## September 21, 2018

### A Pattern That Eventually Fails

#### Posted by John Baez

Sometimes you check just a few examples and decide something is always true. But sometimes even $1.5 \times 10^{43}$ examples is not enough.

## September 20, 2018

### Cartesian Double Categories

#### Posted by Mike Shulman

In general, there are two kinds of bicategories: those like $Cat$ and those like $Span$. In the $Cat$-like ones, the morphisms are “categorified functions”, which generally means some kind of “functor” between some kind of “category”, consisting of functions mapping objects and arrows from domain to codomain. But in the $Span$-like ones (which includes $Mod$ and $Prof$), the morphisms are not “functors” but rather some kind of “generalized relations” (including spans, modules, profunctors, and so on) which do not *map* from domain to codomain but rather *relate* the domain and codomain in some way.

In $Span$-like bicategories there is usually a subclass of the morphisms that *do* behave like categorified functions, and these play an important role. Usually the morphisms in this subclass all have right adjoints; sometimes they are exactly the morphisms with right adjoints; and often one can get away with talking about “morphisms with right adjoints” rather than making this subclass explicit. However, it’s also often conceptually and technically helpful to give the subclass as extra data, and arguably the most perspicuous way to do this is to work with a *double category* instead. This was the point of my first published paper, though others had certainly made the same point before, and I think more and more people are coming to recognize it.

Today a new installment in this story appeared on the arXiv: Cartesian Double Categories with an Emphasis on Characterizing Spans, by Evangelia Aleiferi. This is a project that I’ve wished for a while someone would do, so I’m excited that at last someone has!

## September 19, 2018

*p*-Local Group Theory

#### Posted by John Baez

I’ve been trying to learn a bit of the theory of finite groups. As you may know, Sylow’s theorems say that if you have a finite group $G$, and $p^k$ is the largest power of a prime $p$ that divides the order of $G$, then $G$ has a subgroup of order $p^k$, which is unique up to conjugation. This is called a **Sylow $p$-subgroup** of $G$.

Sylow’s theorems also say a lot about how many Sylow $p$-subgroups $G$ has. They also say that any subgroup of $G$ whose order is a power of $p$ is contained in a Sylow $p$-subgroup.

I didn’t like these theorems as an undergrad. The course I took whizzed through them in a desultory way. And I didn’t go after them myself: I was into group theory for its applications to physics, and the detailed structure of finite groups doesn’t look important when you’re first learning physics: what stands out are *continuous* symmetries, so I was busy studying Lie groups.

Since I didn’t really master Sylow’s theorems, and had no strong motive to do so, I didn’t like them — the usual sad story of youthful mathematical distastes.

But now I’m thinking about Sylow’s theorems again, especially pleased by Robert A. Wilson’s one-paragraph proof of all three of these theorems in his book *The Finite Simple Groups*. And I started wondering if the importance of groups of prime power order — which we see highlighted in Sylow’s theorems and many other results — is all related to localization in algebraic topology, which is a technique to focus attention on a particular prime.

## September 18, 2018

### What is Applied Category Theory?

#### Posted by John Baez

Tai-Danae Bradley has a new free “booklet” on applied category theory. It grew out of the workshop Applied Category Theory 2018, and I think it makes a great complement to Fong and Spivak’s book *Seven Sketches* and my online course based on that book:

- Tai-Danae Bradley,
*What is Applied Category Theory?*

Abstract.This is a collection of introductory, expository notes on applied category theory, inspired by the 2018 Applied Category Theory Workshop, and in these notes we take a leisurely stroll through two themes (functorial semantics and compositionality), two constructions (monoidal categories and decorated cospans) and two examples (chemical reaction networks and natural language processing) within the field.

Check it out!

## September 5, 2018

### A Categorical Look at Random Variables

#### Posted by Tom Leinster

*guest post by Mark Meckes*

For the past several years I’ve been thinking on and off about whether there’s a fruitful category-theoretic perspective on probability theory, or at least a perspective with a category-theoretic flavor.

(You can see this MathOverflow question by Pete Clark for some background, though I started thinking about this question somewhat earlier. The fact that I’m writing this post should tell you something about my attitude toward my own answer there. On the other hand, that answer indicates something of the perspective I’m coming from.)

I’m a long way from finding such a perspective I’m happy with, but I have some observations I’d like to share with other n-Category Café patrons on the subject, in hopes of stirring up some interesting discussion. The main idea here was pointed out to me by Tom, who I pester about this subject on an approximately annual basis.

## August 27, 2018

### Exceptional Quantum Geometry and Particle Physics

#### Posted by John Baez

It would be great if we could make sense of the Standard Model: the 3 generations of quarks and leptons, the 3 colors of quarks vs. colorless leptons, the way only the weak force notices the difference between left and right, the curious gauge group $\mathrm{SU}(3) \times \mathrm{SU}(2)\times \mathrm{U}(1)$, the role of the Higgs boson, and so on. I can’t help but hope that all these facts are clues that we have not yet managed to interpret.

These papers may not be on the right track, but I feel a duty to explain them:

Michel Dubois-Violette, Exceptional quantum geometry and particle physics.

Michel Dubois-Violette and Ivan Todorov, Exceptional quantum geometry and particle physics II.

Michel Dubois-Violette and Ivan Todorov, Deducing the symmetry of the standard model from the automorphism and structure groups of the exceptional Jordan algebra.

After all, the math is probably right. And they use the exceptional Jordan algebra, which I’ve already wasted a lot of time thinking about — so I’m in a better position than most to summarize what they’ve done.

Don’t get me wrong: I’m not claiming this paper is important for physics! I really have no idea. But it’s making progress on a quirky, quixotic line of thought that has fascinated me for years.

Here’s the main result. The exceptional Jordan algebra contains a lot of copies of 4-dimensional Minkowski spacetime. The symmetries of the exceptional Jordan algebra that preserve any one of these copies form a group…. which happens to be exactly the gauge group of the Standard Model!

## August 24, 2018

*Compositionality*: Now Open For Submissions

#### Posted by John Baez

Our new journal *Compositionality* is now open for submissions!

It’s an open-access journal for research using compositional ideas, most notably of a category-theoretic origin, in any discipline. Topics may concern foundational structures, an organizing principle, or a powerful tool. Example areas include but are not limited to: computation, logic, physics, chemistry, engineering, linguistics, and cognition.

*Compositionality* is free of cost for both readers and authors.

## August 22, 2018

### Kan

#### Posted by John Baez

Jake Bian works on the topology and geometry of neural networks. But now he’s created a new add-on—okay, let’s say it, an *extension*—for Firefox, designed to make nLab entries look more like textbook chapters:

- Kan.

## August 10, 2018

### The Philosophy and Physics of Noether’s Theorems

#### Posted by David Corfield

Nicholas Teh tells me that there is to be a conference held in London, UK, on October 5-6, 2018, celebrating the centenary of Emmy Noether’s work in mathematical physics.

2018 brings with it the centenary of a major milestone in mathematical physics: the publication of Amalie (“Emmy”) Noether’s theorems relating symmetry and physical quantities, which continue to be a font of inspiration for “symmetry arguments” in physics, and for the interpretation of symmetry within philosophy.

In order to celebrate Noether’s legacy, the University of Notre Dame and the LSE Centre for Philosophy of Natural and Social Sciences are co-organizing a conference that will bring together leading mathematicians, physicists, and philosophers of physics in order to discuss the enduring impact of Noether’s work.

Speakers include our very own John Baez.

## July 18, 2018

### The Duties of a Mathematician

#### Posted by John Baez

What are the ethical responsibilities of a mathematician? I can think of many, some of which I even try to fulfill, but this document raises one that I have mixed feelings about:

- Ivan Fesenko, Remarks on aspects of pioneering modern mathematics research.

Namely:

The ethical responsibility of mathematicians includes a certain duty, never precisely stated in any formal way, but of course felt by and known to serious researchers: to dedicate an appropriate amount of time to study each new groundbreaking theory or proof in one’s general area. Truly groundbreaking theories are rare, and this duty is not too cumbersome. This duty is especially applicable to researchers who are in the most active research period of their mathematical life and have already senior academic positions. In real life this informal duty can be taken to mean that a reasonable number of mathematicians in each major mathematical country studies such groundbreaking theories.

*Compositionality*: the Editorial Board

#### Posted by John Baez

An editorial board has now been chosen for the journal *Compositionality*, and they’re waiting for people to submit papers.

## July 7, 2018

### Beyond Classical Bayesian Networks

#### Posted by John Baez

*guest post by Pablo Andres-Martinez and Sophie Raynor*

In the final installment of the Applied Category Theory
seminar, we discussed the
2014 paper *“Theory-independent limits on correlations from generalized
Bayesian
networks”*
by Henson, Lal and Pusey.

In this post, we’ll give a short introduction to Bayesian networks, explain why quantum mechanics means that one may want to generalise them, and present the main results of the paper. That’s a lot to cover, and there won’t be a huge amount of category theory, but we hope to give the reader some intuition about the issues involved, and another example of monoidal categories used in causal theory.

## July 4, 2018

### Symposium on Compositional Structures

#### Posted by John Baez

There’s a new conference series, whose acronym is pronounced “psycho”. It’s part of the new trend toward the study of “compositionality” in many branches of thought, often but not always using category theory:

- First Symposium on Compositional Structures (SYCO1), School of Computer Science, University of Birmingham, 20-21 September, 2018. Organized by Ross Duncan, Chris Heunen, Aleks Kissinger, Samuel Mimram, Simona Paoli, Mehrnoosh Sadrzadeh, Pawel Sobocinski and Jamie Vicary.

The Symposium on Compositional Structures is a new interdisciplinary series of meetings aiming to support the growing community of researchers interested in the phenomenon of compositionality, from both applied and abstract perspectives, and in particular where category theory serves as a unifying common language. We welcome submissions from researchers across computer science, mathematics, physics, philosophy, and beyond, with the aim of fostering friendly discussion, disseminating new ideas, and spreading knowledge between fields. Submission is encouraged for both mature research and work in progress, and by both established academics and junior researchers, including students.

More details below! Our very own David Corfield is one of the invited speakers.

## June 27, 2018

### Elmendorf’s Theorem

#### Posted by John Huerta

I want to tell you about Elmendorf’s theorem on equivariant homotopy theory. This theorem played a key role in a recent preprint I wrote with Hisham Sati and Urs Schreiber:

We figured out how to apply this theorem in mathematical physics. But Elmendorf’s theorem by itself is a gem of homotopy theory and deserves to be better known. Here’s what it says, roughly: given any $G$-space $X$, the equivariant homotopy type of $X$ is determined by the ordinary homotopy types of the fixed point subspaces $X^H$, where $H$ runs over all subgroups of $G$. I don’t know how to intuitively motivate this fact; I would like to know, and if any of you have ideas, please comment. Below the fold, I will spell out the precise theorem, and show you how it gives us a way to define a $G$-equivariant version of any homotopy theory.

## June 15, 2018

### ∞-Atomic Geometric Morphisms

#### Posted by Mike Shulman

Today’s installment in the ongoing project to sketch the $\infty$-elephant: atomic geometric morphisms.

Chapter C3 of Sketches of an Elephant studies various classes of geometric morphisms between toposes. Pretty much all of this chapter has been categorified, *except* for section C3.5 about *atomic* geometric morphisms. To briefly summarize the picture:

Sections C3.1 (open geometric morphisms) and C3.3 (locally connected geometric morphisms) are steps $n=-1$ and $n=0$ on an infinite ladder of locally n-connected geometric morphisms, for $-1 \le n \le \infty$. A geometric morphism between $(n+1,1)$-toposes is locally $n$-connected if its inverse image functor is locally cartesian closed and has a left adjoint. More generally, a geometric morphism between $(m,1)$-toposes is locally $n$-connected, for $n\lt m$, if it is “locally” locally $n$-connected on $n$-truncated maps.

Sections C3.2 (proper geometric morphisms) and C3.4 (tidy geometric morphisms) are likewise steps $n=-1$ and $n=0$ on an infinite ladder of n-proper geometric morphisms.

Section C3.6 (local geometric morphisms) is also step $n=0$ on an infinite ladder: a geometric morphism between $(n+1,1)$-toposes is $n$-local if its direct image functor has an indexed right adjoint. Cohesive toposes, which have attracted a lot of attention around here, are both locally $\infty$-connected and $\infty$-local. (Curiously, the $n=-1$ case of locality doesn’t seem to be mentioned in the 1-Elephant; has anyone seen it before?)

So what about C3.5? An atomic geometric morphism between elementary 1-toposes is usually defined as one whose inverse image functor is logical. This is an intriguing prospect to categorify, because it appears to mix the “elementary” and “Grothendieck” aspects of topos theory: a geometric morphisms are arguably the natural morphisms between Grothendieck toposes, while logical functors are more natural for the elementary sort (where “natural” means “preserves all the structure in the definition”). So now that we’re starting to see some progress on elementary higher toposes (my post last year has now been followed by a preprint by Rasekh), we might hope be able to make some progress on it.