## May 26, 2015

### A 2-Categorical Approach to the Pi Calculus

#### Posted by John Baez

*guest post by Mike Stay*

Greg Meredith and I have a short paper that’s been accepted for Higher-Dimensional Rewriting and Applications (HDRA) 2015 on modeling the asynchronous polyadic pi calculus with 2-categories. We avoid domain theory entirely and model the operational semantics directly; full abstraction is almost trivial. As a nice side-effect, we get a new tool for reasoning about consumption of resources during a computation.

It’s a small piece of a much larger project, which I’d like to describe here in a series of posts. This post will talk about lambda calculus for a few reasons. First, lambda calculus is simpler, but complex enough to illustrate one of our fundamental insights. Lambda calculus is to serial computation what pi calculus is to concurrent computation; lambda calculus talks about a single machine doing a computation, while pi calculus talks about a network of machines communicating over a network with potentially random delays. There is at most one possible outcome for a computation in the lambda calculus, while there are many possible outcomes in a computation in the pi calculus. Both the lazy lambda calculus and the pi calculus, however, have as an integral part of their semantics the notion of *waiting* for a sub-computation to complete before moving onto another one. Second, the denotational semantics of lambda calculus in Set is well understood, as is its generalization to cartesian closed categories; this semantics is far simpler than the denotational semantics of pi calculus and serves as a good introduction. The operational semantics of lambda calculus is also simpler than that of pi calculus and there is previous work on modeling it using higher categories.

### SoTFoM III and The Hyperuniverse Programme

#### Posted by David Corfield

Following SoTFom II, which managed to feature three talks on Homotopy Type Theory, there is now a call for papers announced for **SoTFoM III and The Hyperuniverse Programme**, to be held in Vienna, September 21-23, 2015.

Here are the details:

The Hyperuniverse Programme, launched in 2012, and currently pursued within a Templeton-funded research project at the Kurt Gödel Research Center in Vienna, aims to identify and philosophically motivate the adoption of new set-theoretic axioms.

The programme intersects several topics in the philosophy of set theory and of mathematics, such as the nature of mathematical (set-theoretic) truth, the universe/multiverse dichotomy, the alternative conceptions of the set-theoretic multiverse, the conceptual and epistemological status of new axioms and their alternative justificatory frameworks.

The aim of SotFoM III+The Hyperuniverse Programme Joint Conference is to bring together scholars who, over the last years, have contributed mathematically and philosophically to the ongoing work and debate on the foundations and the philosophy of set theory, in particular, to the understanding and the elucidation of the aforementioned topics. The three-day conference, taking place September 21-23 at the KGRC in Vienna, will feature invited and contributed speakers.

I wonder if anyone will bring some category theory along to the meeting. Perhaps they can answer my question here.

Further details:

## May 22, 2015

### PROPs for Linear Systems

#### Posted by John Baez

PROPs were developed in topology, along with operads, to describe spaces with lots of operations on them. But now some of us are using them to think about ‘signal-flow diagrams’ in control theory—an important branch of engineering. I talked about that here on the *n*-Café a while ago, but it’s time for an update.

### How to Acknowledge Your Funder

#### Posted by Tom Leinster

A comment today by Stefan Forcey points out ways in which US citizens can try to place legal limits on the surveillance powers of the National Security Agency, which we were discussing in the context of its links with the American Mathematical Society. If you want to act, time is of the essence!

But Stefan also tells us how he resolved a dilemma. Back here, he asked Café patrons what he should do about the fact that the NSA was offering him a grant (for non-classified work). Take their money and contribute to the normalization of the NSA’s presence within the math community, or refuse it and cause less mathematics to be created?

What he decided was to accept the funding and — in this paper at least — include a kind of protesting acknowledgement, citing his previous article for the *Notices of the AMS*.

I admire Stefan for openly discussing his dilemma, and I think there’s a lot to be said for how he’s handled it.

## May 21, 2015

### The Origin of the Word “Quandle”

#### Posted by John Baez

A quandle is a set equipped with a binary operation with number of properties, the most important being that it distributes over itself:

$a \triangleright (b \triangleright c) = (a \triangleright b)\triangleright (a \triangleright c)$

They show up in knot theory, where the operation $\triangleright$ describes what happens when one strand crosses over another… and the laws are chosen so that the quandle gives an *invariant* of a knot, independent of how you draw it. Even better, the quandle is a complete invariant of knots: if two knots have isomorphic quandles, there’s a diffeomorphism of $\mathbb{R}^3$ mapping one knot to the other.

I’ve always wondered where the name ‘quandle’ came from. So I decided to ask their inventor, David Joyce—who also proved the theorem I just mentioned.

## May 18, 2015

### The Revolution Will Not Be Formalized

#### Posted by Mike Shulman

After a discussion with Michael Harris over at the blog about his book Mathematics without apologies, I realized that there is a lot of confusion surrounding the relationship between homotopy type theory and computer formalization — and that moreover, this confusion may be causing people to react negatively to one or the other due to incorrect associations. There are good reasons to be confused, because the relationship is complicated, and various statements by prominent members of both communities about a “revolution” haven’t helped matters. This post and its sequel(s) are my attempt to clear things up.

## May 13, 2015

### Categorifying the Magnitude of a Graph

#### Posted by Simon Willerton

Tom Leinster introduced the idea of the magnitude of graphs (first at the Café and then in a paper). I’ve been working with my mathematical brother Richard Hepworth on categorifying this and our paper has just appeared on the arXiv.

Categorifying the magnitude of a graph, Richard Hepworth and Simon Willerton.

The magnitude of a graph can be thought of as an integer power series. For example, consider the Petersen graph.

Its magnitude starts in the following way. $\begin{aligned} \#P&=10-30q+30q^{2}+90q ^{3}-450q^{4}\\ &\quad\quad+810q^{5} + 270 q^{6} - 5670 q^{7} +\dots. \end{aligned}$

Richard observed that associated to each graph $G$ there is a bigraded group $\mathrm{MH}_{\ast ,\ast }(G)$, the **graph magnitude homology** of $G$, that has the graph magnitude $# G$ as its graded Euler characteristic.
$\begin{aligned}
#G &= \sum _{k,l\geqslant 0} (-1)^{k}\cdot \mathrm{rank}\bigl (\mathrm{MH}_{k,l}(G)\bigr )\cdot q^{l}\\
&= \sum _{l\geqslant 0} \chi \bigl (\mathrm{MH}_{\ast ,l}(G)\bigr )\cdot q^{l}.
\end{aligned}$
So graph magnitude homology categorifies graph magnitude in the same sense that Khovanov homology categorifies the Jones polynomial.

For instance, for the Petersen graph, the ranks of $\mathrm{MH}_{k,l}(P)$ are given in the following table. You can check that the alternating sum of each row gives a coefficient in the above power series.

$\begin{array}{rrrrrrrrrr} &&&&&&k\\ &&0&1&2&3&4&5&6&7 \\ &0 & 10\\ & 1 & & 30 \\ &2 & && 30 \\ &3 &&& 120 & 30 \\ l &4 &&&& 480 & 30 \\ &5 &&&&& 840 & 30 \\ &6 &&&&& 1440 & 1200 & 30 \\ &7 &&&&&& 7200 & 1560 & 30 \\ \\ \end{array}$

Many of the properties that Tom proved for the magnitude are shadows of properties of magnitude homology and I’ll describe them here.