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Category: personal


Exactly 20 years ago I wrote my first blogpost, ‘a blogging 2004’. I wasn’t using WordPress yet (but something called pMachine), and this blog was not called ‘neverendingbooks’, but ‘’ (the URL of the mac still running this blog).

At the time I wanted to find out whether blogging was something for me. “I’m just starting out. Give me a couple of weeks/months to develop my own style and topics and I’ll change the layout accordingly.”

Well, after 20 years I know what I can, and more important, what I cannot do within this framework. Time to move on.

There are other reasons why this might be the right time to pull the plug.

– I’m on retirement since October 1st and soon I’ll have to vacate my office, containing the webserver on which NeB runs.

– My days are filled with more activities now, and I don’t think you want to read here for example about my struggles with chestnut-farming.

– I like to explore other channels to talk about mathematics. This may happen on Mathstodon, MathOverflow or YouTube. Or it might be through teaching or writing a book, perhaps even a children’s book.

NeB will remain reachable until mid 2024. I’ll check out options to preserve its content after that (suggestions are welcome).

I wish you a better 2024.



A suit with shorts

I’m retiring in two weeks so I’m cleaning out my office.

So far, I got rid of almost all paper-work and have split my book-collection in two: the books I want to take with me, and those anyone can grab away.

Here’s the second batch (math/computer books in the middle, popular science to the right, thrillers to the left).

If you’re interested in some of these books (click for a larger image, if you want to zoom in) and are willing to pay the postage, leave a comment and I’ll try to send them if they survive the current ‘take-away’ phase.

Here are two books I definitely want to keep. On the left, an original mimeographed version of Mumford’s ‘Red Book’.

On the right, ‘Een pak met een korte broek’ (‘A suit with shorts’), a collection of papers by family and friends, presented to Hendrik Lenstra on the occasion of the defence of his Ph.D. thesis on Euclidean number-fields, May 18th 1977.

If the title intrigues you, a photo of young Hendrik in suit and shorts is included.

This collection includes hilarious ‘papers’ by famous people including

  • ‘A headache-causing problem’ by Conway (J.H.), Paterson (M.S.), and Moscow (U.S.S.R.)
  • ‘A projective plain of order ten’ by A.M. Odlyzko and N.J.A. Sloane
  • ‘La chasse aux anneaux principaux non-Euclidiens dans l’enseignement’ by Pierre Samuel
  • ‘On time-like theorems’ by Michiel Hazewinkel
  • ‘She loves me, she loves me not’ by Richard K. Guy
  • ‘Theta invariants for affine root systems’ by E.J.N. Looijenga
  • ‘The prime of primes’ by F. Lenstra and A.J. Oort
  • (and many more, most of them in Dutch)

Perhaps I can do a couple of posts on some of these papers. It might break this clean-up routine.

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Christine Bessenrodt (1958-2022)

We were pretty close once. It is a shock to read about her passing on Twitter.

I met Christine in the late 80ties at some representation meeting in Oberwolfach. Christine was a regular at such meetings, being in the Michler-clique from Essen. I don’t recall why I was invited.

We had a fun time, and had a sneaky plan to be invited more regularly to the same conferences. All we had to do was to prove a good result, together…

Easier said than done. Christine’s field was modular representation theory (over $\overline{\mathbb{F}_p}$), and I was interested in the geometry of quiver moduli-spaces (over $\mathbb{C}$).

The next year I ran a post-graduate course on rationality problems and emailed the notes weekly to Christine. After all, results of Lenstra, Colliot-Thelene and Sansuc reduced the problem of (stable) rationality of algebraic tori to integral representation theory, a half-way meeting ground for both of us.

Around that time, our youngest daughter was born, and Christine graciously accepted to be her godmother.
Over the next years, she and Klaus visited us in Antwerp and we week-ended in their brand new house in the outskirts of Duisburg, close to a lake.

Christine in Oberwolfach

Christine and I were working on the rationality problem for matrix invariants.

A sufficiently general $n \times n$ matrix is diagonalisable and is therefore determined, up to conjugacy, by $n$ free parameters (the eigenvalues), so the corresponding quotient-variety is rational.

Now consider couples of $n \times n$ matrices under simultaneous conjugation. In the mid 80ties, Formanek proved rationality for $n=3$ and $n=4$, by using the theory of algebraic tori, and that was about all that was known.

We were able to reduce the question of stable rationality for $n=5$ and $n=7$ to modular representation theory, after which Christine performed her magic to crack the problem. The paper appeared a year later in Inventiones.

Thirty years later, it is still the best result on rationality of matrix invariants.

So, we had our joint result, but its intended use never happened, and our contacts gradually watered down as our mathematical interests again diverged.

My thoughts go out to Klaus and all her loved ones.

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Kasha-eating dragons

This semester I’m teaching a first course in representation theory. On campus, IRL! It’s a bit strange, using a big lecture room for a handful of students, everyone wearing masks, keeping distances, etc.

So far, this is their only course on campus, so it has primarily a social function. The breaks in between are infinitely more important than the lectures themselves. I’d guess breaks take up more than one third of the four hours scheduled.

At first, I hoped to make groups and their representations relevant by connecting to the crisis at hand, whence the the symmetries of Covid-19 post, and the Geometry of Viruses series of posts.

Not a great idea. I guess most of us are by now over-saturated with Corona-related news, and if students are allowed to come to campus just one afternoon per week, the last thing they want to hear about is, right, Covid.

So I need to change tactics. By now we’ve reached the computation of character tables, and googling around I found this MathOverflow-topic: Fun applications of representations of finite groups.

The highest rated answer, by Vladimir Dotsenko, suggests a problem attributed to Kirillov:

An example from Kirillov’s book on representation theory: write numbers 1,2,3,4,5,6 on the faces of a cube, and keep replacing (simultaneously) each number by the average of its neighbours. Describe (approximately) the numbers on the faces after many iterations.

A bit further down the list, the Lecture notes on representation theory by Vera Serganova are mentioned. They start off with a variation of Kirillov’s question (and an extension of it to the dodecahedron):

Hungry knights. There are n hungry knights at a round table. Each of them has a plate with certain amount of food. Instead of eating every minute each knight takes one half of his neighbors servings. They start at 10 in the evening. What can you tell about food distribution in the morning?

Breakfast at Mars. It is well known that marsians have four arms, a standard family has 6 persons and a breakfast table has a form of a cube with each person occupying a face on a cube. Do the analog of round table problem for the family of marsians.

Supper at Venus. They have five arms there, 12 persons in a family and sit on the faces of a dodecahedron (a regular polyhedron whose faces are pentagons).

Perhaps the nicest exposition of the problem (and its solution!) is in the paper Dragons eating kasha by Tanya Khovanova.

Suppose a four-armed dragon is sitting on every face of a cube. Each dragon has a bowl of kasha in front of him. Dragons are very greedy, so instead of eating their own kasha, they try to steal kasha from their neighbors. Every minute every dragon extends four arms to the four neighboring faces on the cube and tries to get the kasha from the bowls there. As four arms are fighting for every bowl of
kasha, each arm manages to steal one-fourth of what is in the bowl. Thus each
dragon steals one-fourth of the kasha of each of his neighbors, while at the same
time all of his own kasha is stolen. Given the initial amounts of kasha in every
bowl, what is the asymptotic behavior of the amounts of kasha?

I can give them quick hints to reach the solution:

  • the amounts of kasha on each face gives a vector in $\mathbb{R}^6$, which is an $S_4$-representation,
  • calculate the character of this kasha-representation,
  • use the character table of $S_4$ to decompose the representation into irreducibles,
  • identify each of the irreducible factors as instances in the kasha-representation,
  • check that the food-grabbing operation is an $S_4$-morphism,
  • remember Schur’s lemma, and compute the scaling factors on each irreducible component,
  • conclude!

But, I can never explain it better than Khovanova’s treatment of the kasha-eating dragons problem.

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taking stock

The one thing harder than to start blogging after a long period of silence is to stop when you think you’re still in the flow.

(image credit Putnam Consulting)

The Januari 1st post a math(arty) 2018 was an accident. I only wanted to share this picture, of a garage-door with an uncommon definition of prime numbers, i saw the night before.

I had been working on a better understanding of Conway’s Big Picture so I had material for a few follow-up posts.

It was never my intention to start blogging on a daily basis.

I had other writing plans for 2018.

For years I’m trying to write a math-book for a larger audience, or at least to give it an honest try.

My pet peeve with such books is that most of them are either devoid of proper mathematical content, or focus too much on the personal lives of the mathematicians involved.

An inspiring counter-example is ‘Closing the gap’ by Vicky Neal.

From the excellent review by Colin Beveridge on the Aperiodical Blog:

“Here’s a clever way to structure a maths book (I have taken copious notes): follow the development of a difficult idea or discovery chronologically, but intersperse the action with background that puts the discovery in context. That’s not a new structure – but it’s tricky to pull off: you have to keep the difficult idea from getting too difficult, and keep the background at a level where an interested reader can follow along and either say “yes, that’s plausible” or better “wait, let me get a pen!”. This is where Closing The Gap excels.”

So it is possible to publish a math-book worth writing. Or at least, some people can pull it off.

Problem was I needed to kick myself into writing mode. Feeling forced to post something daily wouldn’t hurt.

Anyway, I was sure this would have to stop soon. I had plans to disappear for 10 days into the French mountains. Our place there suffers from frequent power- and cellphone-cuts, which can last for days.

Thank you for upgrading your network to the remotest of places. At times, it felt like I was working from home.

I kept on blogging.

Even now, there’s material lying around.

I’d love to understand the claim that non-commutative geometry may offer some help in explaining moonshine. There was an interesting question on an older post on nimber-arithmetic I feel I should be following up. I’ve given a couple of talks recently on $\mathbb{F}_1$-material, parts of which may be postable. And so on.

Problem is, I would stick to the same (rather dense) writing style.

Perhaps it would make more sense to aim for a weekly (or even monthly) post over at Medium.

Medium offers no MathJax support forcing me to write differently about maths, and for a broader potential audience.

I may continue to blog here (or not), stick to the current style (or try something differently). I have not the foggiest idea right now.

If you have suggestions or advice, please leave a comment or email me.

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a wintry chataigneraie

It took us some time to clear the array of old chestnut trees.

But it paid off. A good harvest easily gives half a ton of chestnuts, including some rare, older varieties.

In the autumn, the coloured leaves make a spectacular view. Now it looks rather desolate.

But, the wind piles up the fallen leaves in every nook, cranny, corner or entrance possible.

My day in the mountains: ‘harvesting’ fallen leaves…

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Brancusi’s advice : avoid vampires

My one and only resolution for 2018: ban vampires from my life!

Here’s the story.

In the 1920’s, Montparnasse was at the heart of the intellectual and artistic life in Paris because studios and cafés were inexpensive.

Artists including Picasso, Matisse, Zadkine, Modigliani, Dali, Chagall, Miro, and the Romanian sculptor Constantin Brancusi all lived there.

You’ll find many photographs of Picasso in the company of others (here center, with Modigliani and Salmon), but … not with Brancusi.

From A Life of Picasso: The Triumphant Years, 1917-1932 (Vol 3) by John Richardson:

“Brancusi disapproved of one of of Picasso’s fundamental characteristics—one that was all too familiar to the latter’s fellow artists and friends—his habit of making off not so much with their ideas as with their energy. “Picasso is a cannibal,” Brancusi said. He had a point. After a pleasurable day in Picasso’s company, those present were apt to end up suffering from collective nervous exhaustion. Picasso had made off with their energy and would go off to his studio and spend all night living off it. Brancusi hailed from vampire country and knew about such things, and he was not going to have his energy or the fruits of his energy appropriated by Picasso.”

I learned this story via Austin Kleon who made this video about it:

Show Your Work! Episode 1: Vampires from Austin Kleon on Vimeo.

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