. Here is a bigger version of the photo.

I will not blog about the workshop any further – I don’t feel like I participated as a mathematician. I miss being a regular participant! Luckily I don’t have to wait long: Next week, I am going to Athens to participate in the Sixth Summer School in Operator Theory in Athens.