The String Coffee Table

I’m packing for tomorrow’s trip to Marseille, where I’ll talk about monads, PROPs, Lawvere’s algebraic theories and reasoning with diagrams to a bunch of people interested in the geometry of calculation. You might like the stuff about the relation between classical and quantum logic - that is, categories with finite products, versus symmetric monoidal categories!

But, for now, just one remark on spans versus bimodules. At first glance spans and bimodules seem pretty different, since a span is a map from some gadget $X$ to some gadget $A\times B$, while a bimodule is some kind of action of $A\times B$ on $X$.

(I want to be a bit vague about what these things $A,B$ and $X$ are, since the issue is very general. I’m also being a bit vague about when we use $B$ and when we use $B^{\mathrm{op}}$; for bimodules of rings we normally say we have an action of $A\times B^{\mathrm{op}}$ on $X$. Maybe I should even be a bit vague about whether I really mean $\times$ or $\otimes$! One can usually sort out all these issues without too much trouble in any specific situation.)

But, we can see spans and bimodules are actually not so different if we think about it right!

First of all, the issue of “spans” versus “bimodules” is really just a special case of the issue of “maps” versus “modules”, as we can see by calling $A\times B$ above simply $P$. Then our span becomes a map $P\to X$, while our bimodule becomes an action of $X$ on $P$.

So, how can maps masquerade as actions and vice versa?

Well, here’s one way. Suppose we have a principal $G$-bundle $P\to X$. Then this is “the same” as a map $X\to \mathrm{BG}$ where $\mathrm{BG}$ is the classifying space of $G$.

And here’s another way… which is secretly the same idea. Suppose we have a functor $P\to X$. Then this is “the same” as a weak 2-functor $X\to \mathrm{Cat}$ assigning to each object of $X$ its “essential preimage” under the functor $P\to X$.

Or, decategorifying: suppose we have a function $P\to X$. Then this is “the same” as a function $X\to Set$, assigning to each point its preimage under the map $P\to X$.

Or, a special case: suppose we have a subset $P\to X$. Then this is “the same” as a function $X\to \mathrm{0,1}$, namely the characteristic function of this subset.

These are all examples of a general construction which I described in week223. And, they all have pretty easy “bimodule/span” versions, where you write $X$ as $A\times B$.

Jeffrey uses this trick in his paper when he thinks of a stuff operator, not as a “matrix of groupoids” $A\times B\to \mathrm{Gpd}$, but (equivalently) as a groupoid $P$ equipped with a functor $P\to A\times B$.

It’s possible that nothing I said in my last comment is interesting to you or relevant to what you’re doing… I’m just gradually catching up with your train of thought, and I can’t help saying a bunch of stuff as I do so.

Your account of Ostrik’s theorem is really fascinating. I’ve mainly thought about module categories of Vect, thanks to the work of Kapranov-Voevodsky and Yetter. Indeed, under some nice assumptions these turn out to be just categories like Vectⁿ, and Vectⁿ is the category of modules of kⁿ - the commutative ring of k-valued functions on the n-point set, where k is our ground field. So, Ostrik is generalizing the heck out of this…..

Of course we can also think of Vectⁿ as the category of modules of any algebra Morita-equivalent to kⁿ, like a direct sum of n matrix algebras. So, Morita equivalence shows up very naturally these questions.

This makes me want to digress a bit….

As you know, Morita equivalence gets along with the operation of tensoring algebras, so we get a monoid of Morita equivalence classes of algebras over k, where the monoid operation comes from tensor product. And sitting inside this monoid is the Brauer group, consisting of all the invertible elements.

The Brauer group of the real numbers, for example, is Z/2, with elements corresponding to R (real numbers) and H (the quaternions).

But, we can also play this game with superalgebras, and then it turns out the superBrauer group of R is Z/8, which is just the Bott periodicity clock! I spoke about this in week211 and week212, based on some preliminary remarks in week209.

It would be fun to fit this into your big picture somehow, especially since it’s connected to how superstrings work best in 10 dimensions.

But I’m still just rambling, not really responding to your ideas… sorry!