# Ordinal category theory?

Just out of curiosity: Is there a notion of $\alpha$-category for an ordinal number $\alpha$, extending the given notions for $\alpha \leq \omega$? If there is none, which one would you propose? Feel free to draw images.

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One can also dream of a notion of x-category for any real (or whatever) number x. – Jonathan Chiche Sep 16 '11 at 16:56
I imagine that the difficulty would be in what you must do for limit ordinals. – Spice the Bird Sep 17 '11 at 5:40
Here is an idea: giving a higher category is equivalent so giving its nerve, which is a simplicial that satisfies some horn filling condition. Now one could define $\alpha$-categories by taking "huge simplical sets" which are indexed not only by the finite ordinals but by ordinals smaller than $\alpha$ and again impose some horn filling condition. – Jan Weidner Sep 18 '11 at 11:28
@Jan: I don't think that would work. That would give you a notion of an $(\alpha,1)$-category. The simplicial models of Verity and Street for (even strict) higher categories with noninvertible higher cells make use of thin cells, which are "marked" cells in a stratified simplicial set. – Harry Gindi Sep 20 '11 at 10:53
This question seems similar to the question I asked here: mathoverflow.net/questions/91213/… – Samuel Reid May 14 '12 at 0:59

Let $\Delta_2$ the 2-tronked simplicial category (objects are the finite orders $0=$(0), $1=$(0,1), $2=$(0, 1, 2) and order-preserving functions). Then $Cat$ (category of small categories)is isomorphically to $Fun_c(\Delta_2^{op}, Set)$ (finite limits preserving functors and natural transformations). More in general the (hyper)category of n-double small categories $n$-$Double$ is isomorphically to $Fun_c((\Delta_2^{op}\times\ldots\times\Delta_2^{op}), Set)$ (n fold products), and $n$-$Cat$ is a subcategory of $n$-$Double$ (elements of $Fun_c((\Delta_2^{op}\times\ldots\times\Delta_2^{op}), Set)$ that send some morphisms to identities). Of course you can generalize this for a infinite cardinal $n$.

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Thanks! Can you give a reference for the isomorphism $\mathrm{Cat} \cong \mathrm{Fun}_c(\Delta^{\mathrm{op}},\mathrm{Set})$? The functor is probably the nerve, which is fully faithful, but I haven't come across this description of the image. – Martin Brandenburg May 13 '12 at 17:04
Its a old folklore property, for example P.Johnstone "Topos theory" p.48, remark. 2.13. That double categories are Cat-object (internal categories) inside Cat (this last is the category of Cat object come from JW Gray, Formal category theory Lnm 391, or R. Brown article "Double categorie, 2.categories, thin-structures, connections" (is mentioned as a result, no more hard than a nice exercise). – Buschi Sergio May 13 '12 at 17:52
YOu can find a more simple algebraic description of $\omega$-category (stright generalizable to infinite cardinals) in R. Street "The algebra of oriented simplexes" J. Pure Appl. Alg.49 (math.mq.edu.au/~street/aos.pdf) – Buschi Sergio May 13 '12 at 17:53