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I have touched H-space, slightly expanded here and there and slightly reorganized it.
A question on terminology:
in the spirit of the terms “H-monoid” and “H-group” it would be natural to say “H-category” for a category object internal to Ho(Top). Are there any texts that say so, explicitly? Or else, is there any term which is semi-standard at least in some circles for categories internal to Ho(Top)?
Ho(Top) doesn’t have a lot of pullbacks, so I’m not exactly sure what you would want to mean by a category internal to it. Do you just mean a category-like thing internal to Top where the associativity and unitality only hold up to incoherent homotopy? I don’t think I remember ever seeing such a thing; do you have some example in mind?
Or perhaps a weak category object in the 2-category with underlying 1-category Top and with equivalence classes of homotopies for 2-arrows? This probably only makes sense for cofibrant topological spaces, though.
Right, I didn’t say that well.
What I mean is the structure in ∞Grpd given by a type X0:Type, a dependent type x0,x1:X0⊢X1(x0,x1):Type, a function x0,x1,x2:X0⊢compx0,x1,x2:X1(x0,x1)×X1(x1,x2)→X1(x0,x2) such that there exists an equivalence x0,x1,x2,x3:X0⊢(compx0,x2,x3∘compx0,x1,x2≃compx0,x1,x3∘compx1,x2,x3) (and similarly for units).
Or maybe that would rather deserve to be called an “H-Segal space” or the like. The term “H-category” would probably best fit for Ho(Top)-enriched categories. Any opinions?
I am asking because I am having discusison with somebody on how to say category internal to HoTT and I would like to make the point that the coherence is important. I kept saying: “notice that for the special 1-object case there is the crucual distinction between an H-monoid and an A∞-space” but I would like to be able to drop the “for the special case” and just say “there is the crucial distinction between an H-category and an A∞-category” – in such a way that I can make “H-category” a pointer to an nLab entry whose existence and title receives a minimum of consensus.
Well, A∞ category usually also refers to the enriched case. I’m dubious of distinguishing any of these notions with a name like “H-category”, unless someone actually has a use for one of them. (Also, it’s a little unfortunate that in HoTT we sometimes prefix things with an “h-” without meaning homotopy incoherence, e.g. h-set and h-prop and h-level.)
Well, A∞ category usually also refers to the enriched case.
You’d never say A∞ for Ho(Top)-enrichment!
unless someone actually has a use for one of them.
It appears all over the place, for instance the H-category underlying a Segal space is one way to define what an equivalence in a Segal space is.
My point was that since A∞ refers to something enriched in spaces, if you wanted to say ’there is the crucial distinction…’ comparing them to H-categories, then H-categories would have to be enriched in, rather than ’internal’ to, Ho(Top).
What about: an H-category is a monad in the homotopy bicategory of spans?
25 other lettes
One day I’ll invent an Umlaut-theory: äu-cohomology for äußerst interessante Kohomologie.
What about something like “A3-category”? (Or maybe A4? I’m not sure how the indexing is aligned for An.)
Ah, that’s a good pont! Yes. I’ll try to get back to that later…
Thanks! Then “A3-category” would be what I meant.
In general, internalization is different from enrichment, and the definitions above for i.e. H-monoid, H-group, H-ring made it fairly clear that the definition of H-category considered here are category objects internal to Ho(Top), rather than Ho(Top)-enriched categories.
Anonymous
Thanks for the alert. But then let’s just add a working link:
Added the recent
Will add to these authors’ pages.
added pointer to:
and pointer to:
Thanks. This used to be mentioned in passing at Sullivan model of loop space but without a reference.
I have expanded a little (here) and cross-linked the two paragraphs.
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