This is a quite short paper (200 full pages shorter than the paper I recently announced), which tells a simple and interesting story: we find that optimal constant , such that every pair of unitaries satisfying the q-commutation relation

dilates to a pair of **commuting** normal operators with norm less than or equal to (this problems is related to the “complex matrix cube problem” that we considered in the summer project half year ago and the one before). We provide a full solution. There are a few ramifications of this idea, as well as surprising connections and applications, so I invite you to check out the nice little introduction.

Michael Skeide and I have recently uploaded our new paper to the arxiv: CP-Semigroups and Dilations, Subproduct Systems and Superproduct Systems: The Multi-Parameter Case and Beyond. In this gigantic (219 pages) paper, we propose a framework for studying the dilation theory of CP-semigroups parametrized by rather general monoids (i.e., semigroups…]]>

Reblogging the announcement of my recent giant paper with Michael Skeide. I want this to be at the head of my blog for a while.

Michael Skeide and I have recently uploaded our new paper to the arxiv: CP-Semigroups and Dilations, Subproduct Systems and Superproduct Systems: The Multi-Parameter Case and Beyond. In this gigantic (219 pages) paper, we propose a framework for studying the dilation theory of CP-semigroups parametrized by rather general monoids (i.e., semigroups with unit), and we use this framework for obtaining new results regarding the possibility or impossibility of constructing or having a dilation, we use it also for obtaining new structural results on the “mechanics” of dilations, and we analyze many examples using our tools. We present results that we have announced long ago, as well as some surprising discoveries.

This is an exciting moment for me, since we have been working on this project for more than a decade.

“Excuse me, did you really say *decade*?”

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Because the previous post was obsolete and because I really did not feel like updating on the sorry situation, I made this blog private for a short while. I am now bringing it back, with this explanation. I am leaving the black banner though, until there is reason to remove it.

I hope to continue to post on mathematics soon.

]]>Solutions to the situation can be found. Right now everywhere in the world meetings are made, decisions are taken, and classes are taught using video conferencing solutions. Many essential services go on as usual (construction, super-markets, the army, the police not to mention hospitals and so forth); they continue to operate, taking care to follow precautions.

There is absolutely no excuse for the newly elected Knesset not to begin operating **immediately** in full capacity, and first things first, replace the current chairman, who is behaving in a shameful way. Later, to approve new laws and a new government, according to the decisions made by the majority of the elected parliament members, and in compliance with the law.

Hopefully, the prime minister and the government will not abuse their power to stall this process or to gain leverage beyond the power they hold in the Knesset.

Here is an article in English (note that it is an **opinion piece**), that contains a lot of the facts:

This is an exciting moment for me, since we have been working on this project for more than a decade.

“Excuse me, did you really say *decade*?”

Yes, I know, it’s hard to believe that one can work on a single paper for ten years. But it’s true. To be precise, we started some email discussions in 2007, and kicked off this as an official project in 2009. The problem of characterizing precisely which CP-semigroups have a dilation turned out to be extremely difficult, but Michael did not feel like giving up, so we continued working and improving our results, and also finding counter-examples to things that we could not prove, which raised new questions, which were extremely difficult, but Michael did not feel like giving up, and so on, etc. I am grateful for this collaboration, because Michael, besides being a genius, has also a very strong dedication for the quality of work. Honestly, if I was in this alone I would have given up long ago (meaning that I would either discard it or publish the wishy-washy results that we could obtain within a year or two).

So, aren’t you curious to hear what we do in this paper? Though the paper has an abstract and an introduction, which were carefully written, I will try here to give a more informal account, allowing myself to be less precise, and to put emphasis on **one** particular key novelty of our paper: the ** superproduct system** of a dilation. I will be a bit sloppy with details, and if the following makes you curious you can find details and/or references in our paper.

Let be a commutative semigroup with unit; for simplicity one may consider a sub-semigroup of (in the paper, we treat general monoids, and in some cases we restrict attention to Ore monoids, and there are interesting aspects that arise from noncommutativity of the semigroup. But here I will ignore noncommutative monoids to make the presentation a bit more readable. In fact, some of the novelties of our paper are interesting *even in the one-parameter case*).

A ** CP-semigroup** is a family of completely positive maps acting on a

A ** dilation** of is a triple , where is a unital C*-algebra containing , is a projection such that , and is an E-semigroup such that

(*)

for all and . Sometimes, we just say that is a dilation of . We say that a dilation is ** strong** if (*) is replaced by

(**)

for all and . The existence of a dilation means that we can understand a CP-semigroup as a “part of” an E-semigroup, which is an object in a more restrictive category, hence presumably better understood. Thus, this notion obeys the general philosophy of “dilation theory”, which was the topic of the survey that I advertised in the previous blog post. (Some applications of dilations are discussed in Section 8.3 of the survey).

It is known that if or then there always exists a dilation, in fact there always is a minimal strong dilation. (I will say something about minimality below). Our mission in this paper was ambitious: to develop dilation theory for general multi-parameter semigroups on general C*-algebras (and beyond), and in particular to completely understand when does there exist a dilation, and of what kind.

Our methods rely on the machinery of Hilbert C*-modules and Hilbert C*-correspondences. Recall, that a ** Hilbert C*-module** (over a C*-algebra ) is a right -module that carries a -valued “inner product” that satisfies the usual properties, e.g., for all and all . Of course, we also require (an inequality taking place in ) and if and only if .

An operator is said to be ** adjointable** if there exists another operator , such that for all . We denote by the algebra of all adjointable operators on . It is a C*-algebra (with the *-operation and the operator norm).

A Hilbert ** C*-correspondence** is a Hilbert C*-module that is in fact a bimodule over where the left action is given by adjointable operators. If we have two correspondences and over , then we can form their

.

We then write for the image of in .

A fundamental tool that we use is ** Paschke’s GNS construction.** Given a CP map , there exists a Hilbert C*-module and a vector such that

(GNS)

for all . One can construct such a pair as follows: first, one constructs the tensor product , and defines on it a semi-inner product by

,

and then extending linearly. One quotients out the space of null vectors and completes, and this gives rise to the C*-module . The vector is defined to be the image of in , and it is easy to check that it satisfies (GNS). We give the structure of a correspondence on by the obvious -actions. Then is generated by , and the pair is determined uniquely as the unique (up to isomorphism) correspondence generated by a vector that satisfy equation (GNS).

Now suppose that we have a CP-semigroup on a C*-algebra . Then for every , we can construct the GNS representation . If , we can make the following computation:

.

Note that here we made use of the semigroup property of . This means that if we write

then is a GNS representation for , and by uniqueness we find that there exists an isomorphism of C*-correspondence that maps to . We can summarize this a bit sloppily by saying that

(I)

and

(II) .

We call a family satisfying (I) a ** subproduct system** (we also require that the identifications are associative – this also follows from the semigroup properties of ) and a family satisfying (II) a

To recap, with every CP-semigroup we have a subproduct system and unit, from which one can completely recover the semigroup. The subproduct system can be considered as a kind of algebraic invariant attached to the semigroup, and we will see below that in principle it contains information about the possibility of constructing a dilation for .

We have just seen that with every CP-semigroup, there is a family of C*-correspondences satisfying equation (I) above (with associative identifications), and we called this a ** subproduct system**. If the family satisfies the stronger requirement

then we say it is a ** product system**. Product systems of Hilbert spaces were introduced by Arveson as the definitive tool for studying and classifying E-semigroups on . Afterwards, product systems of C*-correspondences appeared, and they were shown by Michael Skeide (my co-author in this current paper) to be a complete invariant for E-semigroups on .

In a previous work with Baruch Solel, we showed that for a CP-semigroup to have a full normal dilation, a necessary condition is that the subproduct system can be embedded into a product system. (To be precise, we did not work with the GNS-subproduct system, but rather with another construction, called the *Arveson-Stinepring subproduct system*; see the appendix of the new paper for the connection). Moreover, we showed that for Markov semigroups the embedability of the subproduct system in a product system is a sufficient condition for the existence of a dilation. However, those results only held for normal CP-semigroups on von Neumann algebras (and the monoids parametrizing the semigroups were assumed to be subsemigroups of ). A more dramatic deficiency, was that it could not rule out the existence of non-full dilations.

In the new paper, we obtain a generalization of the above result on normal Markov semigroups as follows (this appears as Theorem 12.9 in our paper):

**Theorem:** *Let be a Markov semigroup over the opposite of an Ore monoid. Then admits a strict full dilation if and only if the GNS-subproduct system of embeds into a product system. *

Explanations: An “Ore monoid” is a monoid that is right reversible and cancellative (any cancellative abelian monoid is an Ore monoid). A “full dilation” is a dilation where is of the form for some -module . Here by “strict” we mean that is continuous in appropriate topology on .

We used the above theorem to construct – for every pair of commuting UCP maps – a dilation consisting of commuting unital *-endomorphisms (recovering and somewhat sharpening an earlier result of Solel).

The above theorem is interesting, but we had expected it and it was obtained early on in our research. Let me say imprecisely (from a historical point of view), that such a result allowed us to construct examples of Markov semigroups over that have no full dilation (this was a somewhat surprising counter example – read the intro of that paper to see why). However, we were not satisfied with the fact that our theorem only treated **full** dilations. We did not know whether or not there exists examples of Markov semigroup acting on a C*-algebra such that there exists no dilation whatsoever . The fact that a dilation is full means that is Morita equivalent to ; there is no apparent reason to restrict the notion of dilations only to Morita equivalent algebras.

To treat non full dilations, we introduced the notion of a ** superproduct system**. In fact, superproduct product system have appeared in the literature without being called that (at least in the work of Claus Kostler, who suggested the terminology), and have appeared under that name, but for a completely different purpose, in the work of Margetts and Srinivasan (I blogged about their work several years ago). We discovered that they arise naturally in the context of dilations.

Let be a dilation (of some CP-semigroup on ). We can define a family of C*-correspondences over as follows:

with a left action and a trivial right action. We can define maps by

.

Some straightforward calculations show that are all isometries, and that they compose associatively (see Theorem 9.1 in our paper). Thus, roughly, we have

.

We call such a family of correspondences a ** superproduct system**. Thus, with every dilation, there is a superproduct system. We obtain the following result (which is a simplification of Theorem 9.3 in our paper):

**Theorem:** *If is a strong dilation of a CP-semigroup , then the superproduct system of the dilation contains the GNS-subproduct system of the . *

It is noteworthy that every dilation of a Markov semigroup is strong; thus we have a necessary condition for the existence of any dilation for a Markov semigroup: the embeddability of the GNS-subproduct system into superproduct system. We don’t know – it is an open problem – whether this condition is also sufficient. We constructed subproduct systems that cannot be embedded into superproduct systems, thereby exhibiting examples of Markov semigroups that have no dilation whatsoever.

The machinery of superproduct systems can be used not only to rule out existence of dilations, but also for studying the structure of dilations.

A dilation of a CP-semigroup is said to be ** compressible** if there exists a projection in such that is still a dilation. We then say that is a

It is clear that if a dilation is compressible, then it cannot really be considered to be “minimal”. There are several notions of minimality that have been studied in the literature or that are natural to consider. In Chapter 21 of our paper we study and compare them.

One of our most difficult results is the connection between the compressions of a (full) dilation and the various (super)product subsystems that exist between the GNS-subproduct system of the dilated semigroup and the (super)product system of the dilation. These results are bit too much to give the details of here.

One thing that I do want to tell you, and that I am very pleased about, is that we were able to use this machinery of superproduct systems to study and to obtain new results even about the one parameter case (where dilations are always known to exist). We used it to show that several different notions of minimality are equivalent in the one-parameter case, recovering and improving on earlier results of Arveson.

Finally, another one of our difficult achievements, is an analysis of the discrete two parameter case, in which we show that the different notions of minimality that have been shown to be equivalent in the one-parameter case, are not equivalent in general.

]]>(Well, these are the usual highs and lows of being a mathematician, but since this is a survey paper and not a research paper, I feel comfortable enough to share these feelings).

This survey was submitted (and will hopefully appear in) to the Proceedings of the International Workshop on Operator Theory and its Applications (IWOTA) 2019, Portugal. It is an expanded version of the semi-plenary talk that I gave in that conference. I used a preliminary version of this survey as lecture notes for the mini-course that I gave at the recent workshop “Noncommutative Geometry and its Applications” at NISER Bhubaneswar.

I hope somebody finds it useful or entertaining

]]>I have two stories two tell about Janos.

When I came to do a postdoc at the University of Waterloo in 2009, I was already working in operator algebras, and my supervisor was Ken Davidson. However, I remembered my functional equations origins, and I was happy to find out that Janos Aczel is an Emeritus Professor there. I was invited to give a talk at the Analysis Seminar, and my plan was to talk about the triumph of my PhD work. However, my talk was planned quite badly, or maybe I was just too excited, and my talk was over before half an hour. For a young mathematician in a new department, fresh out of his PhD, this is a disaster. The seminar leader asked if there were any questions or comments, and there was a brief and awkward silence. But then Janos raised his hand, and when he got permission to make his comment he said: “a good talk should have a good beginning, a good end, *and the two should be close to one another*“, and when he said the word “close” he held his hands open with palms facing each other, indicating a

Speaking of beautiful acts of kindness, this brings me to the second story. Janos and I met several times to discuss functional equations. In one of our meetings he brought up the fact that his wife is Jewish. Janos was a student in Budapest during World War II. He told me that during World War II, the Jews in Hungary at a certain age were sent to forced labor camps. Hungary was an ally of Germany, and had several anti-semitic laws and measures (a number of Jews were deported to Poland where they were murdered) but Jews were not closed in ghettos and for the most part were not sent to concentration camps, until Germany invaded Hungary, in 1944.

Janos told me that some of the Jewish students had to go to forced labor, but then some were released and allowed to go back to university. (About three fourth of the forced laborers did not survive). But they missed a year of classes! So some top students, and Janos was among them, volunteered to teach the returning Jewish students the material that they missed. This might sound like a light anecdote, missing classes! You have to remember Janos acted in a deeply anti-semitic country, and even though there were no deportations to concentration camps yet, there were quotas of how many Jews could work in certain jobs, and there were also “spontaneous” mass executions, before the Germans invaded. I consider this a beautiful act of solidarity and resistance.

One of the students that Janos tutored was Susan, they fell in love, and she later became his wife. I don’t remember if he told me how exactly Susan survived the horrible years to come (did she hide her identity, hide herself, or did she flee). She survived, and they lived together until Susan passed away in 2010.

I am sorry that I did not interrogate Janos for more details at the time and write them down, but I thought I should write down what I remember. I added some historical details using this Wikipedia article (and also the corresponding one in Hebrew).

]]>(Lectures 1,2,3 were board talks).

]]>Recall that the ** matrix range** of a -tuple of operators is the noncommutative set , where

is UCP .

The matrix range appeared in several recent papers of mine (for example this one), it is a complete invariant for the unital operator space generated by , and is within some classes is also a unitary invariant.

The idea for this paper came from my recent (last couple of years or so) flirt with numerical experiments. It has dawned on me that choosing matrices randomly from some ensembles, for example by setting

`G = randn(N);`

`X = (G + G')/sqrt(2*N);`

(this is the GOE ensemble) is a rather bad way for testing “classical” conjectures in mathematics, such as what is the best constant for some inequality. Rather, as increases, random behave in a very “structured” way (as least in some sense). So we were driven to try to understand, roughly what kind of operator theoretic phenomena do we tend to observe when choosing random matrices.

The above paragraph is a confession of the origin of our motive, but at the end of the day we ask and answer honest mathematical questions with theorems and proofs. If is a -tuple of matrices picked at random according to the Matlab code above, then experience with the law of large numbers, the central limit theorem, and Wigner’s semicircle law, suggests that will “converge” to something. And by experience with free probability theory, if it converges to something, then is should be the matrix range of the free semicircular tuple. We find that this is indeed what happens.

**Theorem**: *Let be as above, and let be a semicircular family. Then for all , *

almost surely

*in the Hausdorff metric.*

The semicircular tuple is a certain -tuple of operators that can be explicitly described (see our paper, for example).

We make heavy use of some fantastic results in free probability and random matrix theory, and our contribution boils down to finding the way to use existing results in order to understand what happens at the level of matrix ranges. This involves studying the continuity of matrix ranges for continuous fields of operators, in particular, we study the relationship between the convergence

(*)

(which holds for as above and by a result of Haagerup and Torbjornsen) and

(**) .

To move from (*) to (**), we needed to devise a certain quantitative Effros-Winkler Hahn-banach type separation theorem for matrix convex sets.

]]>