In other words, I’m hosed.
I’ve been thinking about a category of ‘abstract locally complete partitions’. Fix , a disjoint union of copies of indexed by elements of the finite set . My category is then supposed to consist of pairs where is a finite collection of affine spaces, , along with a partition, , of the indexing set , and an affine (on each component) map which is non-locally-constant. I interchangeably let refer to the collection of spaces, or the partition on that collection of spaces.
To describe non-locally-constant, I must first remind you that I let my partition on induce a partition on where iff the component containing is equivalent to the component containing , mod . That is, all points in a component are considered equivalent, and two components are equivalent as determined by . Now being non-locally-constant means that there exists equivalent mod but with .
Now, given such an object in my category, I would like to reduce it to a more restricted type of complete locally affine partition. In particular, I would like to reduce it to the case where
- There are no more than zero-dimensional affine spaces in , and no more than non-zero-dimensional affine spaces in .
- If is a component of that is not zero-dimensional, then the -equivalence class of is just itself.
- My map takes distinct zero-dimensional spaces in to distinct components of . Similarly for non-zero-dimensional spaces.
To complete the description of my big category, I need to describe the morphisms. A map from to will be an affine map such that and – a property I will now further clarify. My consists of a partition of the space . By , I mean the transitive closure of the relation on where whenever in then in . This process gives me an equivalence relation on . Any time I have an equivalence relation on a (disjoint union of) affine space(s), I let denote the finest coarsening of that is “locally affine” – meaning equivalence classes are (disjoint unions of) affine subspaces, and all equivalence classes in a given component are parallel (technically, there’s probably a little more than that, but it’s good enough for now I guess). So now we have the meaning of , and by , I simply mean that is coarser than (by which I mean the equivalence relation on .
So allow me to recap. My is an affine map with a property saying that an appropriate triangle commutes (), and the affine closure of the image of the partition for is coarser than the partition for . Since is a “complete” locally affine partition (any two points in the same component are in the same equivalence class), this also forces to be “complete”.
Of course, I’m not mentioning that this category is really “a category object in the category of topological spaces”. So really I have a space of objects, a space of morphisms, and enough maps between them to make sense of things. I’ll continue not mentioning that, saving it for another day.
Now, like I said, I want to be reducing any in my big category to get it down to a particularly nice form. One of the main steps I had been relying on turns out to not be allowed, because it isn’t functorial. I hardly knew one could write down “obvious” maps that weren’t functorial, but I’ve apparently done so rather frequently lately.
So, what is this construction? Given , one step I want to take is to replace any subset of the components of that are (1) all related by the partition, and (2) all map to the same component of . I want to replace such a subset by the affine span (direct sum in the category of affine spaces) of the spaces in it. This seems entirely reasonable. Given a bunch of affine spaces, and a map to an affine space, I get, for free, a map from the direct sum of the original affine spaces. That’s what direct sums do.
However, since I have disjoint unions as targets of maps , I run into trouble. Consider, for example, where the space consists of three disjoint points, the equivalence relation has them all equivalent, and sends them all to the same component. Consider the same three points, the same , but only two of the points are equivalent. The obvious map has , and the triangle commutes by construction. Now, when I take affine spans as mentioned, I end up with a single space in (a plane, the affine span of 3 points), and two spaces in (a line (the span of two points), and a point). That’s a problem, because I no longer know what to do with . As I mentioned before, given a map from a bunch of affine spaces to a single affine space, I get a map from the affine span to the single space. However, given a map from a bunch of affine spaces to a bunch of affine spaces, I no longer get a map from the affine span to the same bunch of spaces (the single affine span, being connected, can only end up in one of the target spaces).
So that’s upsetting. It’s not even the only thing I’ve written down recently that wasn’t a functor. Back to the drawing board, as a fella says.