August 19, 2016

Compact Closed Bicategories

Posted by John Baez

I’m happy to announce that this paper has been published:

Abstract. A compact closed bicategory is a symmetric monoidal bicategory where every object is equipped with a weak dual. The unit and counit satisfy the usual ‘zig-zag’ identities of a compact closed category only up to natural isomorphism, and the isomorphism is subject to a coherence law. We give several examples of compact closed bicategories, then review previous work. In particular, Day and Street defined compact closed bicategories indirectly via Gray monoids and then appealed to a coherence theorem to extend the concept to bicategories; we restate the definition directly.

We prove that given a 2-category $C$ with finite products and weak pullbacks, the bicategory of objects of $C$, spans, and isomorphism classes of maps of spans is compact closed. As corollaries, the bicategory of spans of sets and certain bicategories of ‘resistor networks” are compact closed.

Posted at 2:10 AM UTC | Permalink | Followups (14)

August 11, 2016

A Survey of Magnitude

Posted by Tom Leinster

The notion of the magnitude of a metric space was born on this blog. It’s a real-valued invariant of metric spaces, and it came about as a special case of a general definition of the magnitude of an enriched category (using Lawvere’s amazing observation that metric spaces are usefully viewed as a certain kind of enriched category).

Anyone who’s been reading this blog for a while has witnessed the growing-up of magnitude, with all the attendant questions, confusions, misconceptions and mess. (There’s an incomplete list of past posts here.) Parents of grown-up children are apt to forget that their offspring are no longer helpless kids, when in fact they have a mortgage and children of their own. In the same way, it would be easy for long-time readers to have the impression that the theory of magnitude is still at the stage of resolving the basic questions.

Certainly there’s still a great deal we don’t know. But by now there’s also lots we do know, so Mark Meckes and I recently wrote a survey paper:

Tom Leinster and Mark Meckes, The magnitude of a metric space: from category theory to geometric measure theory. ArXiv:1606.00095; also to appear in Nicola Gigli (ed.), Measure Theory in Non-Smooth Spaces, de Gruyter Open.

Here I’ll tell you some of the highlights: ten things we used not to know, but do now.

Posted at 9:57 AM UTC | Permalink | Followups (73)

August 10, 2016

Two Miracles of Algebraic Geometry

Posted by John Baez

In real analysis you get just what you pay for. If you want a function to be seven times differentiable you have to say so, and there’s no reason to think it’ll be eight times differentiable.

But in complex analysis, a function that’s differentiable is infinitely differentiable, and its Taylor series converges, at least locally. Often this lets you extrapolate the value of a function at some faraway location from its value in a tiny region! For example, if you know its value on some circle, you can figure out its value inside. It’s like a fantasy world.

Algebraic geometry has similar miraculous properties. I recently learned about two.

Posted at 8:40 AM UTC | Permalink | Followups (47)

August 9, 2016

In Praise of the Gershgorin Disc Theorem

Posted by Tom Leinster

I’m revising the notes for the introductory linear algebra class that I teach, and wondering whether I can find a way to fit in the wonderful but curiously unpromoted Gershgorin disc theorem.

The Gershgorin disc theorem is an elementary result that allows you to make very fast deductions about the locations of eigenvalues. For instance, it lets you look at the matrix

$\begin{pmatrix} 3 &i &1 \\ -1 &4 + 5i &2 \\ 2 &1 &-1 \end{pmatrix}$

and see, with only the most trivial mental arithmetic, that the real parts of its eigenvalues must all lie between $-4$ and $7$ and the imaginary parts must lie between $-3$ and $8$.

I wasn’t taught this theorem as an undergraduate, and ever since I learned it a few years ago, have wondered why not. I feel ever so slightly resentful about it. The theorem is so useful, and the proof is a pushover. Was it just me? Did you get taught the Gershgorin disc theorem as an undergraduate?

Posted at 5:21 PM UTC | Permalink | Followups (21)

August 8, 2016

What is a Formal Proof?

Posted by Mike Shulman

There’s been some discussion recently in the homotopy type theory community about questions like “must type-checking always be decidable?” While the specific phrasing of this question is specific to type theory (and somewhat technical as well), it is really a manifestation of a deeper and more general question: what is a formal proof?

At one level, the answer to this question is a matter of definition: any particular foundational system for mathematics defines what it considers to be a “formal proof”. However, the current discussions are motivated by questions in the design of foundational systems, so this is not the relevant answer. Instead the question is what properties should a notion of “formal proof” satisfy for it to be worthy of the name?

Posted at 9:06 PM UTC | Permalink | Followups (108)

August 6, 2016

Topological Crystals (Part 3)

Posted by John Baez

Last time I explained how to build the ‘maximal abelian cover’ of a connected graph. Now I’ll say more about a systematic procedure for embedding this into a vector space. That will give us a topological crystal, like this:

Some remarkably symmetrical patterns arise this way!

Posted at 10:09 AM UTC | Permalink | Followups (1)

July 27, 2016

Topological Crystals (Part 2)

Posted by John Baez

We’re building crystals, like diamonds, purely from topology. Last time I said how: you take a graph $X$ and embed its maximal abelian cover into the vector space $H_1(X,\mathbb{R})$.

Now let me back up and say a bit more about the maximal abelian cover. It’s not nearly as famous as the universal cover, but it’s very nice.

Posted at 10:30 AM UTC | Permalink | Followups (6)

July 23, 2016

Topological Crystals (Part 1)

Posted by John Baez

Over on Azimuth I posted an article about crystals:

In the comments on that post, a bunch of us worked on some puzzles connected to ‘topological crystallography’—a subject that blends graph theory, topology and mathematical crystallography. You can learn more about that subject here:

I got so interested that I wrote this paper about it, with massive help from Greg Egan:

I’ll explain the basic ideas in a series of posts here.

Posted at 8:18 AM UTC | Permalink | Followups (10)

June 11, 2016

How the Simplex is a Vector Space

Posted by Tom Leinster

It’s an underappreciated fact that the interior of every simplex $\Delta^n$ is a real vector space in a natural way. For instance, here’s the 2-simplex with twelve of its 1-dimensional linear subspaces drawn in:

(That’s just a sketch. See below for an accurate diagram by Greg Egan.)

In this post, I’ll explain what this vector space structure is and why everyone who’s ever taken a course on thermodynamics knows about it, at least partially, even if they don’t know they do.

Posted at 6:30 PM UTC | Permalink | Followups (24)

May 26, 2016

Good News

Posted by John Baez

Various bits of good news concerning my former students Alissa Crans, Derek Wise, Jeffrey Morton and Chris Rogers.

Posted at 4:12 AM UTC | Permalink | Followups (17)

May 20, 2016

Castles in the Air

Posted by Mike Shulman

The most recent issue of the Notices includes a review by Slava Gerovitch of a book by Amir Alexander called Infinitesimal: How a Dangerous Mathematical Theory Shaped the Modern World. As the reviewer presents it, one of the main points of the book is that science was advanced the most by the people who studied and worked with infinitesimals despite their apparent formal inconsistency. The following quote is from the end of the review:

If… maintaining the appearance of infallibility becomes more important than exploration of new ideas, mathematics loses its creative spirit and turns into a storage of theorems. Innovation often grows out of outlandish ideas, but to make them acceptable one needs a different cultural image of mathematics — not a perfectly polished pyramid of knowledge, but a freely growing tree with tangled branches.

The reviewer makes parallels to more recent situations such as quantum field theory and string theory, where the formal mathematical justification may be lacking but the physical theory is meaningful, fruitful, and made correct predictions, even for pure mathematics. However, I couldn’t help thinking of recent examples entirely within pure mathematics as well, and particularly in some fields of interest around here.

Posted at 3:39 AM UTC | Permalink | Followups (26)

May 19, 2016

The HoTT Effect

Posted by David Corfield

Martin-Löf type theory has been around for years, as have category theory, topos theory and homotopy theory. Bundle them all together within the package of homotopy type theory, and philosophy suddenly takes a lot more interest.

If you’re looking for places to go to hear about this new interest, you are spoilt for choice:

For an event which delves back also to pre-HoTT days, try my

Posted at 12:57 PM UTC | Permalink | Followups (3)

May 12, 2016

E8 as the Symmetries of a PDE

Posted by John Huerta

My friend Dennis The recently gave a new description of the Lie algebra of $\mathrm{E}_8$ (as well as all the other complex simple Lie algebras, except $\mathfrak{sl}(2,\mathbb{C})$) as the symmetries of a system of partial differential equations. Even better, when he writes down his PDE explicitly, the exceptional Jordan algebra makes an appearance, as we will see.

This is a story with deep roots: it goes back to two very different models for the Lie algebra of $\mathrm{G}_2$, one due to Cartan and one due to Engel, which were published back-to-back in 1893. Dennis figured out how these two results are connected, and then generalized the whole story to nearly every simple Lie algebra, including $\mathrm{E}_8$.

Posted at 6:30 PM UTC | Permalink | Followups (18)

May 10, 2016

The Works of Charles Ehresmann

Posted by John Baez

There are 630 pages on algebraic topology and differential geometry, 800 pages on local structures and ordered categories, and their applications to topology, 900 pages on structured categories and quotients and internal categories and fibrations, and 850 pages on sketches and completions and sketches and monoidal closed structures.

That’s 3180 pages!

On top of this, more issues of the journal he founded, Cahiers de Topologie et Géométrie Différentielle Catégoriques, will become freely available online.

Posted at 3:51 PM UTC | Permalink | Followups (16)

May 8, 2016

Man Ejected from Flight for Solving Differential Equation

Posted by Tom Leinster

A professor of economics was escorted from an American Airlines flight and questioned by secret police after the woman in the next seat spotted him writing page after page of mysterious symbols. It’s all over the internet. Press reports do not specify which differential equation it was.

Although his suspiciously mediterranean appearance may have contributed to his neighbour’s paranoia, the professor has the privilege of not having an Arabic name and says he was treated with respect. He’s Italian. The flight was delayed by an hour or two, he was allowed to travel, and no harm seems to have been done.

Unfortunately, though, this story is part of a genre. It’s happening depressingly often in the US that Muslims (and occasionally others) are escorted off planes and treated like criminals on the most absurdly flimsy pretexts. Here’s a story where some passengers were afraid of the small white box carried by a fellow passenger. It turned out to contain baklava. Here’s one where a Berkeley student was removed from a flight for speaking Arabic, and another where a Somali woman was ejected because a flight attendant “did not feel comfortable” with her request to change seats. The phenomenon is now common enough that it has acquired a name: “Flying while Muslim”.

Posted at 7:30 PM UTC | Permalink | Followups (22)