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The central amenability constant of a finite group: Part 1 of n

6 May, 2013

tags: amenability, finite-groups, research

As I am currently too besieged by a jumped-up virus to get any proper work done, I suppose I might as well put this blog to one of its nominal uses and actually talk about some mathematics I did recently (last Monday and Tuesday, in fact). Well, I say talk, I really mean “present” (and I am not at all sure I will manage to “explain”). Consider this my first experiment in using the blog for channeling rainwater…

This post will just try to set up enough to state the result, and then if there is ever a follow-up post I will give more of the background story and the necessary details.

Fix a finite group G and look at the centre of its complex group algebra ${\mathbb C}G$ . This algebra, which we call A for now, is commutative and spanned by its minimal idempotents. What are these minimal idempotents? they are scalar multiples of the irreducible characters of G. (A character of G is, for us, the trace of some irreducible representation over ${\mathbb C}$ .) If $\phi$ is such a character then it is not hard to show that the corresponding idempotent in A is $|G|^{-1}\phi(e)\phi$ .

Now an algebra such as A admits a unique so-called separating idempotent, that is, an element ${\bf m} \in A\otimes A$ satisfying $a\cdot {\bf m} = {\bf m}\cdot a$ for all $a \in A$ and $\Delta(m)=1_A$ , where $\Delta:A\otimes A\to A$ is the linearization of the multiplication map. (This separating idempotent is a witness to the fact that $A$ has trivial Hochschild cohomology, or trivial Ext if you prefer to think of it like that.) In fact it is not at all hard to work out what ${\bf m}$ has to be:

$\displaystyle{\bf m} = \sum_{\phi\in{\rm Irr(G)}} |G|^{-1}\phi(e)\phi \otimes |G|^{-1}\phi(e)\phi .$

Let us accept for now that this element should be worth studying, and introduce norms. We can always equip the complex group algebra ${\mathbb C}(G) with the$ latex \ell^1$-norm, that is $\Vert f\Vert = \sum_{x\in G} |f(x)|$ . The resulting Banach algebra $\ell^1(G)$ is usually studied only for infinite G, but if one is interested in quantitative phenomena then the finite case still holds some mysteries and — I claim — some interest. In any case, since we can identify $A\otimes A$ with the centre of the complex group algebra of $G\times G$ , it inherits a natural norm as a subalgebra of $\ell^1(G\times G)$ .

We now define the central amenability constant of G to be the $\ell^1$ -norm of the separating idempotent ${\bf m}$ , and denote this number by ${\rm AM}_{\rm Z}(G)$ . If we wish to write things out explicitly, some more notation will be useful. Let ${\rm Conj}(G)$ denote the set of conjugacy classes in G, and if C is a conjugacy class in G we write $\phi(C)$ for the value $\phi$ takes on one (hence on every) element of C. Then

$\begin{aligned} {\rm AM}_{\rm Z}(G) & = \sum_{x,y\in G} \left\vert { \sum_{\phi \in {\rm Irr}(G)} |G|^{-2}\phi(e)^2 \phi(x) \phi(y) } \right\vert \\ & = \sum_{C,D\in {\rm Conj}(G)} |C| |D| \left\vert { \sum_{\phi \in {\rm Irr}(G)} |G|^{-2}\phi(e)^2 \phi(C) \phi(D) } \right\vert \,. \end{aligned}$

This formula can be found¹ as Theorem 1.8 of a 2009 JFA paper of Ahmadreza Azimifard, Ebrahim Samei and Nico Spronk (and before you complain about paywalls, this one is on the arXiv). At this point I should acknowledge that I first got interested in this invariant of G while listening to some talks given by Ebrahim about 5-6 years ago, and have since had a few enjoyable and educative discussions with both him and Nico on this topic.

A little patience (or some prior knowledge of Banach algebras) shows that if G is abelian then ${\rm AM}_{\rm Z}(G)=1$ . Moreover, by an argument that will be outlined in the next installment, ${\rm AM}_{\rm Z}(G)>1$ whenever G is non-abelian. What is not all obvious, at least to me, is that 1 is an isolated value of this invariant. The following is extracted from the proof of Theorem 1.10 in the aforementioned paper.

Theorem (Azimifard–Samei–Spronk)
There exists δ > 0 such that ${\rm AM}_{\rm Z}(G)\geq 1+\delta$ whenever G is non-abelian.

The authors obtain the existence of such a δ as a special case of a theorem of D. A. Rider (Transactions AMS, 1973). As an aside: for those of you who frequented MathStackExchange, this application is what ultimately prompted me to raise this question. What is rather unsatisfactory is how small the δ given by Rider’s proof is — Rider’s bounds yield δ = 1/300, although in some back of the envelope calculations I thought I could bring it down to around 1/90 — while the smallest value known for ${\rm AM}_{\rm Z}(G)$ when G is nonabelian is 7/4.

After some failed attempts to get a better lower bound, I put the problem on the backburner, although from time to time it would come back to haunt me. More recently, in some joint work with Ebrahim and our PhD student Mahmood Alaghmandan, we had a closer look at the problem of calculating ${\rm AM}_{\rm Z}(G)$ explicitly when you don’t know how to use GAP, and don’t have anyone at hand who does. See this preprint if you want to know more. In the process, I found myself getting interested again in the original problem of getting a lower bound closer to 7/4 than to 301/300…

Theorem (yours truly, some time last Monday)
${\rm AM}_{\rm Z}(G)\geq 4/3$ whenever G is non-abelian.

This bound is still probably not sharp, but it does at least approach what seems to be the right value. However, more important than the improvement in the constant is that the argument now bypasses Rider’s theorem, which is much more general and whose proof I find somewhat hard to understand². Instead of using general non-abelian Fourier analysis like Rider, who was in any case working on compact groups, we can get by with some basic arguments from character theory, together with a little structure theory for finite groups.

Well, this post looks long enough, so further explanation will have to wait for the next post…

Footnotes:

I am cheating a little here, in that those authors have a slightly more conceptual definition of ${\rm AM}_{\rm Z}(G)$ , and have to do a little work to show it gives the same number as the formula above.
One can follow the proof through line by line, but it leaves me rather baffled about what is conceptually going on, and what the intuition is.

“My name is my name”

5 May, 2013

tags: rants

Possibly NSFW for language, depending on your melon-farming place of employment, I suppose:

Anyway, a well-meant correction made to proofs of a recent paper has reminded me of the following nicety.
([Link])

The following information is provided to meet the requirements of HEFCE:

The legal name and address of the University are:

The University of Newcastle upon Tyne
King’s Gate
Newcastle upon Tyne
NE1 7RU

Since February 2006, the University has adopted the trading title of Newcastle University.

This was first pointed out to me by Antony Mee, a contemporary at Newcastle. (I’ve never understood why the original name was deemed a “defective brand”, and in any case, it just invites confusion for some of us with this University of Newcastle.)

So what was Dr. Blind’s question, then?

24 January, 2013

tags: research

I suspect I must have blogged this quote already, but since I can’t find such a post, I may as well repeat it.

Dr Blind (pronounced ‘Blend’) was about ninety years old and had taught, for the past fifty years, a course called ‘Invariant Subspaces’ which was noted for its monotony and virtually absolute unintelligibility, as well as for the fact that the final exam, as long as anyone could remember, had consisted of the same single yes-or-no question. The question was three pages long but the answer was always ‘Yes.’ That was all you needed to know to pass Invariant Subspaces.

He was, if possible, even a bigger windbag than Dr Roland. Together, they were like one of those superhero alliances in the comic books, invincible, an unconquerable confederation of boredom and confusion. I murmured an excuse and slipped away, leaving them to their own formidable devices.

(From Donna Tartt’s excellent novel The Secret History, which has nothing to do with mathematics.)

Update 25th Jan 2013: Professor Blind, meet Professors Cowen and Gallardo.

further update 5th Feb 2013: As you were; nothing to see here; move along.

I leave you to draw the parallels

9 October, 2012

tags: research

From Jenny Diski, some reflections from 2011:

Why, since I’m living the dream with 17 books published, and a 25 year career as a writer, do I not feel fulfilled? All I wanted was to ‘be a writer’ when I grew up. I am. However, nothing about writing and being published has fulfilled me, justified me, or made me feel better about myself — but then I never thought that writing was supposed to make me feel better about myself. The satisfaction you get is ten minutes in the bath feeling relief that a manuscript is finished and sent away, and then it’s all anxiety about what didn’t work in the last one and whether you can make the next one more like the book you really wanted to write.

My daydream of writing comes from a time (the 1950s and 60s) when angst was in fashion, when writing was angelic and crazy… It was a vocation, the writer was the monk, the nun, who devoted themselves to the written word, to extracting some sort of special insight about existence from the combining of discrete letters of the alphabet. It was a dogged quest that was conducted by keeping as still and alone as possible. Something to live and die for. Both at the same time, always. And that was the writer I wanted to be. It is, as any modern publisher will tell you (and has told me), a hopelessly sentimental view of the literary world. But I did know people who lived that sort of life, who were published in spite of selling only a few copies, because their publishers were excited about what they did. They didn’t make much or even any money, and they weren’t mobbed in the streets. They didn’t, unless they were poets, go about doing readings or book signings. They didn’t figure which genres they should write according to market demands. They didn’t attend focus groups (as novelists have done recently) of ‘ordinary readers’ set up by publishers to ask chapter by chapter how they were doing and what should be altered. They just wrote and worried.

Je veux qu’on rie, je veux qu’on danse, je veux qu’on s’amuse comme des fous

6 October, 2012

tags: rants

After one instance too many of getting needlessly irked by certain views on MathStackExchange, and belatedly realising that what seems to be the majority ethos is one I feel deeply out of step with…

… je l’ai quitté pour soigner mon âme.

Adieu l’Antoine, je t’aimais pas bien.

Adieu l’Antoine, je t’aimais pas bien, tu sais:

J’en crève de crever aujourd’hui -

Alors que toi, tu es bien vivant,

Et même plus solide que l’ennui…

Rorschach test

13 August, 2012

I’ll start … with the case against [basket-weaving]. Suppose you were exposed to that subject as a sub-cabalistic ritual of manipulating [toves] and magical [borogoves] according to rules justified (if at all) only by a transparently false origin myth — that is to say, you had to endure what is still an all-too-common sort of intro. [basket-weaving] class — or, perhaps worse, a “research methods” class whose content had fossilized before you were born. Suppose you then looked at the genuinely impressive things done by the best of those who call themselves “[trendy container architects]“. Well then, no wonder you think “This is something new and wonderful”; and I would not blame you in the least for not connecting it with [basket-weaving]. Perhaps you might find some faint resemblance, but it would be like comparing a child’s toy wagon to a Ducati.

Modern [basket-weaving] is not like that, and has not been for decades.

With thanks, and apologies, to this post of Cosma Shalizi, which is well worth reading.

update 2012-08-17: not so much a Rorschach test as a spam magnet, it would seem. Comments now off.

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Since it is not …

… we simply try to teach mathematics.

The central amenability constant of a finite group: Part 1 of n

“My name is my name”

So what was Dr. Blind’s question, then?

I leave you to draw the parallels

Je veux qu’on rie, je veux qu’on danse, je veux qu’on s’amuse comme des fous

Rorschach test

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