## December 13, 2019

### Applied Category Theory Postdocs at NIST

#### Posted by John Baez

Here is an advertisement for postdoc positions in applied category theory at the National Institute of Standards and Technology.

## December 11, 2019

### Random Permutations (Part 9)

#### Posted by John Baez

In our quest to understand the true nature of random permutations, Part 7 took us into a deeper stratum: their connection to Poisson distributions. I listed a few surprising facts. For example, these are the same:

- The probability that $n$ raindrops land on your head in a minute, if on average one lands on your head every $k$ minutes.
- The probability that a random permutation of a huge finite set has $n$ cycles of length $k$.

Here the raindrops are Poisson distributed, and ‘huge’ means I’m taking a limit as the size of a finite set goes to infinity.

Now let’s start trying to understand what’s going on here! Today we’ll establish the above connection between raindrops and random permutations by solving this puzzle:

**Puzzle 12.** Treat the number of cycles of length $k$ in a random permutation of an $n$-element set as a random variable. What do the moments of this random variable approach as $n \to \infty$?

First, let me take a moment to explain moments.

## December 8, 2019

### Random Permutations (Part 8)

#### Posted by John Baez

Last time we saw a strong connection between random permutations and Poisson distributions. Thinking about this pulled me into questions about *partitions* and the moments of Poisson distributions. These may be a bit of a distraction — but maybe not, since every permutation gives a partition, namely the partition into cycles.

Here’s a good puzzle to start with:

**Puzzle 11.** Suppose raindrops are falling on your head, randomly and independently, at an average rate of one per minute. What’s the average of the *cube* of the number of raindrops that fall on your head in one minute?

## December 7, 2019

### Random Permutations (Part 7)

#### Posted by John Baez

Last time I computed the expected number of cycles of length $k$ in a random permutation of an $n$-element set. The answer is wonderfully simple: $1/k$ for all $k = 1, \dots, n$.

As Mark Meckes pointed out, this fact is just a shadow of a more wonderful fact. Let $C_k$ be the number of cycles of length $k$ in a random permutation of an $n$-element set, thought of as a random variable. Then as $n \to \infty$, the distribution of $C_k$ approaches a Poisson distribution with mean $1/k$.

But this is just a shadow of an even *more* wonderful fact!

### A Visual Telling of Joyal’s Proof Of Cayley’s Formula

#### Posted by Tom Leinster

Cayley’s formula says how many trees there are on a given set of vertices. For a set with $n$ elements, there are $n^{n - 2}$ of them. In 1981, André Joyal published a wonderful new proof of this fact, which, although completely elementary and explicit, seems to have been one of the seeds for his theory of combinatorial species.

All I’m going to do in this post is explain Joyal’s proof, with lots of pictures. In a later post, I’ll explain the sense in which Cayley’s formula is the set-theoretic analogue of the linear-algebraic fact I wrote about last time: that for a random linear operator on a finite vector space $V$, the probability that it’s nilpotent is $1/\#V$. And I’ll also give a new (?) proof of that fact, imitating Joyal’s. But that’s for another day.

## December 5, 2019

### Position at U. C. Riverside

#### Posted by John Baez

The Department of Mathematics at the University of California, Riverside invites application to a faculty position at the full professor level in the general area of pure mathematics. The successful candidate is expected to fill the F. Burton Jones Chair in Pure Mathematics. The search seeks candidates with national and international recognition, an outstanding research track record and outlook, a strong record in mentoring and teaching, and leadership in promoting diversity and serving the professional community.

A PhD in mathematics is required. It is expected that at UCR, the candidate will lead a rigorous research program, will develop and teach graduate and undergraduate courses, will advise and mentor graduate and undergraduate students, will vigorously promote diversity, and will engage in both campus and professional service activities.

A start-up package will be provided. The F. Burton Jones endowment fund will be made available to support the candidate’s research and mentoring activities. This position will start between July 1, 2020 and July 1, 2021.

### Topoi from Lawvere Theories

#### Posted by John Baez

Often we get a classifying topos by taking a finite limits theory $T$ and forming the category of presheaves $\hat{T} = Set^{T^{op}}$. For example, in their book, Mac Lane and Moerdijk get the classifying topos for commutative rings from the finite limits theory for rings this way. My student Christian Williams suggested an interesting alternative: what if we take the category of presheaves on a Lawvere theory?

For example, suppose we take the category of presheaves on the Lawvere theory for commutative rings. We get a topos. What is it the classifying topos for?

## December 4, 2019

### A Couple of Talks in Vienna

#### Posted by David Corfield

I’m in Vienna to give a couple of talks, one yesterday and one later this afternoon. Draft slides of both talks are on this page.

The first sees me take up again the work of Michael Friedman, in particular his Dynamics of Reason. I see that I was lamenting the difficulty of getting my original article published back in 2007. (But then I was still a couple of months away from finding out that I had a permanent job after a very long search, and so evidently fed up.)

In any case, I think the story I’m telling is improving with age - we now appear to have modal homotopy type theory and M-theory in harmony. But irrespective of whether Urs Schreiber’s Hypothesis H bears fruit, there’s still an important story to tell about mathematics modifying its framework.

The second talk introduces ideas from my forthcoming Modal homotopy type theory: The prospect of a new logic for philosophy.

## December 3, 2019

### Counting Nilpotents

#### Posted by Tom Leinster

What is the probability that a random linear operator is nilpotent?

Recall that a linear operator $T$ on a vector space $V$ is called “nilpotent” if $T^n = 0$ for some $n$. Assuming that $V$ is finite-dimensional and over a finite field, there are only finitely many operators on $V$. Choose one at random. It turns out that the probability of it being nilpotent is

$\frac{1}{\# V}$

— the reciprocal of the number of elements of $V$.

I’ll explain why. Along the way, we’ll meet some dynamical linear algebra, some $q$-formalism, and a kind of generating function adapted to linear algebra over a finite field.

This post was inspired by John’s recent series on random permutations, but is free-standing and self-contained. Enjoy!

## December 1, 2019

### Random Permutations (Part 6)

#### Posted by John Baez

Now I’ll tackle a really fundamental question about random permutations — one whose answer will quickly deliver the solutions to many others!

**Puzzle 7.** What is the expected number of cycles of length $k$ in a random permutation of an $n$-element set?

The answer is beautiful: it’s $1/k$. More precisely, this holds whenever the question is reasonable, meaning $1 \le k \le n$.

Note that this answer passes a simple sanity check. If on average there’s $1$ cycle of length $1$, $1/2$ cycles of length $2$, and so on, the average number of elements in our $n$-element set must be

$1 \cdot 1 + \frac{1}{2} \cdot 2 + \cdots + \frac{1}{n} \cdot n = n$

Whew!