## September 16, 2019

### Partial Evaluations 2

#### Posted by John Baez

*guest post by Carmen Constantin*

This post belongs to the series of the Applied Category Theory Adjoint School 2019 posts. It is a follow-up to Martin and Brandon’s post about partial evaluations.

Here we would like to use some results by Clementino,
Hofmann, and Janelidze
to answer the following questions: *When can we compose
partial evaluations?* and more generally *When is the
partial evaluation relation transitive?*

## September 12, 2019

### Stellenbosch is Hiring

#### Posted by John Baez

*guest post by Bruce Bartlett*

The Mathematics Division at Stellenbosch University is advertising two permanent faculty positions at the level of Senior Lecturer and Professor.

Quoting from the advertisement (Senior Lecturer position, Professor position):

The Mathematical Sciences Department is responsible for teaching and research in Mathematics, Applied Mathematics and Computer Science at Stellenbosch University. The Mathematics Division is keen to strengthen its research in Algebra, Analysis, Category Theory, Combinatorics, Logic, Number Theory, and Topology. The Faculty of Science will offer a good research establishment grant for the first two years.

## September 11, 2019

### The Riemann Hypothesis (Part 3)

#### Posted by John Baez

Now I’ll say a little about the Weil Conjectures and Grothendieck’s theory of ‘motives’. I will continue trying to avoid all the technical details, to convey some general flavor of the subject without assuming much knowledge of algebraic geometry.

I *will* start using terms like ‘variety’, but not much more. If you don’t know what that means, imagine it’s a shape described by a bunch of polynomial equations… with some points at infinity tacked on if it’s a ‘projective variety’. Also, you should know that a ‘curve’ is a 1-dimensional variety, but if we’re using the complex numbers it’ll look 2-dimensional to ordinary mortal’s eyes, like this:

This guy is an example of a ‘curve of genus 2’.

Okay, maybe now you know enough algebraic geometry for this post.

## September 10, 2019

### The Riemann Hypothesis (Part 2)

#### Posted by John Baez

Now let’s dig a tiny bit deeper into the Riemann Hypothesis, and the magnificent developments in algebraic geometry it has inspired. My desire to explain this all rather simply is making the story move more slowly than planned, but I guess that’s okay.

## September 7, 2019

### The Riemann Hypothesis (Part 1)

#### Posted by John Baez

I’ve been trying to understand the Riemann Hypothesis a bit better. Don’t worry, I’m not trying to *prove* it — that’s a dangerous quest. Indeed Ricardo Pérez-Marco has a whole list of things *not* to do if you want to prove the Riemann Hypothesis, such as:

Don’t expect that the problem consists in resolving a single hard difficulty. In this kind of hard problem many enemies are on your way, well hidden, and waiting for you.

and

Don’t go for it unless you have succeeded in other serious problems. “Serious problems” means problems that have been open and well known for years. If you think that the Riemann Hypothesis will be your first major strike, you probably deserve failure.

Taken together, his pieces of advice are sufficiently discouraging that he almost could have just said “don’t try to prove the Riemann Hypothesis”.

But trying to understand what it means, and how people have proved vaguely similar conjectures — that seems like a more reasonable hobby.

In what follows I want to keep things as simple as possible, because I’m finding, as I study this stuff, that people are generally too eager to dive into technical details before sketching out ideas in a rough way. But I will skip over a lot of standard introductory stuff on the Riemann zeta function, since that’s easy to find.

## September 6, 2019

### Homotopy Type Theory Electronic Seminar Talks

#### Posted by John Baez

Learn cool math without flying around making the planet hotter! The Homotopy Type Theory Electronic Seminar Talks (HoTTEST) will be returning in Fall 2019. The speakers are:

- October 9: Andrej Bauer
- October 23: Anders Mörtberg
- November 6: Andrew Swan
- November 20: Benno van den Berg
- December 4: Christian Sattler (TBC)
- December 11: Richard Garner

This semester, the seminar will be meeting on alternating Wednesdays at 11:30 Eastern Time. For updates and instructions how to attend, please see

The seminar is open to everyone, but some prior familiarity with homotopy type theory will be assumed.

## September 3, 2019

### The Narratives Category Theorists Tell Themselves

#### Posted by David Corfield

Years ago on this blog, I was exploring the way narrative may be used to give direction to a tradition of intellectual enquiry. This eventually led to a book chapter, Narrative and the Rationality of Mathematical Practice in B. Mazur and A. Doxiades (eds), Circles Disturbed, Princeton, 2012.

Now, someone recently reading this piece has invited to me to speak at a workshop, *Narrative and mathematical argument*, listed here. Reflecting on what I might discuss there, I settled on the following:

The narratives category theorists tell themselves

Category theory is an attempt to provide general tools for all of mathematics. Its history, dating back to the 1940s, is characterised by ambitious attempts to reformulate branches of mathematics and even mathematics as a whole. It has since moved on to influence theoretical computer science and mathematical physics. Resistance to this movement over the years has taken the form of accusations of engaging in abstraction for abstraction’s sake. Here we explore the role of narrative in forming the self-identity of category theorists.