## October 25, 2016

### The Kan Extension Seminar returns

#### Posted by Emily Riehl

In early 2014, the $n$-Category Café hosted the Kan Extension Seminar, a graduate reading course in category theory modeled after Daniel Kan’s eponymous reading course in algebraic topology at MIT. My experience with the seminar, described here, was overwhelming positive, so I am delighted to announce that we’re back. Alexander Campbell, Brendan Fong, and I are organizing “Kan II” in early 2017 and we are currently soliciting applications for seminar participants.

## October 20, 2016

### Linear Algebraic Groups (Part 2)

#### Posted by John Baez

This time we show how projective geometry ‘subsumes’ Euclidean, elliptic and hyperbolic geometry. It does so in two ways: the projective plane includes all 3 other planes, and its symmetry group contains their symmetry groups.

By the time we understand this, we’re almost ready to think about geometry as a subject that depends on a choice of group. But we’re also getting ready to think about algebraic geometry (for example, projective varieties).

## October 17, 2016

### Linear Algebraic Groups (Part 1)

#### Posted by John Baez

I’m teaching an elementary course on linear algebraic groups. The main aim is not to prove a lot of theorems, but rather to give some sense of the main examples and the overall point of the subject. I’ll start with the ideas of Klein geometry, and their origin in old questions going back almost to Euclid.

John Simanyi has been taking wonderful notes in LaTeX, so you can read those!

## October 10, 2016

### Jobs at Edinburgh

#### Posted by Tom Leinster

I’m pleased to announce that we’re advertising two Lectureships in “algebra, geometry & topology and related fields such as category theory and mathematical physics”. Come and join us! We’re a happy, well-resourced department with a very positive atmosphere. The algebra/geometry/topology group provides an excellent home for a category theorist.

To be clear, these positions are for practical purposes permanent, i.e. as close as the UK gets to tenure. There’s no one-to-one correspondence between UK and US job titles, so I’ll just say that Lecturer is the usual starting position for someone in their first permanent job, followed by Senior Lecturer, Reader, then Professor. The ad adds “Exceptionally, the appointments may be to Readership”.

## October 4, 2016

### Mathematics Research Community in HoTT

#### Posted by Emily Riehl

I am delighted to announce that from June 4-10, 2017, there will be a workshop on homotopy type theory as one of the AMS’s Mathematical Research Communities.

The MRC program, whose workshops are held in the “breathtaking mountain setting” of Snowbird Resort in Utah,

nurtures early-career mathematicians — those who are close to finishing their doctorates or have recently finished — and provides them with opportunities to build social and collaborative networks to inspire and sustain each other in their work.

The organizers for the HoTT MRC include our fearless leader Chris Kapulkin, Dan Christensen, Dan Licata, Mike Shulman and myself.

## September 26, 2016

### Euclidean, Hyperbolic and Elliptic Geometry

#### Posted by John Baez

There are two famous kinds of non-Euclidean geometry: hyperbolic geometry and elliptic geometry (which almost deserves to be called ‘spherical’ geometry, but not quite because we identify antipodal points on the sphere).

In fact, these two kinds of geometry, together with Euclidean geometry, fit into a unified framework with a parameter $s \in \mathbb{R}$ that tells you the curvature of space:

when $s \gt 0$ you’re doing elliptic geometry

when $s = 0$ you’re doing Euclidean geometry

when $s \lt 0$ you’re doing hyperbolic geometry.

This is all well-known, but I’m trying to explain it in a course I’m teaching, and there’s something that’s bugging me.

It concerns the precise way in which elliptic and hyperbolic geometry reduce to Euclidean geometry as $s \to 0$. I know this is a problem of deformation theory involving a group contraction, indeed I know all sorts of fancy junk, but my problem is fairly basic and this junk isn’t helping.

## September 19, 2016

### Logical Uncertainty and Logical Induction

#### Posted by Qiaochu Yuan

Quick - what’s the $10^{100}$th digit of $\pi$?

If you’re anything like me, you have some uncertainty about the answer to this question. In fact, your uncertainty probably takes the following form: you assign a subjective probability of about $\frac{1}{10}$ to this digit being any one of the possible values $0, 1, 2, \dots 9$. This is despite the fact that

- the normality of $\pi$ in base $10$ is a wide open problem, and
- even if it weren’t, nothing random is happening; the $10^{100}$th digit of $\pi$ is a particular digit, not a randomly selected one, and it being a particular value is a mathematical fact which is either true or false.

If you’re bothered by this state of affairs, you could try to resolve it by computing the $10^{100}$th digit of $\pi$, but as far as I know nobody has the computational resources to do this in a reasonable amount of time.

Because of this lack of computational resources, among other things, you and I aren’t **logically omniscient**; we don’t have access to all of the logical consequences of our beliefs. The kind of uncertainty we have about mathematical questions that are too difficult for us to settle one way or another right this moment is **logical uncertainty**, and standard accounts of how to have uncertain beliefs (for example, assign probabilities and update them using Bayes’ theorem) don’t capture it.

Nevertheless, somehow mathematicians manage to have lots of beliefs about how likely mathematical conjectures such as the Riemann hypothesis are to be true, and even about simpler but still difficult mathematical questions such as how likely some very large complicated number $N$ is to be prime (a reasonable guess, before we’ve done any divisibility tests, is about $\frac{1}{\ln N}$ by the prime number theorem). In some contexts we have even more sophisticated guesses like the Cohen-Lenstra heuristics for assigning probabilities to mathematical statements such as “the class number of such-and-such complicated number field has $p$-part equal to so-and-so.”

In general, what criteria might we use to judge an assignment of probabilities to mathematical statements as reasonable or unreasonable? Given some criteria, how easy is it to find a way to assign probabilities to mathematical statements that actually satisfies them? These fundamental questions are the subject of the following paper:

Scott Garrabrant, Tsvi Benson-Tilsen, Andrew Critch, Nate Soares, and Jessica Taylor, Logical Induction. ArXiv:1609.03543.

Loosely speaking, in this paper the authors

- describe a criterion called
**logical induction**that an assignment of probabilities to mathematical statements could satisfy, - show that logical induction implies many other desirable criteria, some of which have previously appeared in the literature, and
- prove that a computable logical inductor (an algorithm producing probability assignments satisfying logical induction) exists.

Logical induction is a weak “no Dutch book” condition; the idea is that a logical inductor makes bets about which statements are true or false, and does so in a way that doesn’t lose it too much money over time.

## September 15, 2016

### Disaster at Leicester

#### Posted by Tom Leinster

You’ve probably met mathematicians at the University of Leicester, or read their work, or attended their talks, or been to events they’ve organized. Their pure group includes at least four people working in categorical areas: Frank Neumann, Simona Paoli, Teimuraz Pirashvili and Andy Tonks.

Now this department is under severe threat. A colleague of mine writes:

24 members of the Department of Mathematics at the University of Leicester — the great majority of the members of the department — have been informed that their post is at risk of redundancy, and will have to reapply for their positions by the end of September. Only 18 of those applying will be re-appointed (and some of those have been changed to purely teaching positions).

It’s not only mathematics at stake. The university is apparently on a process of “institutional transformation”, involving:

the closure of departments, subject areas and courses, including the Vaughan Centre for Lifelong Learning and the university bookshop. Hundreds of academic, academic-related and support staff are to be made redundant, many of them compulsorily.

If you don’t like this, sign the petition objecting! You’ll see lots of familiar names already on the list (Tim Gowers, John Baez, Ross Street, …). As signatory David Pritchard wrote, “successful departments and universities are hard to build and easy to destroy.”

## September 13, 2016

### HoTT and Philosophy

#### Posted by David Corfield

I’m down in Bristol at a conference – HoTT and Philosophy. Slides for my talk – *The modality of physical law in modal homotopy type theory* – are here.

Perhaps ‘The modality of differential equations’ would have been more accurate as I’m looking to work through an analogy in modal type theory between necessity and the jet comonad, partial differential equations being the latter’s coalgebras.

The talk should provide some intuition for a pair of talks the following day:

- Urs Schreiber & Felix Wellen: ‘Formalizing higher Cartan geometry in modal HoTT’
- Felix Wellen: ‘Synthetic differential geometry in homotopy type theory via a modal operator’

I met up with Urs and Felix yesterday evening. Felix is coding up in Agda geometric constructions, such as frame bundles, using the modalities of differential cohesion.

## September 12, 2016

#### Posted by John Baez

I’m now trying to announce all my new writings in one place: on Twitter.

Why? Well…

## September 9, 2016

### Barceló and Carbery on the Magnitude of Odd Balls

#### Posted by Simon Willerton

In Tom’s recent post he mentioned that Juan Antonio Barceló and Tony Carbery had been able to calculate the magnitude of any odd-dimensional Euclidean ball. In this post I would like to give some idea of the methods they use for calculating the magnitude.

Tony and Juan Antonio calculate the magnitude of an odd dimensional ball of a given radius using a potential function rather than a weighting, I think that if you know much about magnitude then you will have some idea what a weighting is but not much idea about what a potential function is, so I will explain that below, the theory having been developed by Mark Meckes.

I intend to brush over technical details about distributions, I hope that I do not do so in too egregious a fashion.

In Tony and Juan Antonio’s paper, various aspects of mine and Tom’s Convex Magnitude Conjecture are confirmed [see the comments below]; however, the calculations for the five-ball provide a counterexample to the conjecture in general. This raises lots of new and interesting questions, but I won’t go into them in this post.

### The Ultimate Question, and its Answer

#### Posted by John Baez

David Madore has a lot of great stuff on his website - videos and discussion of rotating black holes, a math blog whose only defect is that half is in French, and more.

He has has an interesting story that claims to tell you the Ultimate Question, and its Answer:

- David Madore, Toward enlightenment.

No, it’s not 42. I like it, but I can’t tell how much sense it makes. So, I’ll ask you.

## September 6, 2016

### Magnitude Homology

#### Posted by Tom Leinster

I’m excited that over on this thread, Mike Shulman has proposed a very plausible theory of magnitude homology. I think his creation could be really important! It’s general enough that it can be applied in lots of different contexts, meaning that lots of different kinds of mathematician will end up wanting to use it.

However, the story of magnitude homology has so far only been told in that comments thread, which is very long, intricately nested, and probably only being followed by a tiny handful of people. And because I think this story deserves a really wide readership, I’m going to start afresh here and explain it from the beginning.

Magnitude is a *numerical* invariant of enriched categories.
Magnitude homology is an *algebraic* invariant of enriched categories. The
Euler characteristic of magnitude homology is magnitude, and in that sense,
magnitude homology is a categorification of magnitude. Let me explain!

## September 3, 2016

### Economy of Style

#### Posted by Tom Leinster

John Regehr writes: “holy cow this Cousot+Cousot paper achieves a density I’ve never before seen.” Me neither!

Much of the paper looks like the snippet shown, except for the part where they take the time to explain that “e.g.” means “for example”. Read this Twitter thread for speculation on how this state of affairs came to be.

## August 28, 2016

### Topological Crystals (Part 4)

#### Posted by John Baez

Okay, let’s look at some *examples* of topological crystals. These are what got me excited in the first place. We’ll get some highly symmetrical crystals, often in higher-dimensional Euclidean spaces. The ‘triamond’, above, is a 3d example.