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this page not only has query boxes, but it also has a long discussion section at the end of the article, most of which would probably be better suited in the nForum rather than on the page.
Removing query box discussion from article:
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Urs Schreiber: I haven’t looked at Marco Grandis’ book yet: does it say anything about the homotopy hypothesis in the context of the definition of directed space used there?
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Don’t know why Tim Porter’s comment got cut off but the full discussion is
Urs Schreiber: I haven’t looked at Marco Grandis’ book yet: does it say anything about the homotopy hypothesis in the context of the definition of directed space used there?
Tim Porter: No.
The following text was originally in a “Discussion” section on this article:
The above defined directed topological spaces. My impression is that Eric was interested in more general concepts. But the above definition has a straightforward generalization away from topological spaces. The general strategy is really: start with a category with interval object and consider then the category whose objects are pairs $(X, d X)$ for $X$ an object and $d X$ a subobject of $[I,X]$, and whose morphisms are morphisms $X \to Y$ that take $d X$ to $d Y$.
For instance, let’s define directed sets: make the ordinary category Set a category with interval object by , say, taking the interval object to be the set $I := [n]$ of $n$ elements. A map from $I$ into any other set can be regarded as an $n$-step path in that set. Then pairs consisting of a set and a subset of all such maps model “directed sets”.
Eric says: Yes, exactly :) That sounds like a good plan. By the way, what you say about $I := [n]$ reminds me a lot of simplicial sets.
Eric says: We have directed spaces and we may soon have directed sets. This makes me wonder if we should have a directed category internal to another category? This way
Would that make sense?
Urs: Let’s see, before getting into this idea of realizing a directed space as a space internal to something else or the like, I don’t see what you want to mean by a “directed category”. See, the point is that a category already is supposed to be a combinatorial model for a directed space. Just as a groupoid is a combinatorial model for an undirected space. This is the very motivation for defining directed spaces: to fill in the question marks in
This is why a directed space is defined such that its “thing of all paths in it” is not, in general, a fundamental groupoid but a fundamental category.
Methinks that for the application which you have in mind you want to be studying posets and these are special cases of categories and in particular naturally interpreted as combinatorial models for directed space, in exactly the way in which you are thinking of them as directed spaces! So it seems to me you don’t actually need to be looking for what you seem to be looking for, since it is already quite easily there. But of course maybe I misunderstand what you are after.
Eric: I doubt that what I am looking for is new. If you could help put a name on it, that would be great. I’m not exactly sure what I mean by directed category either other than a “category with a direction” :|
Urs: but a category is directed! Recall that underlying every category is a directed graph (it is a directed graph equipped with a composition operation). So I am still puzzled by what you are looking for, because a “directed category” would have underlying it a “directed directed graph”. What’s that supposed to be? And why do you want it?
Eric: Sorry for being so dense. We can delete this once I get a clue. But for now, I’m still confused. Maybe what I wanted to say is more along the line (but probably still not correct)
“A directed space has a fundamental category”
“A directed set has a fundamental category”
“A directed object has a fundamental category”
Ack! light bulb! (those hurt sometimes)
I think that is probably precisely why you defined directed object.
Could we say (and be correct!) that
“a directed space is a directed object in Top”?
“a directed set is a directed object in Set”?
If so, I think I am making some progress.
Urs: Yes, a directed space should be a directed object in the category of possibly directed topological spaces! (In Top itself there are no directed spaces. Every ordinary topological space is undirected). I think I listed that as a should-be example. To make it a proper example one will have to say a few more probabaly straightforward things about directed homotopies etc. But the idea is certainly this, yes, a directed space is a directed object in the category of possibly directed spaces.
And as for categories: the generic category is a directed object in the category of categories. Unless it happens to be a groupoid. In which case it is an undirected object there.
(All this with respect to the “canonical” choice of interval object. The notion of directedness depends on which interval object you choose to test with. For instance the point itself satisfies the axioms of an interval object. But using it of course everything will look undirected.)
Eric: Ugh. I didn’t want a directed space to be a directed object in the category of directed spaces. That is boring :) A set is an object in the Set too, but it doesn’t tell you anything. Hmm. It looks like what I wanted isn’t going to work as is, i.e. a directed space is not a directed object in Top because there are no directed objects in Top apparently.
Urs: I think you do want that. Just don’t let the terminology let mix you up. An ordinary space is already called a space. While from your perspective an ordinary space ought to be called an undirected space. Then “space” could be assigned to mean “not-necessarily but possibly directed space” and then a directed space could be called a directed object in spaces.
But convention is different. So a directed space is a directed object in the category of “not necessarily but possibly drected spaces”.
Toby: Even here, I don't think that you're really using the terminology ideally. The proper term for what you're calling a “not-necessarily but possibly directed space” is just directed space! Much like a non-associative algebra might happen to be associative, so a directed space might happen to be undirected. (In terms of Grandis's definition, any space $X$ defines a directed space where $d$ consists of only the constant paths.)
Urs: Right, Toby, I think that is my point. I was just trying to convince Eric that there is nothing wrong or cheating or boring about the fact that “a directed space is a directed object in the category of directed spaces”.
But maybe the the true issue is whether we want to speak of “directed objects” over at directed object or rather restrict to speaking about undirected objects. Then every object would be a directed object, possibly with trivial direction information, while those objects which are propertly directed would be the not undirected objects.
I consider you as an authority on such issues of logical rigour. You should say how we should fix the terminology and we’ll implement that.
Toby: I'll discuss this at directed object.
Removed an old query box:
But for that to work we need the structure of a directed topological space on $C(I_d,(X,d X))$. This requires that $X$ has directed homotopies! Does Grandis discuss higher directed paths, too? —Urs
Toby: I don't think that you need internal homs and all that. But see my edits to directed object.
Urs: I think we need directed homotopies to check if a “constructed” directed space is actually a directed object in the original definition: that original definition asks us to check if the internal hom $[I,X]$ is weakly equivalent to $X$. Well, I made up this definition because I think it is the right abstraction, but there is room of course to debate this. But if we accept it then we should try to define the internal hom of Grandis’ directed spaces. There is an obvious solution which one should check the details of: namely a directed topological space should be one which singles out not only subsets of $hom(I,X)$ but subsets of $hom(I^{\times n}, X)$ for all $n$, closed under the obvious reparameterization and gluing. This would induce an obvious notion of directed homotopies and should induce in an obvious way an internal hom for directed topological spaces. I’d think. But I don’t feel like investing much time into finalizing this idea right now…
Tim Porter: As I have now looked at Marco’s book, there are results on exponentiable d-spaces.(p.59). I can give details if anyone is still interested.
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