## February 28, 2022

### Hardy, Ramanujan and Taxi No. 1729

#### Posted by John Baez

In his book *Ramanujan: Twelve Lectures on Subjects Suggested by His Life and Work*, G. H. Hardy tells this famous story:

He could remember the idiosyncracies of numbers in an almost uncanny way. It was Littlewood who said every positive integer was one of Ramanujan’s personal friends. I remember once going to see him when he was lying ill at Putney. I had ridden in taxi-cab No. 1729, and remarked that the number seemed to be rather a dull one, and that I hoped it was not an unfavourable omen. “No,” he replied, “it is a very interesting number; it is the smallest number expressible as the sum of two cubes in two different ways.”

Namely,

$10^3 + 9^3 = 1000 + 729 = 1729 = 1728 + 1 = 12^3 + 1^3$

But there’s more to this story than meets the eye.

## February 25, 2022

### Applied Category Theory 2022

#### Posted by John Baez

The Fifth International Conference on Applied Category Theory, **ACT2022**, will take place at the University of Strathclyde from 18 to 22 July 2022, preceded by the Adjoint School 2022 from 11 to 15 July. This conference follows previous events at Cambridge (UK), Cambridge (MA), Oxford and Leiden.

Applied category theory is important to a growing community of researchers who study computer science, logic, engineering, physics, biology, chemistry, social science, linguistics and other subjects using category-theoretic tools. The background and experience of our members is as varied as the systems being studied. The goal of the Applied Category Theory conference series is to bring researchers together, strengthen the applied category theory community, disseminate the latest results, and facilitate further development of the field.

## February 15, 2022

### Questions About the Néron–Severi Group

#### Posted by John Baez

A friend of mine with good intuitions sometimes says things without proof, and sometimes I want to know why — or even whether — these things are true.

Here are some examples from algebraic geometry.

## February 12, 2022

### Ergodic Set Theory is Trivial

#### Posted by Tom Leinster

On the one hand, an ultrafilter on a set can be seen as a primitive sort of probability measure, in which every subset is assigned a probability of either 0 or 1 and the measure only has to be *finitely* additive.

On the other, ergodic theory studies measure-preserving endomorphisms of probability spaces.

What happens if you put the two together? That is, given an ultrafilter $\mathcal{U}$ on a set $X$, what can be said about the measure-preserving endomorphisms of $(X, \mathcal{U})$?

The answer is: very little. The situation is trivial. Essentially the only measure-preserving endomorphism of $(X, \mathcal{U})$ is the identity. A bit more exactly, such an endomorphism must be the identity almost everywhere.

Here’s why.

## February 10, 2022

### Submission to arXiv

#### Posted by John Baez

Philip Helbig is an astrophysicist who wrote a paper called The flatness problem and the age of the Universe. It’s a good review of some very important problems, but the arXiv refused to accept it in the category where it belongs, astro-ph. Instead they tried to shunt it off to gen-ph. This is their usual strategy for dealing with bad papers, designed to keep the more important categories clean. Helbig protested, and this blog article is the story of what happened next.

Briefly: by now his paper has been published in the *Monthly Notices of the Royal Astronomical Society*, but the arXiv still refuses to accept his paper in astro-ph. Recently Stein Sigurðsson, the Scientific Director of the arXiv, gave this cryptic response:

As I have noted, we do not discuss decisions on individual papers with third parties. When people express concerns, we discuss process and broader issues that affect the processes. The SCOAP3 agreement constrains how appeals for published submissions are handled.

and

SCOAP3 impacts all appeals to arXiv based on the submission having been published in a journal. arXiv gets sued, our policies and processes are constrained by advice of counsel and rulings in those cases. I believe in all case arXiv prevailed, but judge rulings provide guidance.

I don’t understand this. Is he claiming that the arXiv might get sued if they put Helbig’s paper on the arXiv now… *because* it’s been published?

## February 8, 2022

### Concepts and Profunctor Nuclei

#### Posted by Simon Willerton

*Guest post by Matthew Di Meglio and Owen Lewis*

This article is part of the ongoing work at the 2021 Adjoint School. Thanks to Sophie Libkind, David Jaz Myers, Simon Willerton, Tai-Danae Bradley, Callum Reader, and participants of the 2021 Adjoint School for helpful discussions and feedback.

Previously on this blog, Simon Willerton introduced the notion of the nucleus of an enriched profunctor, and alluded to the fact that the formal concept lattice of a relation is a particular instance of this general categorical construction. Continuing from Simon’s posts, we will present, through the lens of enriched category theory, a generalization of Formal Concept Analysis called Quantitative Concept Analysis (QCA). The material in this blog post is based on Dusko Pavlovic’s Quantitative Concept Analysis and Jonathan Elliot’s On the Fuzzy Concept Complex.

## February 7, 2022

### Compositional Thermostatics (Part 2)

#### Posted by John Baez

*guest post by Owen Lynch*

In Part 1, John talked about a paper that we wrote recently:

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

and he gave an overview of what a ‘thermostatic system’ is.

In this post, I want to talk about how to compose thermostatic systems. We will not yet use category theory, saving that for another post; instead we will give a ‘nuts-and-bolts’ approach, based on examples.