## December 4, 2021

### Surveillance Publishing

#### Posted by John Baez

Björn Brembs recently explained how

“massive over-payment of academic publishers has enabled them to buy surveillance technology covering the entire workflow that can be used not only to be combined with our private data and sold, but also to make algorithmic (aka ‘evidenceled’) employment decisions.”

Reading about this led me to this article:

- Jefferson D. Pooley, Surveillance publishing.

It’s all about what publishers are doing to make money by collecting data on the habits of their readers. Let me quote a bunch!

## December 1, 2021

### Mysterious Triality

#### Posted by David Corfield

When we started this blog back in 2006 my co-founders were both interested in higher gauge theory. Their paths diverged as Urs looked to adapt these constructions to formulate the elusive M-theory.

Over the years I’ve been following this work, which has taken up a proposal by Hisham Sati in Framed M-branes, corners, and topological invariants, Sec 2.5 that M-theory be understood in terms of 4-cohomotopy, culminating in what they call Hypothesis H. I even chipped in sufficiently to one article to be included with them as an author:

- David Corfield, Hisham Sati, Urs Schreiber, Fundamental weight systems are quantum states.

Philosophically speaking, I’ve been intrigued by the idea that the novel mathematical framework of twisted equivariant differential cohomology theory, required for Hypothesis H, may be formulated via modal homotopy type theory. This is the line of thought I mentioned a few weeks ago in Dynamics of Reason Revisited.

But I’ve also been thinking that you can’t add something important to fundamental physics without it causing ripples through mathematics. So I was interested to see appear yesterday:

- Hisham Sati, Alexander Voronov,
*Mysterious triality*(arXiv:2111.14810).

## November 23, 2021

### Compositional Thermostatics

#### Posted by John Baez

At the Topos Institute this summer, a group of folks started talking about thermodynamics and category theory. It probably started because Spencer Breiner and my former student Joe Moeller, both working at the National Institute of Standards and Technology, were talking about thermodynamics with some people there. But I’ve been interested in thermodynamics for quite a while now — and Owen Lynch, a grad student visiting from the University of Utrecht, wanted to do his master’s thesis on the subject. He’s now working with me. Sophie Libkind, David Spivak and David Jaz Myers also joined in: they’re especially interested in open systems and how they interact.

Prompted by these conversations, a subset of us eventually wrote a paper on the foundations of equilibrium thermodynamics:

- John Baez, Owen Lynch and Joe Moeller, Compositional thermostatics.

## November 17, 2021

### Large Sets: The Movie

#### Posted by Tom Leinster

Earlier this year, I wrote a series of blog posts on large sets — or large cardinals, if you prefer — in categorical set theory. Thinking about large sets in Glasgow’s beautiful green spaces, writing those posts, and chatting about them with people here at the Café was one of the highlights of my summer.

Juan Orendain at the Universidad Nacional Autónoma de México was kind enough to invite me to give a talk in their category theory seminar, which I did today. I chose to speak about large sets, first giving a short introduction to categorical set theory, and then explaining some of the key points from this summer’s blog posts.

You can watch the video or read the slides.

## November 8, 2021

### Causality in Machine Learning

#### Posted by David Corfield

Back when we started the Café in 2006, I was working as a philosopher embedded with a machine learning group in the Max Planck Institute in Tübingen. Here I am reporting on my contribution to a NIPS workshop, held amongst the mountains of Whistler, on how one may still be able to learn when the distributions from which data is drawn for training and testing purposes differ. My proposal was that background knowledge, much of it causal, had to be deployed. It turns out that a video of the talk is still available – links to this and the resulting book chapter, *Projection and Projectability*, are here.

I was reminded of this work recently after seeing the strides taken by the machine learning community to integrate causal graphical models with their statistical techniques in Towards Causal Representation Learning and Causality for Machine Learning. Who knows? Perhaps my talk, which was after all addressed to some of these people, sowed a seed.

## October 30, 2021

### Firoozbakht’s Conjecture

#### Posted by John Baez

The Iranian mathematician Farideh Firoozbakht made a strong conjecture in 1982: the $n$th root of the $n$th prime keeps getting smaller as we make $n$ bigger! For example:

$\sqrt[1]{2} \; > \; \sqrt[2]{3} \; > \; \sqrt[3]{5} \; > \; \sqrt[4]{7} \; > \; \sqrt[5]{11} \; > \; \cdots$

It’s been checked for all primes up to about 18 quintillion, but nobody has a clue how to prove it. In fact, some experts think it’s probably false.

## October 28, 2021

### Learn Applied Category Theory!

#### Posted by John Baez

Do you like the idea of learning applied category theory by working on a project, as part of a team led by an expert? If you’re an early career researcher you can apply to do that now!

- Mathematical Research Community: Applied Category Theory, meeting 2022 May 29–June 4. Details on how to apply: here. Deadline to apply: Tuesday 2022 February 15 at 11:59 Eastern Time.

After working with your team online, you’ll take an all-expenses-paid trip to a conference center in upstate New York for a week in the summer. There will be a pool, bocci, lakes with canoes, woods to hike around in, campfires at night… and also whiteboards, meeting rooms, and coffee available 24 hours a day to power your research!

Later you’ll get invited to the 2023 Joint Mathematics Meetings in Boston.

## October 25, 2021

### The Kuramoto–Sivashinsky Equation (Part 2)

#### Posted by John Baez

I love the Kuramoto–Sivashinsky equation, beautifully depicted here by Thien An, because it’s one of the simplest partial differential equations that displays both chaos and a visible ‘arrow of time’. Now we’ll see that it’s also invariant under ‘Galilean transformations’ — that is, transformations into a moving frame of reference.

## October 22, 2021

### The Kuramoto–Sivashinsky Equation (Part 1)

#### Posted by John Baez

I love this movie showing a solution of the Kuramoto–Sivashinsky equation, made by Thien An. If you haven’t seen her great math images on Twitter, check them out!

I hadn’t known about this equation, and it looked completely crazy to me at first. But it turns out to be important, because it’s one of the simplest partial differential equations that exhibits chaotic behavior and an ‘arrow of time’: that is, a difference between the future and past.

## October 20, 2021

### What is the Uniform Distribution?

#### Posted by Tom Leinster

Today I gave the Statistics and Data Science seminar at Queen Mary University of London, at the kind invitation of Nina Otter. There I explained an idea that arose in work with Emily Roff. It’s an answer to this question:

What is the “canonical” or “uniform” probability distribution on a metric space?

You can see my slides here, and I’ll give a lightning summary of the ideas now.

## October 19, 2021

### Topos Institute Postdoc

#### Posted by John Baez

The Topos Institute is trying to hire a postdoc to work on polynomial functors! Here is the ad, written by David Spivak.

## October 15, 2021

### Dynamics of Reason Revisited

#### Posted by David Corfield

A couple of years ago, I mentioned a talk reporting my latest thoughts on a very long-term project to bring Michael Friedman’s *Dynamics of Reason* (2001) into relation with developments in higher category theory and its applications.

While that Vienna talk entered into some technicalities on cohomology, last week I had the opportunity of speaking at our departmental seminar in Kent, and so thought I’d sketch what might be of broader philosophical interest about the project.

You can find the slides here.

## October 4, 2021

### Stirling’s Formula

#### Posted by John Baez

Stirling’s formula says

$\displaystyle{ n! \sim \sqrt{2 \pi n} \, \left(\frac{n}{e}\right)^n }$

where $\sim$ means that the ratio of the two quantities goes to $1$ as $n \to \infty.$

Where does this formula come from? In particular, how does the number $2\pi$ get involved? Where is the circle here?

## September 27, 2021

### Weakly Globular Double Categories: a Model for Bicategories

#### Posted by Emily Riehl

*guest post by Claire Ott and Emma Phillips as part of the Adjoint School for Applied Category Theory 2021.*

As anyone who has worked with bicategories can tell you, checking coherence diagrams can hold up the completion of a proof for weeks. Paoli and Pronk have defined weakly globular double categories, a simplicial model of bicategories which is a sub-2-category of the 2-category of double categories. Today, we’ll introduce weakly globular double categories and briefly talk about the advantage of this model. We’ll also take a look at an application of this model: the SIRS model of infectious disease.

## September 23, 2021

### Axioms for the Category of Hilbert Spaces (bis)

#### Posted by Tom Leinster

*Guest post by Chris Heunen*

Dusk. The alley is secreted in mist. The miserly reflection of the single lamp on the cobblestones makes you nervous. The stranger approaches, hands inside trenchcoat. Why on Earth did you agree to meet here? The transfer happens, the stranger walks away without a word. The package, it’s all about the package. You have it now. You’d been promised it was the category of Hilbert spaces. But how can you be sure? You can’t just ask it. It didn’t come with a certificate of authenticity. All you can check is how morphisms compose. You leg it home and verify the Axioms for the category of Hilbert spaces!