## February 5, 2023

### Applied Category Theory 2023

#### Posted by John Baez

You can now submit a paper if you want to give a talk here:

## January 31, 2023

### Talk on the Tenfold Way

#### Posted by John Baez

There are ten ways that a substance can have symmetry under time reversal, switching particles and holes, both or neither. But this fact turns out to extend far beyond condensed matter physics! It’s really built into the fabric of mathematics in a deep way.

On Monday February 6, 2023 I’m giving a talk about this. It’s at 10 am Pacific Time, or 18:00 UTC. To attend, you need to register here. You can see my slides already here.

Posted at 7:19 AM UTC | Permalink | Followups (9)

## January 26, 2023

### Mathematics for Humanity

#### Posted by John Baez

I mentioned this earlier, but now it’s actually happening! I hope you can think of good workshops and apply to run them in Edinburgh.

Posted at 9:42 PM UTC | Permalink | Followups (2)

## January 23, 2023

### Question on Condensed Matter Physics

#### Posted by John Baez

The tenfold way is a mathematical classification of Hamiltonians used in condensed matter physics, based on their symmetries. Nine kinds are characterized by choosing one of these 3 options:

• antiunitary time-reversal symmetry with $T^2 = 1$, with $T^2 = -1$, or no such symmetry.

and one of these 3 options:

• antiunitary charge conjugation symmetry with $C^2 = 1$, with $C^2 = -1$, or no such symmetry.

(Charge conjugation symmetry in condensed matter physics is usually a symmetry between particles - e.g. electrons or quasiparticles of some sort - and holes.)

The tenth kind has unitary “$S$” symmetry, a symmetry that simultaneously reverses the direction of time and interchanges particles and holes. Since it is unitary and we’re free to multiply it by a phase, we can assume without loss of generality that $S^2 = 1$.

What are examples of real-world condensed matter systems of all ten kinds?

Posted at 7:43 PM UTC | Permalink | Followups (9)

## January 17, 2023

### The Tenfold Way (Part 8)

#### Posted by John Baez

Last time I explained a wonderful one-to-one correspondence between the 10 Morita equivalence classes of Clifford algebras and Cartan’s 10 infinite families of compact symmetric spaces.

This time I’ll do something different. I’ll explain a wonderful one-to-one correspondence between the 10 Morita equivalence classes of Clifford algebras and Cartan’s 10 infinite families of compact symmetric spaces.

Yes, it’s different! Not only will the details of the construction look very different, it gives a different correspondence! And I hope you can help me figure out what’s going on.

I thank Claude Schochet for pointing out that these two constructions don’t match.

Posted at 11:05 PM UTC | Permalink | Followups (4)

## January 16, 2023

### The Tenfold Way (Part 7)

#### Posted by John Baez

Last time I reviewed a bit of Bott periodicity. Now I want to start leading up to a question about it. It will take a while.

So, this time, I will explain a wonderful one-to-one correspondence between the 10 Morita equivalence classes of Clifford algebras and Cartan’s 10 infinite families of compact symmetric spaces.

Then, next time, I will explain a wonderful one-to-one correspondence between the 10 Morita equivalence classes of Clifford algebras and Cartan’s 10 infinite families of compact symmetric spaces.

Unfortunately these correspondences are not the same! And my question will be: why? Or, perhaps better: what’s the relationship between them?

Posted at 12:15 AM UTC | Permalink | Followups (3)

## January 15, 2023

### The Tenfold Way (Part 6)

#### Posted by John Baez

I’ve been studying Bott periodicity on and off since 1979, when I did a term paper on Clifford algebras in an undergrad course on group representation theory and physics taught by Valentine Bargmann. He was 71 at the time. Some of the students mocked him for being a bit slow — but if we’d known he’d been Einstein’s assistant from 1937 to 1946, we might have shown him a bit more respect, and asked him what working with Einstein was like!

I still have that term paper somewhere. Now I’m getting a bit slow, and I still don’t understand Bott periodicity quite as well as I want. So I have some questions. But in this part I’ll mainly just explain a bunch of stuff.

Posted at 11:17 PM UTC | Permalink | Followups (12)

### Total Freedom

#### Posted by John Baez

Wow! I just learned an objective reason why sets and vector spaces are special!

Of course we all know math relies heavily on set theory and linear algebra. And if you know category theory, you can say various things about why the categories $\mathsf{Set}$ and $\mathsf{Vect}$ are particularly convenient frameworks for calculation. But I’d never known a theorem that picks out these categories, and just a few others.

Briefly: these are categories of algebraic gadgets where all the objects are free!

We could call these ‘totally free’ algebraic gadgets.

Posted at 5:59 PM UTC | Permalink | Followups (17)

## January 6, 2023

### Topos Institute Positions

#### Posted by John Baez

The Topos Institute is doing some remarkable work in applying category theory to real-world problems. And they’re growing!

They want to hire a Finance and Operations Manager and a Research Software Engineer. For more information, go here.

And if you’re a grad student, you definitely want to check out their summer research positions! For more information on those, go here. Applications for these are due February 15th, 2023.

## January 4, 2023

### A Curious Integral

#### Posted by John Baez

On Mathstodon, Robin Houston pointed out a video where Oded Margalit claimed that it’s an open problem why this integral:

$\displaystyle{ \int_0^\infty\cos(2x)\prod_{n=1}^\infty\cos\left(\frac{x}{n} \right) d x }$

is so absurdly close to $\frac{\pi}{8}$, but not quite equal.

Posted at 4:26 PM UTC | Permalink | Followups (14)

## December 26, 2022

### Guillotine Partitions and the Hipparchus Operad

#### Posted by John Baez

If you dissect a square into $n$ similar rectangles, what proportions can these rectangles have? Folks on Mathstodon figured this out for $n \le 7$, and I blogged about it here recently. But I was left feeling that some deeper structure governed this problem.

Various people on Mathstodon, including Steven Stanicki, David Eppstein and Rahul Narain, convinced me of the importance of a certain class of dissections called ‘guillotine partitions’. I started suspecting that these were connected to an operad I once blogged about here: the ‘Hipparchus operad’. And last night I put some of the pieces together… though there is still more to do.

Posted at 9:42 PM UTC | Permalink | Followups (2)

## December 22, 2022

### Dividing a Square into Similar Rectangles

#### Posted by John Baez

If you divide a square into some fixed number of similar rectangles, what proportions can these rectangles have? We’ve been having fun thinking about this on Mathstodon, and here is a progress report.

Posted at 6:15 PM UTC | Permalink | Followups (21)

## December 21, 2022

### Free Idempotent Rigs and Monoids

#### Posted by John Baez

I’ve been having a lot of fun on Mathstodon lately, and here’s an example.

A rig $R$ has a commutative associative addition, an associative multiplication that distributes over addition, an element $0$ with $r+0 = r$ and $0r = 0 = r0$ for all $r \in R$, and an element $1$ with $1r = r = r1$ for all $r \in R$.

A rig is idempotent if $r r = r$ for all $r \in R$.

Is the free idempotent rig on $2$ generators finite? If so, how many elements does it have?

Morgan Rogers raised this issue on the Category Theory Community server, and after a bit of progress I posed this as a puzzle on Mathstodon. By now three people there have independently figured out the answer.

Posted at 10:38 AM UTC | Permalink | Followups (6)

## December 18, 2022

#### Posted by John Baez

Are you interested in applying category-theoretic methods to problems outside of pure mathematics? Apply to the Adjoint School!

Apply here. And do it soon.

• January 9, 2023. Application Due.

• February - July, 2023. Learning Seminar.

• July 24 - 28, 2023. In-person Research Week at University of Maryland, College Park, USA.

## December 3, 2022

### Neutrino Dark Matter

#### Posted by John Baez

I talked to Neil Turok at a café today. He used to be the head of the Perimeter Institute, but now he’s at the University of Edinburgh.

He coauthored a paper arguing that dark matter is very heavy right-handed neutrinos:

It’s very natural to add right-handed neutrinos to the Standard Model, and if they’re heavy they can make the observed left-handed neutrinos light via the ‘see-saw mechanism’. The problem is to keep them from decaying too fast!

Posted at 10:27 AM UTC | Permalink | Followups (23)